Abstract: Obesity is an escalating problem in many countries. There is a need to reduce the average body weight of a population, not simply treat the extremes. This requires consideration of societal factors which affect body weight. Weight increases with age, the increase leading to a higher proportion of body fat to lean tissue. Causes include progressive inactivity and dietary fat which often accompany increasing affluence. A body mass index (BMI) of 20–25 is generally advised, but some suggest a BMI of 20–22 for lowest risk of disease. The risk of smoking when thin exceeds the risk of being overweight, so dietary advice is important on cessation of smoking. Genetic susceptibility to weight gain is strongly influenced by nutrient interactions at all stages of life. Foetal and neonatal nutrition may modify body weight in later life. Familial trends of reduced activity and low metabolic rate need to be addressed, as does the problem of lower metabolic rates and lower food needs after weight loss. Appetite control is poorly understood but dietary fat and energy density of food and exercise are important factors. For those who miss breakfast, the loss of a low fat meal of cereals and toast is significant. The strong influence of dietary fat on obesity and associated disease risks has led to expert committees to develop population nutrient goals which limit dietary fat. An energy discrepancy of only 2% a day can explain an increase in body weight of 5 kg/year. It is easier to understand why people become fat than to tackle the societal issues which affect the prevalence of obesity.
