Obesity is one of the most important causes of life-style related diseases, and recently its pathophysiology is emphasized as metabolic syndrome. Preventing obesity by good dietary habit is a key to achieve healthy longevity. However, a lean body is not always good for health. There is an ideal body size for each person. This ideal body size differs according to age. Especially in the elderly, to prevent weight loss is more important for maintaining health and longevity than to be obese. Malnutrition is a critical factor of diseases and death in the elderly. Problems in nutritional status, and dietary intake, and methods of nutritional assessment in the elderly are discussed. Ideal body size for health and longevity, the relationship of body fat distribution and intra-abdominal fat accumulation health, and the effects of rapid weight change are also discussed to clarify the association of dietary habit and nutrition with longevity.