Introduction

Food is a typical realization of the strong ties between environment and people (Blache 1970) and provides not only calories and nutrients, but also cultural meaning. (Diaz et al. 2018). Adequate management of regional landscapes and biodiversity enriches food diversity, which is essential for human nutrition and resilience in a regional food system (Lachat et al. 2018). As a representative instance, food-sharing networks not only secure nutritional resources, but also enhance cultural identity and social capital (Cantarero et al. 2013; Kamiyama et al. 2016). Within a food-sharing network, community members give and receive foods without receiving payment (Morton et al. 2008; Davies et al. 2010). These foods, called “non-market food” (Kamiyama et al. 2016), are self-produced, harvested and/or received with non-market transactions. Many scientists worldwide have recognized the contribution of non-market foods to human well-being. Non-market food promotes the nutrition of women, children and older adults in multiple ways (Girard et al. 2012; Quandt et al. 2001) and diversifies nutrient ingestion and household income (Talukder et al. 2010). The sharing of home-grown food is important for strengthening social cohesion, environmental sustainability and food re-localization in developed countries (Jehlicka and Danek 2017). However, the actual nutritional value of the food exchanged has not been investigated (Quandt et al. 2001).

Food-sharing activities can be understood as collective practices which provide people the basic needs of everyday life, newly discussed in the notion of sustainable materialism in the post-postmaterial era (Schlosberg and Coles 2015). These multiple benefits brought by food-sharing activities are not accounted in the conventional economy yet. Thus we recognized these hidden values as “shadow benefits” and have been investigating how local natural capital benefits people and these benefits are shared through social networks in Japan. Collecting, processing and gifting wild local foods reinforce human relations and the inheritance of local food culture (Nakawaza et al. 2014). In an online web survey, Saito et al. (2018) concluded that food-sharing cultures contribute to enhanced resilience against socio-economic changes and natural disasters.

The main references of this study are Kamiyama et al. (2014), Saito et al. (2015) and Kamiyama et al. (2016), who tried to quantify the shadow benefits of non-market food in two typical rural areas in Japan (the Noto Peninsula and Hachijo Island). Kamiyama et al. (2014) identified the proportions of non-market food consumptions in representative food categories (vegetables/potatoes/beans, fruit, rice, forest products, seafood/seaweed, and egg/dairy products), and evaluated their economic values. By interview and questionnaire surveys on Hachijo Island, Saito et al. (2015) identified the shared food species in detail, and found that non-market foods occupied approximately half of the total food consumption during the high cropping and harvesting season of agricultural and marine products. Kamiyama et al. (2016) identified the socio-geographic factors affecting the sharing mechanism. From the above previous studies, they found that connections to rural communities enhance the varieties and quantities of non-market food consumed, and that food-species diversity contributes to human well-being. However, the structure of food-sharing networks, including the shared-food species in detail, has not been fully investigated and clarified. The quantities of non-market food consumptions have not been exhaustively systematized into food categories consistent with official statistics. Additionally, the shadow benefits of non-market foods have been evaluated only in monetary units, and must be linked to current accounting systems for a more comprehensive evaluation.

To overcome these challenges, this study aims to (1) identify the structure of food-sharing networks and non-market food species, (2) quantify the composition of food distribution channels by systematic food categories, and (3) elucidate the comprehensive shadow benefits provided by non-market transactions.

Materials and methods

Case study site: Hachijo Island

The case study site was Hachijo Island, Japan. Hachijo Island is located in the Izu Islands, approximately 300 km south of Tokyo (Fig. 1). It occupies an intermediate position between Tokyo Metropolis and the Ogasawara Islands, and is accessible by direct flight from Tokyo. Among the many isolated islands in Japan, Hachijo Island is large in both size (69 km2) and population (7591 people and 4392 households in 2017) (GSI 2017; Hachijo Island Department of Financial Planning 2017). The island has a subtropical climate with an annual average temperature, precipitation, and humidity of 17.8 °C, 3202 mm and 80%, respectively. The island has two mountains, Mt. Hachijo Fuji (854 m) and Mt. Mihara (700 m) (Hachijo Town 2017). Hachijo Island includes five communities: Mitsune, Okago, Kashitate, Nakanogo, and Sueyoshi. Generally, material exchange between an isolated island and the main island are limited, so the Hachijo people depend on various products imported from the main island while sharing food within their local communities (Saito et al. 2015). The food-sharing activity in Hachijo Island is mainly interactive and driven by social norm, not by barter economy (Saito et al. 2015). Especially, although rice is the staple food in Japan, there is no rice field on Hachijo Island. Thus, the residents receive all their rice from the mainland. Thirty-eight percent of the population are seniors (aged over 65). The main industries are flower horticulture, fishing, and tourism-related industries (Hachijo Island Department of Financial Planning 2017). The gross agricultural production in 2016 was 1.8 billion JPY (1.7 million USD, 109 JPY USD−1), of which 84% comprised horticultural products such as Phoenix roebelenii, Rumohra adiantiformis, Freesia and Sandersonia aurantiaca, and 15% comprised agricultural crops such as Angelica keiskei, vegetables, and potatoes. The fish catches in 2016 weighed 1060 t and produced 940 million JPY (8.6 million USD, 109 JPY USD−1). Red snapper, flying fish, and mackerel scad are the main marine products.

Fig. 1
figure 1

Map of Hachijo Island

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In this study, we conducted interview and questionnaire surveys about non-market food. According to Kamiyama et al. (2016)’s definition, non-market food is food obtained through non-market transaction without financial transaction. In other words, non-market food includes residents produced at their home garden, foods they harvested by themselves, and foods they received from others. The foods received from others include foods others produced, others harvested, and others bought.

Interview survey

Hachijo residents were surveyed by structured and open-ended interviews. The interview survey was executed from Nov. 30 to Dec. 2, 2016. The interviewees (15 residents) were recommended by the Hachijo Island Tourism Association, Hachijo’s town office, and the Hachijo Island Women’s Association. The interviewees’ ages ranged from the 20 s to the 80 s (mean age 54 years, s.d. 16.4 years). Thirteen interviewees were female with a thorough knowledge of food production, purchase, and preservation. The interviewees covered four communities of Hachijo Island and most of them were native to the island. The interview comprised seven questions (Table 1). Each interview lasted 40 to 60 min. After all interviews, the food species were categorized by season and the food-sharing networks were determined. The unknown seasonal information was complemented by the comments of other interviewees, internet sources (Foods Link 2017; Yasainavi 2017) and studies on Hachijo Island foods (Yamasaki 1976). The standard climatological seasons were assumed: March to May for spring, June to August for summer, September to November for autumn and December to February for winter (Japan Meteorological Agency 2017).

Table 1 Questions in the interview (I1–I7) and questionnaire surveys (Q1, Q2)
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Questionnaire survey

Our questionnaire survey was designed to identify the usual distribution channels of the foods acquired by residents. Three hundred questionnaires were distributed to the island residents through the Hachijo Island Tourism Association, Hachijo’s town office and the Hachijo Island Women’s Association from Oct. 10 to Nov. 30 2016, covering different age groups and years of residence on the island. The number of returned questionnaires was 251 (response rate: 84%), equivalent to 3.8% of the adult population of Hachijo Island (Hachijo Island Department of Financial Planning 2017). Eighty-four percent of all respondents were female. The respondents fitted into the following age brackets: 20s (2%), 30s (8%), 40s (13%), 50s (12%), 60s (31%), 70s (24%), and over 80s (10%). The residential community breakdown of the respondents was Mitsune (47%), Okago (20%), Kashitate (14%), Nakanogo (14%) and Sueyoshi (5%), and the occupation breakdown was full-time homemakers (25%), company and government employees (22%), part-time workers (22%), unemployed (15%), self-employed (farmers, fishers and forestry workers) (7%), self-employment (commerce, industry and service industry) (6%) and others (3%). The household compositions were couples (35%), parents and children (30%), single persons (28%), parents, children and grandchildren (4%) and others (2%). In our samples, the proportion of female respondents in the 60s and 70s age brackets was 1.8 times larger than in the 2016 demographic statistics (Hachijo Island Department of Financial Planning 2017), and the proportion of under 30s respondents was reduced. This can be explained by the large number of respondents (84%) recruited through the Hachijo Island Women’s Association. We acknowledge this bias in the respondents’ age and gender occupation composition as a limitation of the study. In particular, they have many social connections, so that they are deeply involved in food-sharing networks than the average persons in the island. It is possible that the proportion of non-market food was overestimated.

The questionnaire survey consisted of Q1 and Q2 in Table 1, and Q3–Q5 in Table 5 of the Appendix. To quantify the food amounts acquired through non-market transactions, we requested the distribution channels of the respondents’ food sources [grown at home (channel a), acquired through the community food network (channel b), or purchased from markets (channel c)]. Channel b includes both self-produced foods by Hachijo people and processed foods, as defined in Saito et al. (2018). According to our interview survey, most of the received foods on Hachijo Island were self-produced in home gardens. Therefore, the sum of the proportions of distribution channels (a) and (b) is the total proportion of the non-market food. The food categories in our questionnaires were defined following the National Health and Nutrition Survey (MHLW 2016), which reports the caloric and nutrient values of 17 food categories per daily intake (crops, potatoes, sugar, beans, nuts, vegetables, fruits, mushrooms, seaweed, seafood, meat, eggs, dairy products, fats and oils, confectionery, beverages, and seasonings and spices). The food species in each food category are shown in Table 6 of the “Appendix”. It should be noted that in Japan, the dominant food species in the crop category is rice. To estimate the shadow benefits created by non-market food, the proportions in each category obtained from the questionnaire survey were integrated with the existing statistics. However, the proportions stated in the survey were not merely numerical–physical, caloric, or monetary records, but also included subjective impressions. Q2–Q5 were designed to clarify the motivation for participating in food-sharing networks on Hachijo Island. Some essential products are mainly imported from the main island. Therefore, if Hachijo residents promote local production for local consumption, they conserve the costs of obtaining imported products and circulate their economy within the local community. To reveal the progress of local production for local consumption on Hachijo Island, Q2 requested the proportion of received foods that were produced on the island, while Q3 requested the supplementary purchase channels of local production for local consumption. Q4 and Q5 asked residents why they buy outside products rather than use local products. The respondents’ answers to these answers can guide the enhancement of local production for local consumption. At the end of the questionnaire, the attributes of the respondents (basic demographic attributes and lifestyle demographics such as residential years and place of origin) were requested.

Statistical analysis

After extracting the shared foods, the text data obtained from the interview survey were visualized as directed graphs representing the food-sharing networks. The variety of non-market food, the relationship between the interviewees and their sharing partners, and the number of sharing partners, were classified by season and direction of the sharing activities.

Second, using the data obtained from questionnaire surveys, the proportions of the distribution channels for each food category were calculated by Eq. (1). The three distribution channels were (a) household food production (hp), (b) sharing networks (sn), and (c) purchase (pc).

$${R_{i,j}}={{\left( {\mathop \sum \limits_{{k=1}}^{N} \frac{{{r_{i,j,k}}}}{{\mathop \sum \nolimits_{j} {r_{i,j,k}}}}} \right)} \mathord{\left/ {\vphantom {{\left( {\mathop \sum \limits_{{k=1}}^{N} \frac{{{r_{i,j,k}}}}{{\mathop \sum \nolimits_{j} {r_{i,j,k}}}}} \right)} N}} \right. \kern-0pt} N},$$
(1)

where Ri,j is the proportion of distribution channel j [j = 1–3, j = 1: household food production (hp), j = 2: food-sharing network (sn), j = 3: purchase (pc)] in food category i (i = 1 to 17, Table 6), ri,j,k is the proportion of distribution channel j in food category i of respondent k (k = 1–251), and N is the number of respondents (N = 251). Thus, the proportion of non-market transactions (Rnm) is the sum of the proportions of household food production and food-sharing networks:

$${R_{{\text{n}}{{\text{m}}_i}}}={R_{{\text{h}}{{\text{p}}_i}}}+{R_{{\text{s}}{{\text{n}}_i}}},$$
(2)

where \({R_{{\text{n}}{{\text{m}}_i}}}\), \({R_{{\text{h}}{{\text{p}}_i}}}\), and \({R_{{\text{s}}{{\text{n}}_i}}}\) denote the proportions of food category i in non-market transactions, exclusively produced by households, and exchanged through the sharing network, respectively.

Third, the economic savings of non-market food were estimated by multiplying the proportion of non-market transactions by the annual food expenditure of Hachijo Island households obtained from ZGI (2016):

$${E_{{\text{n}}{{\text{m}}_i}}}={E_{{\text{ha}}{{\text{c}}_i}}} \times {R_{{\text{n}}{{\text{m}}_i}}},$$
(3)

where Enmi is the money saved by non-market food i [JPY household−1 year−1], and Ehaci is the household expenditure on food i on Hachijo Island [JPY household −1 year−1].

Next, the proportion of non-market transactions \({R_{{\text{n}}{{\text{m}}_i}}}\) in Eq. (2) was multiplied by the daily caloric intake per person on Hachijo Island as follows:

$${C_{{\text{n}}{{\text{m}}_i}}}={C_{{\text{ha}}{{\text{c}}_{\text{i}}}}} \times {R_{{\text{n}}{{\text{m}}_i}}},$$
(4)

where \({C_{{\text{n}}{{\text{m}}_i}}}\) is the caloric intake of foods acquired from non-market transactions in food category i [kcal day−1 person−1], and \({C_{{\text{ha}}{{\text{c}}_{\text{i}}}}}\) is the daily caloric intake of food category i per Hachijo Island resident [kcal day−1 person−1].

Similarly, the daily nutrient intake per person through non-market transactions was estimated as

$${N_{{\text{n}}{{\text{m}}_{i,l}}}}={N_{{\text{ha}}{{\text{c}}_{_{{i,l}}}}}} \times {R_{{\text{n}}{{\text{m}}_{_{i}}}}},$$
(5)

where Nnmi,l, and Nhaci,l are the daily intakes of nutrient l per Hachijo Island resident obtained through non-market transactions and from all sources, respectively, of food category i [mass day−1 person−1]. The selected nutrient species were those defined in the Overview of Dietary Reference Intakes for Japanese (MHLW 2014).

Note that the annual food expenditure of Hachijo Island households [\({E_{{\text{ha}}{{\text{c}}_i}}}\) in Eq. (3)] was available from (ZGI 2016), but the daily caloric and nutrient intakes per person [\({C_{{\text{ha}}{{\text{c}}_i}}}\) and \({N_{{\text{ha}}{{\text{c}}_{i,l}}}}\) in Eqs. (4) and (5), respectively] were not directly available for Hachijo Island residents. Instead, the basic units were derived from the national averages (\({C_{{\text{na}}{{\text{t}}_i}}}\), \({Q_{{\text{na}}{{\text{t}}_i}}}\) and \({N_{{\text{na}}{{\text{t}}_{i,l}}}}\), respectively) and multiplied by the Kanto averages (Qkantoi) by the following conversion equations:

$${C_{{\text{ha}}{{\text{c}}_{_{i}}}}}={Q_{{\text{kant}}{{\text{o}}_i}}} \times \frac{{{C_{{\text{na}}{{\text{t}}_i}}}}}{{{Q_{{\text{na}}{{\text{t}}_i}}}}},$$
(6)
$${N_{{\text{ha}}{{\text{c}}_{i,l}}}}={Q_{{\text{kant}}{{\text{o}}_i}}} \times \frac{{{N_{{\text{na}}{{\text{t}}_{i,l}}}}}}{{{Q_{{\text{na}}{{\text{t}}_i}}}}},$$
(7)

where \({C_{{\text{na}}{{\text{t}}_i}}}\) and \({N_{{\text{na}}{{\text{t}}_{i,l}}}}\) define the national averages of the daily caloric intake and the daily intake of nutrient l, respectively, from food category i per person [kcal day−1 person−1] and [mass day−1 person−1], respectively. \({Q_{{\text{kant}}{{\text{o}}_i}}}\) and \({Q_{{\text{na}}{{\text{t}}_i}}}\) denote the daily per-person intakes [g day−1 person−1] in the Kanto and national regions, respectively. All terms on the right-hand side in Eqs. (6) and (7) were obtained from the National Health Nutrition Statistics (MHLW 2016).

Table 2 summarizes the annual household expenditures and daily caloric and nutrient intakes of each food category per person on Hachijo Island. In this analysis, the shadow benefits of non-market food were defined as the money saved by receiving non-market food \({E_{{\text{n}}{{\text{m}}_i}}}\) in Eq. (3), the caloric intake through non-market transactions \({C_{{\text{n}}{{\text{m}}_i}}}\) in Eq. (4), and the daily nutrient intake per person through non-market transactions \({N_{{\text{n}}{{\text{m}}_{i,l}}}}\) in Eq. (5).

Table 2 Annual household expenditure on food, and daily calorie and nutrient intakes, of Hachijo Island residents
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Results and discussion

Visualization of food-sharing networks

Figure 2 shows the food-sharing networks throughout the year. The abbreviations in the circle nodes represent the five communities. The directional edges between the nodes represent the non-market transactions. The edge widths and directions reflect the number of sharing partners and the distribution routes of the non-market foods, respectively. Table 3 lists the food items in the food-sharing networks by season and direction. The contents of each cell are the shared food species.

Fig. 2
figure 2

Food-sharing networks in all seasons. OI Outside Hachijo Island, MN Mitsune, OG Okago, KT Kashitate, NG Nakanogo, SY Sueyoshi. Green arrow head: from, red arrow head: to

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Table 3 Food items disseminated through the food-sharing networks, organized by season and direction
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All interviewees in all communities shared foods within and beyond the island. The 151 shared food species were dominated by vegetables (29%), fruit (21%) and seafood (15%). The interviewees often exchanged food with sharing partners outside the island, especially in summer and winter. This reflects the traditional Japanese customs of Chugen and Seibo, defining the summer and end-of-year gift-giving festivities, respectively. The offered gifts demonstrate one’s appreciation of daily support from close friends and relatives. During Chugen and Seibo, the Hachijo Island residents grew vegetables and fruits in their home gardens and gifted them to their sharing partners outside the island. In summer, the representative food gifts were vegetables (e.g., okura and green peppers), seaweeds (e.g., kelp) and fruits (e.g., melon, citrus hassaku, and cherries). The year-end gifts were dominated by potatoes (including taros), vegetables (e.g., cauliflowers, ginger and Japanese radishes) and fruits (e.g., kumquats, apples, and pears). These species are seasonal fresh foods in winter. From outside of Hachijo Island, the residents mainly received confectioneries (e.g., western-style and Japanese cakes), beverages (e.g., tea, coffee and alcoholic drinks) and meat (e.g., roast beef and roast pork) in summer, and confectioneries (confections, and western-style and Japanese cakes), beverages (e.g., tea) and fats and oils (e.g., salad oil) at the end of the year. Most of the foods received from outside the island were processed foods. These non-market transactions play an important role in complementing the lives of the residents and enhancing their food-species diversity.

Throughout the year, the Hachijo people shared carbohydrates such as rice and sugar, seaweed, seafood, livestock products, luxury food, and seasoning and spice. Seafood (e.g., flying fish, mackerel shad and bonito), fruits (e.g., citrus reticulata Siranui, kiyomi oranges and Sweet Watson pomelos) and vegetables (e.g., broccoli, cabbages and carrots) were widely shared in spring (Table 3). As these food species are perishable, the Hachijo people share them with their neighbors. In summer, the shared food species were seasonal fresh foods in Japan, including vegetables (e.g., cucumbers, eggplants and green peppers), fruits (e.g., watermelons and other melons) and seafood (e.g., yellowtail amberjack). In autumn, the residents shared potatoes (standard potatoes, sweet potatoes and taros), seafood (e.g., greater amberjack and pacific saury) and fruits (e.g., Japanese persimmons, guavas, and Vitis ficifolia). Potatoes are especially shared among the Hachijo residents, as they are often grown in home gardens and distributed through the food-sharing network. In winter, the Hachijo residents shared vegetables (e.g., Japanese radishes, ginger and cabbage), seafood (e.g., red bream, yellowtail and mahi-mahi) and fruits (e.g., bananas, oranges, and apples). Especially, when Hachijo fisher-people catch a large quantity of seafood such as flying fish or mackerel shad, they usually distribute them to close residents, who secondarily distribute them to their neighborhoods. In this way, seafood becomes widely shared across Hachijo Island (NANKAI TIMES 2014). Vegetables, potatoes, beans and processed foods were the main shared foods. The same tendency was observed by the survey on Noto Peninsula, which has large forest areas (Kamiyama et al. 2014), and the previous questionnaire survey conducted with 664 participants in Hachijo Island (Saito et al. 2015). However, mushrooms were largely shared on Noto Peninsula, whereas seafood was distributed in large proportion on Hachijo Island.

In summary, food-sharing activities provide both local staples and specialty foods from the regional natural capital. Our spatiotemporally detailed research revealed that the summer and end-of-year gift-giving culture, coupled with the usual food-sharing culture, enhances the non-market food distribution on Hachijo Island. Shared processed foods arrived from outside the island, whereas seasonal foods were mainly shared among the island residents. Seasonal fruits and vegetables are highly nutritious (Tomita and Mizutani 2012), and seasonal vegetables contain much more vitamin C and carotene than those cultivated out-of-season (Enomoto 2008). Additionally, Lachat et al. (2018) discovered that dietary species richness, defined as the number of different food species consumed, increases the adequacy of nutrient intake. Hence, it can be said that sharing various kinds of seasonal foods promotes the health of the Hachijo Island residents.

One interviewee mentioned that she grew vegetables and fruits for the purpose of sharing them with relatives and friends beyond the island. As reported by another interviewee, a shipping fee of approximately 50,000 JPY (459 USD, 109 JPY USD−1) is payable when sending foods from the island. Despite the high shipping fee, some residents continue growing and sharing their foods through the food-sharing networks. All interviewees growing foods in their home gardens shared most of their home-grown foods both within and beyond Hachijo Island. The Hachijo people accepted the offered foods even when growing the same food themselves, because denying the offer breaks the linkage. Furthermore, residents who did not grow food in their own gardens (some interviewees in their 20s and 30s, and those inhabiting the island for less than 4 years), continued to receive vegetables, fruits and fish from their neighbors, friends, and coworkers.

Osusowake is the Japanese practice of food-sharing and gift-giving on a small scale. Social relations are an important aspect of Japanese culture, and gifting maintains and strengthens the social network ties of Japanese people (Befu 1968; Suzuki 1988; Ichikawa 1989). Food-sharing activities are also valued because they maintain reciprocity in social relations and create a feeling of community membership (Quandt et al. 2001). Household food production and personal connection establishes cultural identity and maintains traditional knowledge (Galhena et al. 2013; United Nations University 2013; Nakazawa et al. 2014). According to the interview, the food-sharing activity was basically interactive, but the main motivation was maintaining human relations, not barter economy. Similar trends were observed in different areas (Kamiyama et al. 2016) and food sharing was carried out without interaction in both directions. As shown in Table 3, the distribution of shared food species within and beyond Hachijo Island depends on the season, but food-sharing activities are continued throughout the year, maintaining the social relations among the residents. Summarizing the above, social capital supports the food-sharing networks of the Hachijo residents, and is itself strengthened through the maintenance of food-sharing networks.

Proportions of food distribution channels and local production for local consumption

Figure 3 shows the proportions of the distribution channels and local production for local consumption in each food category. The household food production was dominated by potatoes (31%), followed by vegetables (21%) (Fig. 3a). The sharing networks contributed to 35% of the potato consumption and 21% of the vegetable consumption. The purchased products were dominated by dairy products, meat, mushrooms, fats and oils, and eggs. The Hachijo residents usually purchased these foods from their local supermarkets (14.6 times month−1; S.D. = 8.22), and not by mail order (Q3 and Q4 in Table 5). As mentioned in questionnaire survey method, the respondents’ distribution of our study was biased, but the non-market food contributions in each food category were consistent with those of Noto Peninsula with balanced sample of respondents (Kamiyama et al. 2014). In both rural areas (which typify rural areas in Japan), the residents sourced approximately 40% of their staple foods by non-market transactions. Non-market transactions also accounted for 20% of their side dishes of regional specialty foods. The same tendency was reported in a Czech Republic study (Jehlicka and Danek 2017). On Hachijo Island, most of the potatoes, vegetables and seafood received through the food-sharing networks were produced on the island (Fig. 3b). Many Hachijo residents home-grew seasonal potatoes and vegetables for consumption by their households and their neighbors. Most of the Hachijo residents did not harvest seafood, but received seasonal seafood from the Hachijo Island fishers, who captured many types of seafood from the near oceans and distributed them through the food-sharing networks (NANKAI TIMES 2014; interview). On the contrary, foods such as sugar, dairy products, seasonings and spices were rarely distributed through the food-sharing networks, and were produced outside the island and purchased on the market. Other foods such as confectionery, beverage, and crops made in outside the island were also infrequently distributed within the networks. Beside the food sharing within the island, the linkage with the main island supported the food accessibility and diversity of Hachijo’s people.

Fig. 3
figure 3

Proportions of distribution channels and local-production-for-local-consumption organized by food categories. b Proportions of products produced on Hachijo Island to total quantities of shared food. Non-responses were excluded

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As mentioned above, Hachijo people grow seasonal local foods and actively distribute them to relatives, neighbors, and friends throughout the year. Our survey on detailed food categories quantitatively confirmed that the custom of sharing homemade foods greatly enhances the accessibility of seasonal foods and local production for local consumption. Cleveland et al. (2017) found that household vegetable gardens could reduce greenhouse gas emissions in terms of life cycle assessment such as the reduction of vegetables purchased from the conventional agrifood system. Consuming seasonal vegetables in a local-production-for-local-consumption system conserves production and transportation energy, and reduces GHG emission (Tsuda et al. 2005). Also, Hara et al. (2013) suggested that a decrease of energy inputs could be achieved through wider adoption of local organic farming for local consumption. Therefore, food-sharing networks not only strengthen social networks and support peoples’ livelihoods and wellness, but also contribute to energy saving and carbon management. Moreover, Plieninger et al. (2017) analyzed “place-based food networks” in Japan and Europe, which are the food production and consumption practices such as non-market food sharing and exchange. They pointed out that Japanese cases are motivated to promote physical well-being such as nutrition outcomes and local economies, whereas European cases are motivated by biodiversity conservation and socio-cultural traditions. Thus, we should broaden the perspective of food-sharing activities including biodiversity conservation and sustainable use of ecosystem services. These environmental co-benefits are recognized as “quiet sustainability” (Smith and Jehlicka 2013). Our method only captured main benefits of non-market food. However, comprehensive benefits including the context of quiet sustainability should be scoped in the accounting system.

Accounting for the shadow benefits of non-market food distribution

When Hachijo residents grow foods in their home gardens and receive them through food-sharing networks, they reduce their food expenditure and thus save money. Figure 4 shows the estimated annual monetary savings per household and the daily caloric intake of non-market foods per person. The left and right stacked bars shows the total monetary savings and total caloric intakes, respectively, when producing and receiving foods in each category. The consumption of non-market food saved 219,543 JPY household−1 year−1 (2,018 USD, 109 JPY USD−1). Ninety percent of these savings were contributed by the sharing of vegetables, seafood, confectionery, crops, beverages, and fruits. These foods are generally expensive, and account for 69% of the total annual food expenditure on Hachijo Island. In summary, the non-market food saved as much as 25% of the annual food expenditure on Hachijo Island. Similar savings were reported on Noto Peninsular (28%, Kamiyama et al. 2014).

Fig. 4
figure 4

Monetary savings (left) and caloric values (right) of non-market foods

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The caloric values of the non-market food and all food consumed by Hachijo Island residents were 324 and 1,876 kcal person−1 day−1, respectively (see Table 2). That is, the non-market food accounted for 17% of the daily caloric intake. Rice crop was the major contributor, providing 46% of the caloric intake. Crop foods (especially rice) are the main staple foods in Japan and support almost half of the daily caloric intake. Thus, crop is the dominant contributor from a caloric viewpoint although rice constitutes a low proportion of the non-market foods. The other calorie-dominant foods were seafood, vegetables, potatoes, and confectionery, which comprise a large proportion of the non-market food.

Figure 5 shows the proportion of the estimated nutrient intake of each non-market food. The horizontal axis represents the estimated nutrient intake of the non-market food, relative to the whole nutrient intake in Hachijo Island. The vertical axis categorizes the nutrient species into the five major nutrient groups (MHLW 2008). The source food categories are separated by color. Non-market foods provided 9–32% of the total nutrient intake, confirming that food-sharing networks provide health benefits to the Hachijo residents. The top five nutrients in the non-market foods, providing the largest proportions of nutrient intake, were vitamin C, soluble dietary fiber, insoluble dietary fiber, total dietary fiber and vitamin D.

Fig. 5
figure 5

Daily nutrient intakes of non-market food per person

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Protein maintains the condition of muscles, organs, skin and hair, and also adjusts the levels of hormones, enzymes, and antibodies. Seafood and crops were the largest contributors to protein intake, followed by vegetables and meat. Fat is an energy source and a constituent of cell membranes and physiologically active substances. Similarly to protein, seafood and confectionery provided half of the fat intake; the remainder was contributed by crops, meats, and fats and oils. Carbohydrates such as sugar are main energy sources and dietary fiber regulates the function of the intestines. The Hachijo residents sourced most of their essential energy from vegetables (including potatoes) and crops. Vitamins are organic compounds that regulate chemical reactions in the human body, but cannot be synthesized in the body. Vitamins were largely sourced from vegetables and seafood, with additional contributions from crops, potatoes, and fruit. Vegetables, crops, and seafood provided most of the minerals, essential nutrients with various functions such as building bones and supporting muscles. Minerals were also contributed by seasonings, spices, and potatoes. Overall, the sharing of vegetables, seafood, and crops greatly promotes the wellness of Hachijo inhabitants.

Table 4 shows the nutrients provided by non-market food relative to the national recommended diets of MHLW (2014). In this analysis, we referred to the MHLW criteria and weighted the means by the population composition of Hachijo Island. This table shows the criteria, the nutrient intakes from non-market food on Hachijo Island, and the nutrients in the non-market food relative to the MHLW (2014) criteria. The blue cells in this table highlight the non-market foods that largely contributed to the nutrient species. Non-market foods provided over 40% of the recommended dietary intake of vitamin B12 (which regulates metabolism in the body) and vitamin K (which stabilizes the body). To deliver health benefits, nutrients should be consumed at levels exceeding their dietary criteria. The levels of vitamin A (for body stabilization), vitamin B1 (for body metabolism), and calcium and magnesium (for bone and teeth formation) in the Hachijo residents were below the national criteria, but these deficiencies were mitigated by the seafood, vegetables, and crops provided through the food-sharing network. Nutritional quality is measured by the mean adequacy ratio (MAR), calculated as the arithmetic mean of the proportions of consumed nutrients (total nutrient intakes in Hachijo Island in Table 4) per its daily requirement for each individual (criteria in Table 4) (Lachat et al. 2018). The MAR provided by the market-based food on Hachijo Island was 0.84. When the non-market food was included, the MAR improved dramatically to 0.95. Therefore, the food-sharing activities make a significant nutritional contribution.

Table 4 Proportions of nutrients in the non-market foods, referenced to the national recommended diets
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Limitations

By analyzing the above results, we systematically and quantitatively revealed (1) the proportions of non-market foods, (2) the origins of the food, (3) the cultural driving force of sharing, and (4) the shadow benefits (economic and health benefits) of the non-market foods. We think that our method can be applied to other regions. However, the characteristics of shared-food species depend on the geographical and ecological factors of the island, and the proportions of non-market foods might differ by social factors. The composition of a shared-food category might affect the total calorie and nutrient intakes.

Cross-cutting issues

Food-sharing network in Hachijo Island can regarded as one of the alternative food networks (AFN) which is newly emerging networks of producers, consumers, and other actors that embody alternatives to the more standardized industrial mode of food supply (Plieninger et al. 2017). Zoll et al. (2017) developed a typology of AFN consumers: (a) community supported agriculture, (b) Food Coop, (c) self-harvest garden. Dansero et al. (2017) classified AFN into three types: (a) solidarity purchasing groups, (b) farmers market, (c) on-farm direct sale. However, these typologies mainly focus on market-based AFN. As Jehlicka and Danek (2017) pointed out, the informal, non-market form of the sharing of food produced by households is blind spot, especially in Global North. Thus, we need to focus on non-market-based activity such as food-sharing networks in Hachijo Island and establish the accounting method which is comparable to conventional market-based activities.

Sharing activities can be categorized into three types: the “sharing for money” model, the “sharing for charity” model, and the “sharing for the community” model (Michelinia et al. 2018). The food-sharing activities in Hachijo Island can be regarded as “sharing for the community” model. Both market-based players and participants in food-sharing networks should enforce a seamless partnership. For instance, market-based agents should stably distribute the surplus of shared non-market foods among the markets. The Hachijo inhabitants hesitated to buy local market foods because of their high price, poor diversity, short market period and limited accessibility of specific products (see Q5 in Table 5). On the other hand, food-sharing networks are threatened by globalization and urbanization (Saito et al. 2018). Therefore, a regional sustainable production and consumption system with food-sharing networks should be strategically balanced against the global market.

The societal awareness of food-sharing networks and the shadow benefits should be raised to fulfill important public interests and cultural identities (Plieninger et al. 2017). Disclosing the shadow benefits in a scientific report will promote the awareness of actual self-sufficiency capability which is not measured only using official statistics. Moreover, for example as an educational activity, the governmental office of Hachijo Island is promoting locally produced foods in school feeding programs from safety and security viewpoints (Hachijo Town 2016). Besides this, the Hachijo Island Women’s Association distributes lunch boxes using local foods to elderly people as a welfare activity. Policies to couple the food-sharing networks and these existing activities can help people better understand the shadow benefits brought by the sharing culture of non-market foods.

Many scientists have already suggested that food-sharing networks can strengthen resilience against future socio-economic changes and natural disasters (Quandt et al. 2001; Kamiyama et al. 2014; Saito et al. 2015, 2018; Boafo et al. 2016). We quantified the contributions of food-sharing networks to the balances of specific calories and nutrients, thereby providing a large quantitative basis for designing local food resilience. Thus, a fusion of regional disaster management systems and food-sharing networks is crucially required.

Conclusion

This study graphically visualized the food-sharing networks on Hachijo Island and identified the seasons and directions of the food species distributed through the non-market transactions. The Hachijo residents grow seasonal foods in their home gardens and share them within and beyond the island throughout the year. The study also identified the food distribution channels and quantified the contributions of household food production and sharing networks to the dietary habits of the residents. Hachijo Island residents produce large amounts of potatoes, vegetables, fruits, and seafood, which are widely shared and consumed across the island. Other foods such as confectionery, beverages, and crops are sourced from outside the island and shared through the food-sharing networks. By integrating the present quantification and the available statistical data, this study comprehensively described the shadow benefits of non-market food as monetary, caloric, and nutritional values. Non-market foods account for approximately one-fifth of each household’s finance and energy intake. They also supply a wide variety of nutrients and a large portion of the required vitamins. The sharing food practice has become a traditional culture on Hachijo Island, by which the inhabitants access nutritional foods and maintain their social relationships. By examining these findings, we explored the future perspectives of food-sharing networks: the balance between market-based and non-market food provisions, the promotion of local production for local consumption, and the establishment of local food resilience. Our works systematically and quantitatively help with revealing the relationship between food-sharing networks and their shadow benefits (economic and health benefits). The environmental benefits of food sharing and gifting, such as reducing greenhouse gas emissions through local production for local consumption and enhancing ecosystem services by maintaining an agricultural landscape, are also important.