Large variations in physical activity hardly affect body weight; if anything, body composition might be affected. The implication is that eating more or less compensates for an activity-induced change in energy requirement. Extremes in energy intake seem to be explained by extremes in physical activity. The question is whether this is correct. Are people claiming they can eat whatever they like without getting fat, the ones who are always on the move; and, those who claim they get fat from whatever they eat, the ones with an extremely low energy requirement. Overweight is caused by energy intake exceeding energy expenditure. Energy from food is provided by the three macronutrients: fat, carbohydrate and protein. A next question is whether an extreme in fat, carbohydrate or protein intake with food underlies disturbance of the balance between energy intake and energy expenditure? The last question we will examine in this chapter is whether energy expenditure responds to underfeeding or overfeeding. Extremes in reported energy intake do not correspond with simultaneously measured energy expenditure with doubly labelled water. Subjects claiming they can eat whatever they want without getting fat are not necessarily metabolically less efficient or extremely physically active. The apparent success of low-fat and low-carbohydrate foods in the prevention of overweight can be linked to a reduction of food intake by the deliberate limitation of food choice and by a higher thermogenesis-induced satiety when the reduction of fat or carbohydrate intake results in an increase in protein intake. Overfeeding has little effect on energy expenditure, resulting in 80–90 % of excess intake being stored, mainly as body fat. Underfeeding results in a pronounced decrease in all three components of total energy expenditure where the main reduction is by decreased activity induced energy expenditure through less body movement or an insufficient increase in body movement to compensate for the decreased cost to move a lighter body. Thus, eating more has larger consequences for energy balance than eating less.