The Effect of Low-calorie Diets on Metabolic Rate

Michael Rosenbaum, MD

Disclosures

March 20, 2001

Question

I am looking for medical references or scientific studies that "prove" the commonly stated "fact" that a person's metabolism "shuts down" if one eats too few calories. Although the literature is replete with successful weight loss studies that used low-calorie and very-low-calorie diets (LCD, VLCD), there is an "urban legend" that states that if an individual drops below an intake of about 1400 calories/day (or perhaps 1200), the metabolism will "shut down" and he or she will "hold on to fat" and will not lose weight. This is a common argument against LCD and VLCD in weight loss discussions. So far, I have yet to find medical/scientific verification of this phenomenon.

Joseph Pastorek, MD

Response from Michael Rosenbaum, MD

Your inquiry essentially encompasses the following 3 questions:

1. Are there articles in the literature that indicate weight loss or the maintenance of a reduced body weight invokes a hypometabolic state?

2. Are there articles in the literature that indicate weight loss or the maintenance of a reduced body weight is associated with a decline in the oxidation of fat or increased partitioning of stored calories as fat?

3. Is it a myth that there is an absolute caloric intake below which one cannot lose any more weight or fat?

The answer to all 3 questions is "yes."

Numerous studies have demonstrated that the maintenance of a reduced body weight and/or the process of weight loss are associated with significant declines in 24-hour energy expenditure (weight-maintenance caloric requirements) beyond those expected solely on the basis of weight loss.[1,2,3,4,5,6,7,8,9,10,11,12,13] A smaller number of studies, however, have also reported that 24-hour energy expenditure does not remain suppressed during maintenance of a reduced body weight.[14,15,16,17] Numerous studies have also demonstrated that weight loss is associated with impaired fat oxidation during exercise and/or preferential storage of dietary calories as fat following and during weight loss.[18,19,20,21] States of undernutrition -- particularly low carbohydrate, or ketogenic, diets like the Atkins diet -- on the other hand, will result in an increase in absolute rate of oxidation of fats as a source of fuel.[12]

Lipolysis and lipogenesis occur simultaneously. Whether a person is gaining or losing fat mass depends on the relative rates at which these processes are occurring. Whether the net storage of calories as fat, carbohydrate, and protein is increasing or decreasing (ie, gaining or losing weight) also depends on the relative rates of caloric intake and energy expenditure.

Most studies have shown that weight loss and the maintenance of a reduced body weight are associated with a decline in weight-maintenance caloric requirements that is significantly less (about 300 kcal/day less) than what would be predicted solely on the basis of weight lost or compared with body composition-matched control subjects. However, this remains a very active area of research. Conflicting results can often be attributed to methodological differences between studies. For example, the results of studies evaluating the effects of weight loss on energy expenditure may be strongly influenced by the lack of weight stability among subjects (declines in energy expenditure may be accentuated by weight loss). In addition, results are somewhat dependent on whether or not authors have controlled for changes in physical activity.

Weinsier and colleagues[15] reported that women who had lost weight did not experience significant associated reductions in their weight-maintenance caloric requirements, normalized for changes in body weight. However, even though energy expenditure did not decline, the reduced-obese women increased the amount of daily physical activity by about 30% following weight loss. Rosenbaum and colleagues [22] and Astrup and colleagues[23] review these issues in separate papers.

Dr. Pastorek, your characterization of an absolute level of caloric intake at which no further weight loss or fat-loss can occur as an "urban myth" is absolutely correct. The first law of thermodynamics (Net Energy Stored = Energy Intake - Energy Expended) cannot be violated. Thus, if an individual consumes fewer calories than he expends, he must lose weight.

Small discrepancies between energy intake and expenditure will, over time, produce big changes in body weight. If an individual were to reduce caloric intake by 150 calories/day (about one 8 oz glass of milk) relative to energy expenditure and do this every day for a year, they should theoretically lose about 55,000 calories worth of stored energy (about 17 pounds of fat). Similarly, a 300 calorie/day decline in energy expenditure (the average decline beyond that predicted on the basis of weight lost observed in weight-reduced individuals) relative to intake will result in a yearly weight gain of about 34 pounds of fat. In fact, fat is a much more efficient form of energy storage than protein or carbohydrate. You will, in fact, lose fewer pounds if you lose weight as fat than if you lose the same number of calories as protein (muscle).

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