Introduction

Magnesium is a major mineral that exists in the human body at a level of approximately 25 g. Of the 25 g, 50∼60% of the magnesium exists in the skeletal structure, approximately 30% is contained in intracellular fluid, and approximately 1% exists in extracellular fluid [1]. In particular, magnesium that exists inside the human body takes part in the formation of the skeletal structure. Also, as a constituent of ATP, magnesium acts as a cofactor in over 300 enzyme systems, such as the synthesis of proteins and nucleic acids, as well as in lipid metabolism; therefore, it takes part in a variety of biochemical and physiological processes inside the human body [2, 3]. These facts clearly show that the magnesium is closely related to maintaining health and preventing disease.

On the other hand, the magnesium, as a constituent of chlorophyll, is contained in large quantities of green leafy vegetables. While the magnesium contents in nuts, legumes, and cereals are sufficiently high, the contents in meats, fish, milk, and fat are relatively lower [4, 5]. In general, magnesium intake through ordinary meal consumption is sufficient enough to prevent deficiency. However, 80∼96% of the magnesium contained in food items is lost during the course of polishing [6]. So, the magnesium intake may become lower in diets that are highly dependent on refined or processed food items. Therefore, the present dietary life of Korea has become more convenient and gradually simplified due to the westernization of food choice, which may result in an increased incidence of magnesium deficiency.

In Korea, the estimated average requirement (EAR), recommended intake (RI), and tolerable upper intake level of magnesium were first established in 2005 after evaluating the dietary reference intakes (DRI) for Koreans [7]. With their diets centered around vegetables, magnesium intake would sufficiently meet the RI. However, since the DRI for magnesium was established only recently, in-depth studies conducted on the daily intake and nutritional statuses of magnesium in Korean people were seen as insufficient. One of the reasons for this lack of study is due to an insufficient database of the magnesium content in foods whose magnesium intake could be measured.

The Food Composition Table by the National Rural Resources Development Institute [8], whose database of magnesium content in food is used in Korea, provides magnesium contents in 918 varieties of food items. However, with the exception of 104 food items analyzed in Korea in a study by the Korea Health Industry Development Institute [9], the remainder of the data were taken from Japan [10] and the USA [11]. In order to obtain a more accurate evaluation of magnesium intake, the magnesium content of foods produced and ingested in the same region should be evaluated. As such, currently there are insufficient data in Korea to carefully evaluate magnesium intake. This problem is primarily due to the fact that there are not enough data on the magnesium composition of newly developed functional or processed food items. Therefore, it is necessary to conduct studies to analyze magnesium contents in commonly ingested food items. A comprehensive nutrient database that accurately evaluates magnesium intake is important for studies on health-related functions of magnesium, and a study that accurately evaluates magnesium intake by healthy ordinary meal-taking adults is also necessary.

Therefore, in this study, we intended to analyze magnesium contents in the most commonly consumed food items within Korea and, use the data, to evaluate daily magnesium intake by adult males and females who take ordinary meals each day. This study will contribute to the food composition data with which magnesium intake can be estimated and will be continuously utilized in the studies on nutritional evaluation of magnesium.

Materials and Methods

Chemicals

Using the commonly consumed food items in the Korean National Health and Nutrition Examination Survey (KNHANES) of 2005 [12], we selected 366 varieties of food frequently favored by Korean people. We purchased the selected food items, with preference to domestically grown products, from two large discount malls located in Chungnam region, and for imported products, we noted the places of origin. We obtained magnesium ICP standard from AnApex (Dae Jeon, South Korea), and all other reagents used were of the highest purity available.

Analysis on Magnesium Contents

We weighed the above-mentioned food items, removed the parts to be disposed of, and calculated the percentage of disposal and the percentage of edible parts. For two samples of each food item, we mixed them evenly by taking all edible parts from a food item such as an apple and by taking 100 g or more of food items eaten in consistent amounts, such as rice or meat. Then, by taking a prescribed amount of the evenly mixed sample, we created test solutions by breaking them down through a microwave digestion system (Ethos touch control, Milestone Inc., Italy). On the test solutions, we conducted quantitative analyses of magnesium content by using inductively coupled plasma atomic emission spectroscopy (ICP-AES; Thermoelemental Ltd., UK). The ICP-AES system was operated at 1,150 W forward power with a coolant flow of 12 L min−1, sample uptake rate of 49 rpm, auxiliary gas flow rate of 0.5 L min−1, and nebulizer gas flow rate of 0.6 L min−1 with PFA nebulizer. The relative standard deviation obtained from four repetitive analyses on the same sample was less than 3%. Therefore, reproducibility was outstanding. Analytical recovery of the method has been checked by a parallel analysis of rice flour CRM 108-04-001 and oyster tissue powder 108-01-001 (Standard Reference Material, KRISS, South Korea), the certified reference materials. The detection limit and recovery of this method were 0.0003 mg Mg/kg and 102.01%, respectively. All instruments used in the experiment were washed thoroughly to prevent cross-contamination of minerals. Plastic products and glass products were, respectively, submerged in 0.4% EDTA solution and undiluted nitric acid solution for 24 h or longer. They were washed three times or more in secondary distilled water and desiccated in a dryer.

Drawing Up Database of Magnesium Contents

We categorized the results of our analysis on magnesium content per food group and created a database using the categorized results. A total of 366 food items consisted of 51 varieties of grains, seven varieties of potatoes and starches, seven varieties of sugar, 12 varieties of legumes, 11 varieties of nuts and seeds, 68 varieties of vegetables, seven varieties of mushrooms, 33 varieties of fruits, 13 varieties of meat, four varieties of eggs, 48 varieties of fishes and shellfishes, seven varieties of seaweed, 16 varieties of milk, eight varieties of fat, 27 varieties of beverages and liquors, 34 varieties of seasonings, and 13 varieties of other items. We gave five-digit food codes to each food item to be analyzed, and the codes were partially based on their groupings. The first two digits represent 17 food groups, and the remaining three digits represent the serial numbers in alphabetical order of the English names of food items for analysis. The analysis data were expressed as mg magnesium per 100 g fresh-edible weight. Also, of the food items in the Food Composition Table from the Korea National Rural Resources Development Institute (2006), we presented the data of magnesium content together with the values of contents analyzed in this study.

Survey Subjects

We conducted a survey of 239 subjects, of which 100 were adult males and 139 were adult females, who were healthy, without any diseases, and did not take any medications or health supplements from Chungnam region. All participants gave their consent to participate in the survey. The age distribution of study targets ranged from 30 to 94 years, with no significant difference between men and women. The average age, height, weight, and body mass index were, respectively, 60.1 years of age, 166.7 cm, 66.4 kg, and 23.5 kg/m2 for males and 59.5 years of age, 152.1 cm, 55.4 kg, and 23.9 kg/m2 for the females. The survey study was approved by the Medical Research Ethnics Committee of the College of Medicine, Chung-Ang University in Seoul, Korea.

Dietary Intake Survey

The pre-trained research staff conducted dietary intake surveys through one-on-one interviews using the 24-h recall method. Surveys were conducted on types and quantities of food items consumed for breakfast, lunch, dinner, and snacks during the course of 1 day from the time of waking up in the morning to bedtime at night on the day before the survey. In order to standardize the surveys on meal intake, the research staff assisted survey targets in accurately remembering quantities of food items taken by presenting them with pictures and models of the food items prepared in advance. Then, using CAN-Pro 3.0 (The Korean Nutrition Society, South Korea), intakes of energy and major nutrients with magnesium exception were analyzed. Also, using magnesium contents in the food items analyzed in this study, we calculated magnesium intake.

Statistical Analysis

In relation to all results obtained in this study, we calculated the mean values and standard deviations by using an SAS program (version 9.1, SAS Inc., Cary, NC, USA). Differences in results of analysis between male and female subjects were analyzed through an unpaired t test. All results were verified as significant at a level of p < 0.05.

Results

Magnesium Contents in the Commonly Consumed Foods

Magnesium content in commonly ingested food items are as shown in Table 1. Of the 51 varieties of grains, magnesium contents were in the range of 1.15∼138.55 mg/100 g. The food item that showed the highest magnesium content per 100 g was Job’s tears. The magnesium content of milled rice was 10.70 mg/100 g. In potatoes and starches, the magnesium content of potatoes and sweet potatoes were 17.98 mg/100 g and 13.70 mg/100 g, respectively. Korean vermicelli and devil’s tongue jelly showed magnesium contents of 10 mg/100 g or less. For sugar items, the magnesium content of candy, honey, and sugar were, respectively, 0.52 mg/100 g, 0.81 mg/100 g, and 0.98 mg/100 g. Among legume items, the magnesium content of soybean curd and black bean were, respectively, 110.07 mg/100 g and 143.68 mg/100 g. With magnesium contents of 100 mg/100 g or higher, green beans, red beans, and small black beans displayed higher levels of magnesium content compared to other food groups. As for nuts and seeds, magnesium contents were in the range of 42.01 mg/100 g for chestnuts to 379.71 mg/100 g for pumpkin seeds. Compared with legumes, nuts and seeds were found to have higher magnesium content. Among the 68 varieties of vegetables, magnesium content was lowest at 4.72 mg/100 g for lotus root and highest at 265.67 mg/100 g for amaranth. The magnesium content of cabbage kimchi was 31.39 mg/100 g, while for radish and onion, it was 5.58 mg/100 g and 7.78 mg/100 g, respectively. For fungi, the magnesium content of all mushrooms with the exception of Juda’s ear mushroom (187.73 mg/100 g) were within the range of 10.03∼45.21 mg/100 g. As for 33 varieties of fruits, the magnesium content of jujube and raisin were, respectively, 61.52 mg/100 g and 43.62 mg/100 g, which were higher than those of other fruits. In most varieties of fruits with the exception of jujube, raisin, avocado, banana, and kumquat, magnesium content was found to be 20 mg/100 g or less.

Table 1 Magnesium Contents in Commonly Consumed Foods by Koreans
Full size table

Among the 13 varieties of meat, the magnesium content of Bulgogi ham was the lowest at 12.25 mg/100 g, and it is highest in jerked beef with 38.09 mg/100 g. In most meats, the magnesium content was low at 40 mg/100 g or less. Of eggs, the magnesium content of chicken eggs and quail eggs were, respectively, 7.18 mg/100 g and 9.94 mg/100 g, and the magnesium content in egg yolks was 9.85 mg/100 g. In 48 varieties of fishes and shellfishes, the magnesium contents of dried seafood, such as dried anchovy, dried Alaska pollack, dried squid, dried icefish and dried shrimp, etc., were in the high level of 122.16∼295.18 mg/100 g. However, in fish such as hair tail, mackerel, Spanish mackerel, yellow croaker and Pacific saury, etc., the magnesium content was low at 0.16∼34.60 mg/100 g. In seaweed, the magnesium content was higher than in other food groups, with measurements of 328.79 mg/100 g in laver, 741.22 mg/100 g in sea mustard, and 461.46 mg/100 g in sea tangle. In dairy, magnesium content was 11.67 mg/100 g in milk and 4.70 mg/100 g in yogurt. Of the 16 varieties of dairy products, the lowest magnesium content of 1.78 mg/100 g was displayed in butter, while magnesium content in corn ice cream was the highest at 38.14 mg/100 g. Magnesium contents in fat, such as sesame oil, soybean oil, and corn oil, were at lower levels of 2 mg/100 g or less. However, the magnesium content of peanut butter was high at 153.18 mg/100 g.

Of the 27 varieties of beverages and liquors tested, black tea, carbonated drink, green tea, isotonic drinks, and Soju were displayed low at the magnesium contents of 1 mg/100 g or less. On the other hand, the magnesium content of instant coffee powder was 346.04 mg/100 g, which was the highest found among all beverages and liquors. The second highest magnesium content was in cocoa (89.56 mg/100 g). Among the 34 varieties of seasonings, pepper, red pepper powder, and flakes in instant Chinese noodle and Cheonggukjang showed magnesium contents, respectively, at 179.44 mg/100 g, 167.91 mg/100 g, 156.72 mg/100 g, and 112.59 mg/100 g. On the other hand, the magnesium content in vinegar, Worcester sauce, Miwon, Mirim, and coleslaw dressing were low at 10 mg/100 g or less. As for other food items, the magnesium contents in ginseng and fresh ginseng were high at 53.85 mg/100 g and 53.45 mg/100 g, respectively. Precooked items of thick beef soup, sea mustard soup, and beef bone broth displayed low magnesium contents of 3 mg/100 g or less.

The magnesium contents analyzed in this study were compared with corresponding items from the Korean Food Composition Table [8] shown in Table 1. A highly significant and positive correlation was found between our values and their counterparts from the Korean Food Composition Table (data not shown).

Status of Magnesium Intake

Table 2 shows the results of evaluating the status of magnesium intake by 239 adult males and females using the data to analyze magnesium contents in 366 varieties of food items commonly taken by Korean people. The daily average calorie intake was 7,698.9 kJ for males, which was significantly higher than 6,301.6 kJ for females (p < 0.001). As for daily average magnesium intake, 306.5 mg of male was significantly higher than 259.1 mg of female (p < 0.001). However, magnesium intake per 1,000 kcal was found to be 172.9 mg/1,000 kcal and 171.2 mg/1,000 kcal for males and females, respectively. Therefore, there was no significant difference between the sexes. Also, on average, males and females consumed, respectively, 87.6% and 92.4% of RIs for magnesium. The percentage of magnesium intake less than 75% of the RI was 43.0% in males and 46.8% in females. As for the percentage of magnesium intake less than the EAR, more than half of males (54.0%) and females (55.4%) had magnesium intakes less than the EAR.

Table 2 Daily Intakes of Energy and Magnesium in the Subjects
Full size table

Discussion

Recently, there is a growing concern about mineral deficiency due to excessive energy intake. With studies conducted on the functionality of minerals in relation to chronic diseases [13, 14], the need for studies of minerals is further highlighted. Of the minerals, magnesium is known to be an essential nutrient in skeletal formation as well as for nucleic acid, protein, and lipid metabolism [2]. However, studies of magnesium intake by Korean people have been very limited and insufficient. One reason is the lack of a comprehensive database of the magnesium content of a wide variety of food items. As a constituent of chlorophyll, magnesium is contained in large quantities of green leafy vegetables as well as in seeds, legumes, and grains. On the other hand, the magnesium content of meat, fish, milk, and fat are relatively lower [4, 5]. Therefore, in the dietary life of Korean people, which mainly entered on the intake of vegetables, magnesium can be supplied through a variety of source food items such as green leafy vegetables, legumes, and grains, etc. Also, the magnesium intake can be easily estimated when data on the magnesium content in food items that mainly contribute to magnesium intake are available. In this study, we analyzed the magnesium content of 366 food items commonly ingested by Korean people that were selected according to their frequency as seen in the KNHANES of 2005 [12]. The Food Composition Table [8] lists the magnesium content of 918 varieties of food items, which is greater than the number of food items analyzed in this study. However, in many cases, a single food item was subdivided per species, part, and cooking method. So, of the 366 food items analyzed in this study, 104 were not found in the Food Composition Table (28.4%). Also, with the exception of 104 varieties of food items analyzed in Korea [9], most data in the Food Composition Table cited composition data from Japan [10] and the USA [11]. Therefore, it is a problem to utilize these data in estimating magnesium intake by Korean people. So, more accurate evaluation of magnesium intake will be achieved by jointly using the results of this study in which analysis was carried out on the frequently consumed food items together with the Food Composition Table [8]. Furthermore, in order to completely establish a comprehensive database on the food composition of magnesium, it is necessary to accumulate results through repetitive analyses. Also, by comparatively reviewing accumulated data, the reliability of the data must be assessed and studied, and the selection of representative values must be conducted in the future.

In Korea, the daily recommended magnesium intake for 30 years old and over in adult males and females was set as 350 and 280 mg, respectively, as of 2005 [7]. Of the studies evaluating the status of magnesium intake, a study by Yoon et al. [15] was conducted on male elementary students reporting that their magnesium intake was 178.8 mg/day. Also, a study targeted to women aged 20∼24 [16], by Sung et al. [17] targeted for adult males and females aged 26∼59, and postmenopausal women [18] reported the magnesium intakes to be 228.5, 240.4, and 345.9 mg/day, respectively. As such, since no DRI for magnesium was set prior to 2005 in Korea, only the absolute level of magnesium intake was analyzed and reported. Even the KNHANES did not analyze magnesium intake. Therefore, there is insufficiency of more comprehensive evaluations on the status of magnesium intake. The National Nutrition Survey of Japan [19] conducted a survey on 9,825 healthy adults aged 20 or higher and reported that daily magnesium intake per male was 287 mg and per female was 258 mg. Also, the study by Peacock et al. [20] on 7,731 Americans aged between 45 and 64 reported that the daily magnesium intake per males and females to be, respectively, 241∼252 and 270∼279 mg. In this study, the daily average magnesium intake by 239 adult males and females was found to be 279.2 mg, which was 90.4% of the RI and this can be evaluated to be appropriate. However, the percentage of target people with magnesium intake lower than the EAR was 54.8%, and the percentage of target people with magnesium intake less than 75% of the RI was 45.2%. As such, the percentage of target people with inappropriate magnesium intake was found to be high. Therefore, in order to enhance the magnesium intake level, it will be necessary to administer appropriate nutritional education by identifying people with insufficient magnesium intake.

There was no significant difference between magnesium intake evaluated with the database of magnesium content in 366 varieties food items analyzed in this study and with 918 varieties of food items in the database of Food Composition Table [8]. On the other hand, the two results displayed significantly positive correlation (data not shown). The reason for this result despite the number of data for magnesium contents in food items analyzed in this study being smaller than that of the Korean Food Composition Table was because reliability was high for the values of magnesium contents analyzed in this study. In other words, many of the food items analyzed in this study were source items that contained large amounts of magnesium and were frequently used in the current dietary life. Therefore, it is believed that values of this study will be useful in the evaluation of magnesium intake.

Mineral contents in food items differ according to mineral contents in soil. A study by Marr et al. [21] reported that the mineral content in the same variety of brown rice varied according to mineral contents in the soil. Also, there is a difference in mineral content between the East and the West. This is believed to be because of the differences in the species of food items used in daily meal intake. Additionally, for analysis of magnesium content in actual food items, the methods of purchasing, pre-treating, and analyzing food items have differed per researcher. This also is believed to cause the difference in reported magnesium contents. Due to these reasons, in this study, there were also some foods showing a large difference with the database of Food Composition Table [8]. This study reports the analysis of magnesium contents in specific regions, of specific varieties, and by specific methods. Therefore, it has limitations in being utilized as generalized data to estimate intake levels globally. However, with the current insufficient data of magnesium content in Korean food, this research will contribute to promoting studies on magnesium and is significant in the sense that it estimated the true magnesium intake by healthy adult males and females according to their daily meal intake and the source of their food.

So, in conclusion, we analyzed the magnesium content in 366 food items commonly ingested by Korean people using ICP-AES. Using the analysis data of magnesium contents, we evaluated the status of magnesium intake by 239 adult males and females. We found that the daily magnesium intake was 279.2 mg, which was 90.4% of the RI. However, the intake level by 54.8% of the subjects was found to be below the EAR. Also, for 45.2% of the cohort, the magnesium intake was less than 75% of the RI. As such, the percentage of subjects with inappropriate magnesium intake was high.