This paper examines three aspects of hunger disease: the effect of initial fat stores on macronutrient fuel selection during total starvation (no energy) and how it influences survival; the effects of different rates of weight loss on tissue and body function; and the importance of appetite sensations, including hunger, during malnutrition and during enteral and parenteral nutritional support. Long-term starvation studies in humans reveal major differences in fat carbohydrate and protein metabolism between lean and obese subjects, including a 2-4-fold lower contribution of protein oxidation to energy expenditure in obese subjects, which ensures that more of the excess body fat is oxidized. The rate of weight loss, determined by recent dietary intake, can have major effects on tissue and body function, including wound healing, the acute phase protein response, muscle fatigue and psychological/behavioural function in both clinical and non-clinical settings. In depleted states uncomplicated by disease, changes in appetite sensations can result in energy intakes as high as 6000 to 10,000 kcal/day ( 25-42 MJ/day). Long-term enteral tube feeding and parenteral nutrition are associated with frequent disturbances in appetite sensations, and in those able to eat normally they tend to add rather than replace oral intake to an extent that appears to depend on the regimen. It is concluded that 1) differences between lean and obese subjects in macronutrient fuel selection during starvation are adaptive because they optimize survival in both groups of subjects; 2) the rate of weight loss in health and disease has a major effect on certain tissue and body functions, independently of the magnitude of weight loss; and 3) clinically relevant disturbances in appetite sensations are common subjects receiving long-term enteral and parenteral nutrition. The clinical modulation of all these variables would be aided by greater knowledge of the mechanisms involved.