Starvation mode refers to a slowdown in metabolism that can occur when the body receives too few daily calories for an extended period of time. The modern theory of metabolic starvation mode is believed to be based almost entirely on a scientific study often referred to as the Minnesota study. This study, performed in 1950 by researcher Ancel Keys, documented the effects of a starvation diet on a group of healthy young men of normal weight over a period of six months. The study concluded that, when daily calorie intake is reduced to at least half of the normal requirement, base metabolic rate can slow. People operating in starvation mode may generally be expected to continue losing weight, but they will lose weight at a rate somewhat slower than that of a person with a normally functioning metabolism, and they may be expected to experience general lethargy.
Concerns about hampering one's own weight loss progress by eating too few daily calories and going into starvation mode can be common among dieters. Keys' research seems to have established that daily calorie deficits must be severe, and must extend into the long term in order to trigger the physiological changes that cause the metabolic slowdown known as starvation mode. Experts believe that this mode serves a biological purpose, in that it allows the human body to survive through periods of famine, even when stores of body fat are small. Keys' study seems to suggest that starvation mode does not usually begin until the individual reaches a total body fat percentage lower than six percent, for a woman, or lower than five percent, for a man.
The physiological mechanism that leads to starvation mode is believed to rely on the action of a hormone known as leptin. The body's fat cells normally secrete leptin. Blood levels of leptin typically help the brain keep track of the extent of physical fat reserves. Lower calorie intake and increased weight loss can reduce leptin levels, by reducing the number of leptin-secreting fat cells in the body. When body fat percentage falls below the minimal necessary level, reduced leptin levels signal the brain to begin the process of metabolic slowdown.
Starvation mode does not normally prevent further weight loss. Weight loss continues in most cases, though it generally occurs at a rate slower than would be expected in a dieter who is not in this state. People in starvation mode may also experience a severe lack of energy, since they are unlikely to be receiving the calories they need to perform normal daily activities.