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Hardness (difficulty of chewing) of the habitual diet in relation to body mass index and waist circumference in free-living Japanese women aged 18–22 y^1,2,3

¹ From the Nutritional Epidemiology Program, National Institute of Health and Nutrition, Tokyo, Japan (KMurakami, SS, and YT); the Laboratory of Physiological Nutrition, Kagawa Nutrition University, Saitama, Japan (KU); the Department of Health and Nutrition Science, Faculty of Health and Social Welfare Science, Nishikyushu University, Saga, Japan (MY); Department of Human Environmental Science, Fukuoka Women's University, Fukuoka, Japan (HH); Department of Nutrition, School of Food and Nutritional Sciences and COE21, University of Shizuoka, Shizuoka, Japan (TG); Department of Food and Nutrition, Faculty of Home Economics, Tokyo Kasei University, Tokyo, Japan (JO); Department of Nutrition, Mie Chukyo University Junior College, Matsusaka, Japan (KB); Graduate School of Science for Living System, Showa Women's University, Tokyo, Japan (KO); the Department of Food Science and Nutrition, School of Agriculture, Kinki University, Nara, Japan (TK); Course of Food and Nutrition, Department of Human Life Environments, Niigata Women's College, Niigata, Japan (KMuramatsu); and Department of Nutrition Management, Faculty of Health and Nutrition, Minamikyushu University, Miyazaki, Japan (MF)

Background: Animal studies suggest the beneficial effect of hardness of diet on body weight and adiposity. No human studies have examined hardness of diet in relation to obesity.

Objective: We examined cross-sectional associations of hardness of the habitual diet with body mass index (BMI; in kg/m²) and waist circumference in free-living humans.

Design: Subjects were 454 female Japanese dietetic students aged 18–22 y. Dietary hardness was assessed as an estimate of masticatory muscle activity for the habitual diet (ie, the difficulty of chewing the food). The consumption of a total of 107 foods was estimated by means of a self-administered, comprehensive diet history questionnaire, and masticatory muscle activity during the ingestion of these foods was estimated according to published equations. Waist circumference was measured at the level of the umbilicus.

Results: Mean BMI was 21.4 (95% CI: 21.1, 21.6), and mean waist circumference was 73.6 (72.9, 74.3) cm. Mean dietary hardness was 178 (175, 181) mVs/1000 kcal. Dietary hardness was not significantly associated with BMI. However, it was negatively associated with waist circumference (P for trend = 0.005). This association remained after adjustment not only for potential confounding factors (P for trend = 0.028) but also for BMI (P for trend = 0.002).

Conclusions: Whereas no association between dietary hardness and BMI was seen, increasing dietary hardness was associated with lower waist circumference even after adjustment for BMI in free-living young Japanese women. This finding could make innovative contributions to the literature and raise issues for future studies regarding diet and obesity.

Key Words: Hardness of diet • body mass index • waist circumference • Japanese • women • diet history questionnaire • epidemiology