The energy provided by food and the energy consumed by the body are measured in units of kilocalories. When the caloric intake is greater than the energy expenditure over a period of time, the excess calories are stored primarily as fat. Vitamins and elements serve primarily as cofactors and coenzymes. A and D have specific functions but share similar mechanisms of action, activating nuclear receptors and regulating genetic expression. The body tissues can use circulating energy substrates, including glucose, fatty acids, ketone bodies, lactic acid, amino acid, and others, for cell respiration.
Different organs have different preferred energy sources.
Eating behavior is regulated, at least in part, by the hypothalamus. Lesions of the ventromedial area of the hypothalamus produce hyperphagia, whereas lesions of the lateral hypothalamus produce hypophagia. These include the endorphins, norepinephrine, serotonin, and cholecystokinin. Adipose cells, or adipocytes, are both the targets of hormonal regulation and themselves endocrine in nature. In children, circulating saturated fatty acids promote cell division and differentiation of new adipocytes.