Introduction

Although various definitions for metabolic syndrome (MetS) have been published by different organizations, it typically includes abdominal obesity, hypertension, hypertriglyceridemia, hyperglycemia, and dyslipidemia. MetS is a risk factor of cardiovascular and metabolic complications that is associated with cardiovascular disease, type 2 diabetes, and nonalcoholic fatty liver disease [1,2,3,4], and thus an important public health target. However, the prevalence of MetS remains high worldwide [5,6,7,8,9,10,11,12]; therefore, it is important to find effective strategies for controlling MetS [13].

Lifestyle change, including eating behaviors, is a recommended strategy to control lipid levels and address-associated metabolic abnormalities [14]. Among eating behaviors, eating quickly has been considered an especially important factor that contributes to the development of obesity and diabetes [15, 16], because eating quickly has been associated with increased total energy intake [17], reduced satiety [18], and insulin resistance [19]. Recent observational studies have shown that in addition to obesity and diabetes, eating quickly is also associated with MetS [20, 21]. However, whether improvements in eating quickly contribute to the prevention of MetS remains unclear.

In 2008, the Japanese Ministry of Health, Labour, and Welfare started a new health care strategy that targeted the early diagnosis of MetS and interventions to decrease its associated morbidity. Under the new health care system, people diagnosed with MetS receive standardized and individualized lifestyle guidance, followed by additional repeated counselling over a 6-month period. However, according to a national survey in Japan, more than 3 million middle-aged people have still MetS [22]. Therefore, more effective preventive strategies are desired to reduce the morbidity associated with MetS. Here, we hypothesize that health guidance combining improvements in eating quickly with or guidance would have additive effects in preventing MetS-related morbidity. The aim of this study was to investigate the effects of health guidance combining improvements in eating quickly with standard guidance on MetS compared with standard guidance alone among participants in the health care system in Japan.

Materials and methods

Residents (n = 24,996) aged 40–74 years in two adjacent cities in Kagawa Prefecture in Japan were informed of the public health service including MetS checkups; among these residents, 9065 participated in the checkups. Among the residents who were diagnosed with MetS according to the Japanese criteria [23], 259 received health guidance. Inclusion criteria were having MetS and agreeing to participate. Exclusion criteria included the residents who are incapable of participating. After explaining the study aims and procedures, a sufficient number of participants were enrolled on a voluntary basis in both cities (n = 100 in one city and n = 93 in the other). The recruitment period was from July to October 2009. We excluded participants who dropped out (n = 43) and had incomplete data (n = 9). Finally, data from 141 participants (mean age, 65.0 ± 6.7 years; range, 41–74 years) were analyzed (Fig. 1). The study protocol was approved by the Ethics Committee of Okayama University Graduate School of Medicine, Dentistry, and Pharmaceutical Sciences (No. 306, 383), and written informed consent was obtained from all participants.

Fig. 1
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Flow diagram

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Following the recommendations from the Japanese Committee for the Diagnostic Criteria of MetS [23], we defined MetS as the presence of central obesity [waist circumference (WC) ≥ 85 cm in males or ≥ 90 cm in females], as well as two or more of the following risk factors: lipid abnormality [triglycerides (TG) ≥ 150 mg/dL and/or high-density lipoprotein cholesterol (HDL-C) < 40 mg/dL or use of medications for dyslipidemia], high blood pressure (BP; systolic BP ≥ 130 mmHg and/or diastolic BP ≥ 85 mmHg or use of medications for hypertension), and hyperglycemia [hemoglobin A1c (HbA1c) ≥ 5.2% or use of medications for diabetes] [24].

This was a controlled, non-randomized, intervention study conducted in two cities. The participants in one city were allocated to the intervention group, and those in the other city were allocated to the control group. Both groups were assessed at baseline (July–October 2009) and again at a 1-year follow-up (July–October 2010).

At baseline, the participants in both the control and intervention groups received standard health guidance about MetS in accordance with the guidelines of the Ministry of Health, Labour and Welfare in Japan. Briefly, physicians explained the following: (1) the pathophysiology of MetS; (2) the risk of future cardiovascular events associated with this condition; (3) the rationale for prescribing weight loss; and (4) its importance in the maintenance of each participant’s health [25]. Trained nurses and dieticians conducted motivational support at baseline and at 1–3 months. The motivational support included the following: (1) recognition of an unhealthy lifestyle based on the results of participants’ health examinations and questionnaires; (2) an explanation of the advantages of having an improved lifestyle and the disadvantages of having an unhealthy lifestyle; (3) guidance regarding nutrition, exercise, and health records to maintain healthy body weight, WC, and levels of physical activity; (4) setting of goals to achieve during the study period; and (5) encouragement for adopting a healthier lifestyle. An intermediate examination was performed at 6 months to check the achievement status of the participants in accordance with the guidelines.

In addition to the standard health guidance about MetS described above, the participants in the intervention group received further health guidance focusing on eating quickly, which included the following: (1) a lecture on the appropriate eating speed for health self-management; and (2) monitoring of the total duration of chewing and number of chews from the start of eating to the first swallow. A dentist (T.K.) performed the 1-h lecture at baseline. The contents of the lecture focused on the importance of mastication for health, appropriate eating speeds, and the ideal number of chews (30 chews) until first swallow. After giving the lecture, the dentist explained how to monitor the total duration of chewing and number of chews until first swallow. Next, the participants received a self-monitoring sheet and checked their behaviors, including the total duration of chewing and number of chews three times a day for 3 months at home, in addition to self-assessments of exercise and body weight.

The primary study outcome was the prevalence of MetS at the 1-year follow-up. Secondary outcomes included changes in body composition, BP, lipid profiles, aspartate aminotransferase, alanine transaminase, creatinine and HbA1c from baseline to 1 year. In addition, we evaluated the participants’ lifestyles at follow-up as described below.

In addition to the data about MetS from the two cities, we obtained other clinical data, including body mass index (BMI) and serum levels of low-density lipoprotein cholesterol (LDL-C), aspartate aminotransferase, alanine transaminase, gamma-glutamyl transpeptidase and creatinine.

In addition to sex and age, responses to self-reported questionnaires regarding past and current medical history and lifestyle (i.e., smoking status, regular exercise, restful sleep, walking at least 1 h/day, walking fast, eating late-night dinners, eating a fourth meal, and skipping breakfast) were examined using part of a standardized questionnaire for specific health examinations in “yes/no” format [21]. For the questionnaire item of eating quickly, participants reported their eating speed relative to others, according to one of three qualitative categories: slow, normal, or fast [26]. We combined the “slow” and “normal” responses into the single category of “eating slow” [16]. For habitual drinking habit, the participants reported their frequency of alcohol intake according to one of three categories: every day, sometimes, or rare (no drinking) [27]. We combined the “every day” and “sometimes” responses into the single category of “habitual drinking habit”. Furthermore, we asked the participants about their desire to improve their lifestyles according to one of five qualitative categories: “never”; “planning within 6 months”; “planning within 1 month”; “starting within 6 months”; and “starting after 6 months”. We then combined the “never”, “planning within 6 months” and “planning within 1 month” responses into the single category of “non-improving lifestyle”, and the “starting within 6 months” and “starting after 6 months” responses into the single category of “improving lifestyle”.

Power analysis and sample size determination were calculated using statistical software (SamplePower ver. 3.0, IBM, Tokyo, Japan) based on the results of a previous study [28]. A two-way table was calculated to detect positive or negative differences in the prevalence of MetS after the interventions. The minimum sample size to detect statistically significant differences in the positive and negative groups was 80 with 80% power and a two-sided significance level of 5%. Assuming an attrition rate of 23.1% [26], the planned sample size was a minimum of 98 participants (49 in each treatment group).

All study personnel, including the clinical examiners, the laboratory staff who performed the blood analyses, and the investigator responsible for the data analysis, were blinded to the treatment assignment. None of the participants were given any information about the other group.

Data were summarized using means (± standard deviations) for continuous variables and frequencies (percentages) for categorical variables. We used an analysis of covariance (ANCOVA) for primary and secondary outcomes. The ANCOVA model included age, sex, BMI, WC, walking habit, walking speed and eating quickly as covariates. Adjusted differences and 95% confidence intervals were determined. P values were based on t tests comparing continuous parameters between the control and intervention groups from ANCOVA. A per-protocol analysis based on data available both at baseline and the follow-up was performed. The significance level was p < 0.05. Data analysis was performed using the Statistical Package for the Social Sciences (SPSS version 23, IBM, Tokyo, Japan).

Results

We analyzed data from 141 participants (follow-up rate: 73.1%). No significant differences were observed for any variables at baseline between participants who were and were not followed up (data not shown). No participants used any medications for dyslipidemia, hypertension, or diabetes at baseline. The intervention in this study was low risk, and no study-related serious adverse events were reported in any of the participants.

Table 1 shows the differences in all variables between the control and intervention groups at baseline. Overall, 39 participants (27.7%) reported eating quickly. The number of participants who reported walking fast at baseline was significantly higher in the intervention group than in the control group (p = 0.020).

Table 1 Differences between control and instruction groups at baseline (n = 141)
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Differences in binary data between the control and intervention groups at follow-up are shown in Table 2. The prevalence of MetS in the intervention group was significantly lower than that in the control group (p = 0.003). The number of participants who had already started improving their lifestyle was significantly higher in the intervention group than in the control group (p = 0.002). However, no significant differences were observed between the two groups in the percentage of participants who reported eating quickly (p > 0.05).

Table 2 Differences in binary data between control and intervention groups at follow-up (n = 141)
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The decreases seen in body weight, BMI, WC and TG from baseline to 1 year were significantly greater in the intervention group than in the control group (p < 0.05) (Table 3). No significant differences were observed between the two groups in any other parameters (p > 0.05).

Table 3 Differences in continuous values between control and intervention groups at follow-up (n = 141)
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Discussion

As MetS is a risk factor for many diseases [1,2,3,4], it has become an important public health target. It is, therefore, important to find effective strategies for controlling MetS [13]. In 2008, the Japanese Ministry of Health, Labour, and Welfare introduced a new health checkup system and health guidance measures to prevent the onset of MetS among adults aged ≥ 40 years. However, whether improvements in eating quickly contribute to the prevention of MetS remains unclear. Therefore, we examined a new strategy, based on the concept of a working hypothesis that eating slowly may decrease total energy intake, induce satiety or improve insulin sensitivity [29]. The results showed that the combination of standard health guidance about MetS and health guidance focusing on eating quickly significantly decreased the prevalence of MetS as well as BMI, WC and TG among middle-aged participants. Our data suggest that the additional effects of health guidance focusing on eating quickly on reducing the prevalence of MetS contribute to public health.

In the intervention group, we implemented health guidance focusing on eating quickly to improve MetS. However, no significant differences were observed between the intervention and control groups in the percentage of participants who reported eating quickly. Nevertheless, in the intervention group, a significant reduction in MetS was seen. The reason for this may depend on the self-monitoring effects of eating behavior. A previous study reported the effects of self-monitoring of risk factors on the prevalence MetS [30]. In the present study, self-monitoring was, therefore, included in health guidance focusing on eating quickly. The participants checked their physical activity levels and body weight, as well as their total duration of chewing and number of chews until first swallow by themselves every day. Furthermore, the number of participants who had already started improving their lifestyles significantly increased in the intervention study. The aspect of self-monitoring in health guidance focusing on eating quickly may contribute to reducing the prevalence of MetS through improved lifestyles.

After 1 year, the prevalence of MetS decreased by 57.1% in the intervention group and by 30.6% in the control group. Previous Japanese intervention studies reported a 34.7–66.6% decrease in the prevalence of MetS using the same Japanese definition [31,32,33]. Although both the experimental period and type of intervention were different in the present study, the rate of decrease in the intervention group was within the same range.

In this study, BMI, WC, and TG significantly decreased in intervention group compared with the control group. Improvements in obesity and dyslipidemia (risk factors for MetS) may contribute to a decrease in the prevalence of MetS. Previous studies have also investigated the effectiveness of lifestyle modification approaches for individuals with MetS using dietary and/or exercise interventions [25, 28, 31,32,33,34,35] The changes in WC and BMI in previous studies using a 1-year period [28, 34, 35] were from − 2.2 to − 4.6 cm and from − 0.3 to − 0.6 kg/m2, respectively. In the present study, these values were − 3.9 cm and − 0.7 kg/m2, respectively, which were similar to those in the previous studies [28, 34, 35]. Among these studies, repeated interventions involving physical activity and diet modification decreased the serum level of TG among Italian participants [28]. Although the type of intervention in that study was different from that in the present study, the results were similar, which suggests that lifestyle modification approaches that include interventions for eating quickly can contribute to improved lipid profiles. Furthermore, even if we excluded the participants who took lipid-lowering drugs during the period, the results were same.

In this study, the changes in BP and HbA1c did not differ between the two groups. Previous studies have reported various results regarding BP and glycemic control [25, 28, 31,32,33,34,35]. In particular, one study suggested that compared with other components of MetS, it is more difficult to improve BP with repeated counselling, and at least a 5% reduction in body weight may be required to control high BP [25]. Because the participants in our intervention study did not achieve a 5% reduction in body weight, we should seek more effective lifestyle guidance that focuses on controlling BP in patients with MetS. On the other hand, only one previous study reported finding an effect of interventions on glycemic control within a 1-year period [28]. We should, therefore, refer to and combine guidance that includes repeated sessions with individual recommendations from trained professionals (e.g., family physicians, nutritionists, and specialists in endocrinology and internal medicine) to improve hyperglycemia [28].

Eating quickly has been associated with excess body weight [36], increased total energy intake [17], reduced satiety [18], and insulin resistance [19]. A recent observational study showed that changes in eating speed can affect changes in obesity in patients with type 2 diabetes [37]. It also suggests a possibility that interventions to reduce eating speed may be effective in preventing obesity. Further studies are required for establishing an intervention program to effectively improve eating quickly.

A major strength of the present study is its controlled design. Blinding (in participants, examiners, and analysts) was performed to reduce the chance of any biases. In addition, we performed a sample size estimation to obtain the necessary and sufficient number of participants. Finally, we encouraged the participants to complete the study, which resulted in a lower than expected dropout rate.

However, our study did have some limitations. First, all participants were recruited from citizens in two small cities in Kagawa Prefecture. This may limit the ability to extrapolate these findings to the general population. Second, although we attempted to minimize the sampling bias, we could not perform a randomized clinical trial. Third, the attrition rate in this study was 26.9%, which was higher than the estimated rate (23.1%). However, there were no significant differences in parameters of MetS at baseline between the retained and dropout groups. Thus, the bias may be small. Fourth, we did not investigate dietary energy intake. The previous studies report that speed of eating is significantly and positively correlated with total energy intake [19, 38]. In this study, the percentage of participants who reported eating quickly did not change during the period, suggesting that dietary energy intake may be stable. Therefore, we may exclude the effect of changes in dietary energy intake on our results. Finally, the follow-up period was short (1 year). Longer term effects should be investigated in future studies.

In conclusion, health guidance combining improvements in eating quickly with standard guidance significantly reduced the prevalence of MetS compared with standard health guidance only. These results suggest that health guidance focusing on eating quickly may be an effective public health strategy for controlling MetS.