Analysis of dietary insoluble and soluble fiber contents in school meal

The health benefit of dietary fiber, as a kind of phytochemical, is no longer an issue any more. In recent, the dietary fiber is considered to be the most important nutrient, especially in preventing obesity, which is a worldwide epidemic disease [1]. Further, the intake of dietary fiber five or six times a day is also believed to prevent chronic diseases, like cancer, cardiovascular disease and diabetes mellitus. Dietary fiber is food's component that are rich in whole grain, various vegetables, and some fruits and seaweeds.

In fact, the main physiological function of dietary fiber is that it expands the stomach to provide a powerful satiety signal, allowing the feeling of being full and satisfied [2]. The other functions of dietary fiber are supporting the fermentation of bacteria and the increment of the fecal volumes, mainly in the large intestine which stimulate peristaltic contraction and thus, allowing the excretion of the feces more easily [3]. This effectiveness of dietary fiber is the key point of treatment for metabolic syndromes, especially obesity. Recently, one of the critical public health problems is the increasing prevalence of obesity in younger population in Korea, as similarly observed many developed countries [4]. Therefore, their consumption of dietary fiber is recommended in school meal in addition to at home.

According to KDRIs [5], the adequate intake (AI) for dietary fiber in all groups over the age of 12 years is determined as 25 g for male, 20 g for female, while AI of the age group between 6 and 11 years is 20 g for boys, and 15 g for girls. It means that each school meal has to provide dietary fiber approximately, from 5 g to 8 g. DRI of dietary fiber was determined by AI of total fiber only, without the differentiation of insoluble and soluble fiber.

Total dietary fiber (TDF) are determined independently, or are obtained from the amount of insoluble dietary fiber (IDF), plus the amount of soluble dietary fiber (SDF) by enzymatic procedures of AOAC method [6]. However, in some samples, soluble fiber determination often has the problem of filtration, so the two methods mentioned above can't reach the same values of dietary total fiber. Therefore, the recovery rate of soluble fiber should be considered to obtain an accurate analysis.

The main objective of this study was to quantify dietary fibers in each school menu, through direct analysis and to figure out the providing amounts of insoluble and soluble fiber from the school meals.

These data suggest that the school meals, per serving size, would provide above 75% KDRI of total dietary fibers, through mainly the soups and the special dishes conclusively. In addition, it might be expected that children could consume more soluble fiber from the meal with special dishes rather than the regular meals.

Nowadays, it is well-known that a high fat, low carbohydrate diet might be one of the more important factors to cause metabolic imbalance that underlies for the problem of obesity, cardiovascular disease and several cancers. In western society, where fat consumption has been relatively higher than in the Asian countries, it is recommended that fat intake of less than 30% of total energy will lower the risk of such related diseases [7]. However, some researchers suggested that there is no evidence of increased risk with the increased fat intake through the national survey in about 13,000 persons [8-9].

Traditionally for Koreans, most energy comes from carbohydrates, and fat has only contributed approximately 15%-20%. However, the food consumption pattern has been changed, gradually for over the past three decades. People tend to consume more animal foods and less plants, and the prevalence of obesity, cardiovascular diseases, diabetes mellitus, along with prostate cancer and breast cancer have increased during that time [10, 11]. In the same period, fat intake increased three times as much, according to the steady increase of animal food intake. According to KNHANES IV, in general, food intakes of Koreans showed to reach the peak in 2001-2005, and after which, mainly the intake of plant foods reduced, except potatoes and sweet products. However, animal foods such as meats, milk & milk products or eggs showed an increasing trend [11].

Therefore, interest in the source of dietary fiber has been growing dramatically. Thus, consumption of vegetables and fruits, as well as seaweeds has been on the rise for the average consumers. These foods are nutrient-rich sources, such as fiber, vitamins or minerals and/or phytochemicals. However, the consumption of seaweeds and vegetables, which are fiber-rich foods, has been reduced since 2001 for ten years. The range of dietary fiber intake were 6.4-7.2 g/d, which was approximately 30% of AI for KDRIs [10, 11].

In school, foods need to provide the children with sufficient overall nutritional dietary intake and to teach them about nutrition, as well as, social aspects of eating. Recently, children have been enjoying chips, burgers and other high-fat content dishes, such as cakes and soft drinks; with only a very select few opting for fruits and vegetables [12]. As a result, the nutritional quality of school meal has been blamed for having too much saturated fat and non-milk extrinsic sugar or salt, and not enough fruits and vegetables etc. [13, 14]. In school meal, the detrimental factor of total dietary fiber intake was the menu selection and the preference for entrees and side-dishes or special dishes. It is much needed to enhance both the interest in food, while improving the variety and the quality of food provided in schools, which will be an effort to reverse some of the deterioration in the dietary habits of school children. Ultimately, halt the increase in obesity and associated health problems [15, 16]. The range of carbohydrates consumed provides the body, not only with glucose to maintain blood levels for central and peripheral nervous system function, but also as a dietary fiber. It is also a food source for colon health [17-19].

The main dietary fiber definition, in current use, highlights the benefit to the physiological functionality. Dietary fiber consists of the plant cell wall-associated fibers, naturally occurring in fruits, vegetables, and cereal products, in addition to the isolated fibers that are added to processed foods [7]. Dietary fiber has been frequently classified as soluble and insoluble. The distinction is due to the chemical properties of fiber sources and analytical quantification; it does not necessarily reflect the physiological effects [20, 21]. The fiber definition also includes lignin and other associated compounds that are intrinsic and intact in the plant cell wall. Insoluble fiber reaches the colon, largely unchanged, and is not fermented by bacteria. The water-holding characteristics of the insoluble fiber meant that together, with the large bacterial mass, the total intra-colonic mass is greater and thereby, leads to an increased peristaltic action, which increases the speed of movement of the contents in the colon. This action reduces overall transit time through the gastrointestinal tract, and contributes to the overall laxation effect [22, 23]. In the large intestine, soluble fiber is acted upon by the bacterial flora. This process of fermentation releases a number of SCFA: propionic acid and acetic acid are metabolized in the liver, but butyric acid is used locally by the gut colonocytes, as an essential source of energy. The multiplication of the bacterial flora increases the bulk and the water content of the stools. Besides, diets that are rich in fiber encourages chewing, which both slows the process of eating and increases saliva flow, which contributes to the satiation and promoting dental health, as well. As dietary fiber travels through the small intestine, bile acid and divalent cations, such as calcium, zinc and iron, are bound to the surface of the fiber, which may reduce their availability for absorption [24-28].

Traditionally, fiber has been analyzed using the AOAC Official Method of Analysis 985.29, or with other AOAC methods. This method is a generally accepted method of analysis for the quantification of dietary fiber. However, there is no consensus of the definition regarding whether the non-digestible polymers with a degree of polymerization in the range of three to nine should be defined as fiber. In the EU definition, these are included, but the Codex definition leaves it up to the national authorities to decide about their inclusion. The European Union and the Codex Committee on Nutrition and Foods for Special Dietary Uses (CCNFSDU) and the Codex Executive Committee Definitions, as well as that of AACC International (formerly the American Association of Cereal Chemists), include isolated fibers and synthetic polymers, but with the condition that they possess beneficial physiological effects [21, 29-31].

Now, the discussion regarding dietary fiber is not an easy task. The chemical heterogeneity may often cause the confusion of as to the definition and the physiological function. Moreover, their physicochemical properties can change during cooking or digestion. Therefore, dietary fiber is continuously needed to develop the analytical methods and epidemiological research for the accurate analysis of specific fiber quantitatively and qualitatively [21].

Nevertheless, considering the above mentioned facts, these results might be used as a nutritional assessment for the dietary intakes of insoluble fiber, as well as, soluble fiber in children via school meal in the near future.