Bone is formed as a result of calcium phosphate deposition onto a collagenous matrix. Calcium is an important constituent of bone, exert various important biological functions, and therefore its serum concentration is strictly regulated by factors such as parathyroid hormone, active vitamin D, and FGF23. Vitamin D insufficiency, which is milder than deficiency, is a risk factor for osteoporotic fracture and a number of other diseases. Vitamin K is an essential cofactor for both blood coagulation factors and bone matrix proteins. Osteoporosis is defined as a state of increased fracture risk, and is influenced by both bone mineral density and bone quality. Hyperhomocysteinemia, caused by insufficiency of folate, vitamin B₁₂, or vitamin B₆, impairs bone quality and is a fracture risk factor. In the Dietary Reference Intakes for Japanese 2020 (DRIs 2020), DG (tentative dietary goal for prevention of lifestyle-related diseases) is not defined for calcium or vitamins, since it has not received attention in the context of osteoporotic fracture prevention. For this reason, the requirements for these nutrients differ between the DRIs 2020 and the Guideline for the Prevention and Treatment of Osteoporosis.

Soy isoflavones are a form of polyphenol contained in soybeans and the kudzu plant. One gram of whole soybeans contains an average of 1.4 mg of soy isoflavones, which include a total of 15 compounds such as the aglycones daidzein, genistein and glycitein, and their conjugated forms with carbohydrates such as glycosyl, acetyl, malonyl and a succinyl form. The daily intake of isoflavones by Japanese is estimated to be 18 mg at the 50th percentile. When ingested by humans, isoflavone glycosides need to be hydrolyzed to aglycone forms in the small intestine prior to absorption. Isoflavones exert biological effects through weak estrogenic activity and/or antioxidant activity. Daidzein is metabolized by intestinal bacteria to equol, which has stronger estrogenic activity. It has been reported that 30‐50％ of ethnic Asians have equol-producing activity. Through their weak estrogenic activity, it has been reported that isoflavones affect bone health, lipid metabolism, hormone-dependent cancers, and menopausal symptoms. This paper summarizes the relationship between soy isoflavone intake and health, especially bone metabolism.

Age-related loss of skeletal muscle mass and function (sarcopenia) reduces functional independence in the elderly and leads to disability. Dietary protein intake is essential for prevention of sarcopenia. Among the essential amino acids in dietary protein, leucine in particular enhances the synthesis of skeletal muscle proteins via mTORC1. In terms of protein intake, attention should be paid to not only total daily intake but also to the amount of protein consumed at each of the three daily meals. Resistance exercise is the only intervention in daily life that can promote significant muscle gain, and adequate protein intake is also essential for maximizing the effects of exercise and promoting muscle gain. Furthermore, since vitamin D insufficiency/deficiency increases the risk of sarcopenia, there is a need to intervene and monitor various types of nutrient status in combination with exercise interventions in the elderly population.

Skeletal muscle metabolizes nutrients and supports physical activity by producing energy and synthesizing its constituent proteins. It also acts as a secretory organ that regulates systemic functions by secreting bioactive factors, referred to as myokines. Myokines can influence health through effects on gut functions and such as hormone secretion, immunity, and carcinogenesis risk, in association with physiological responses and benefits induced by exercise. On the other hand, the metabolic capacity of skeletal muscle is affected by various signals derived from the gut. The gut is responsible for digestion and absorption of nutrients and contributes to the supply of metabolic substrates. In addition, it acts as a barrier by preventing invasion of extrinsic factors such as bacteria, antigens, and endotoxins, thereby preventing excessive inflammatory responses and metabolic disturbances. It is also known that metabolites produced by gut bacteria regulate metabolism directly or indirectly. Therefore, healthy gut function is important for skeletal muscle metabolism, contributing to the prevention of metabolic diseases and frailty, and maintaining exercise performance. The science of the crosstalk between skeletal muscle and gut, i.e. the “Muscle-Gut Axis”, has led to novel perspectives of diet and exercise.