Human Experimental Overfeeding

Experimental overfeeding studies show a highly significant relationship between excess energy ingested and both body weight gain (r2 = 0.82) and fat gain (r2 = 0.89) but with some individual differences. There is a genetic component as shown in the variability of changes in body weight, adiposity, visceral, and hepatic fat. The response to overfeeding is affected by the baseline state of the groups being compared: those with insulin resistance versus insulin sensitivity; those prone to obesity versus those resistant to obesity; and those with metabolically abnormal obesity versus those with "metabolically normal" obesity. Dietary components, such as protein, saturated versus polyunsaturated fat, and carbohydrates, influence weight and fat deposition. Overfeeding affects the endocrine system with increased circulating concentrations of insulin and triiodothyronine frequently present. Growth hormone, in contrast, was rapidly suppressed. Changes in plasma lipids are influenced by diet, exercise, and the magnitude of the weight gain. Adipose tissue and skeletal muscle morphology and metabolism are substantially altered by chronic overfeeding.