Can Chronic Under-Eating or Fasting Slow Down Metabolism?
Yes, chronic under-eating and very low calorie dieting cause resting metabolic rate to decrease, making it difficult to lose weight and maintain weight loss, though the magnitude and clinical significance of this adaptation remains debated. 1
Metabolic Adaptation to Caloric Restriction
Evidence for Metabolic Slowdown
Resting metabolic rate decreases with fasting and very low calorie dieting, as the body modulates metabolic rate based on calories consumed relative to energy expenditure 1
When less energy is available, people lose weight which reduces their overall metabolic rate, and cellular metabolic rate may also decrease through metabolic adaptation, reflected in physiological and endocrinological changes 2
This reduction in resting metabolic rate is problematic because it is the primary component of daily energy expenditure, making it difficult for individuals to continue losing weight and to maintain weight that is lost 1
Magnitude and Composition of Weight Loss
Prolonged fasting for 5-20 days produces mild to moderate weight loss of 2-10%, but approximately two-thirds of the weight lost is lean mass and only one-third is fat mass 3
The excessive lean mass loss suggests that prolonged fasting may increase the breakdown of muscle proteins, which is a major concern for metabolic health 3
Bed rest in sedentary individuals leads to a reduction in resting metabolic rate, and in highly trained runners, cessation of daily exercise training lowers resting metabolic rate by about 7-10% 1
Time-Restricted Eating vs. Prolonged Fasting
Safer Approach: Time-Restricted Eating
The American College of Cardiology recommends an 8-12 hour eating window rather than prolonged fasting, as this balances metabolic benefits with cardiovascular safety 4, 5
Time-restricted eating naturally decreases energy intake by 10-30% from baseline, leading to weight loss of 1-8% without the severe metabolic suppression seen with prolonged fasting 4
Well-timed eating and fasting windows (8-10 hours eating/>14 hours fasting) enhance fat loss and improve metabolic efficiency by synchronizing circadian clocks involved in energy expenditure and fat oxidation 6, 4
Critical Safety Warning
Eating windows shorter than 8 hours daily are associated with significantly higher cardiovascular mortality risk in both general and cardiovascular disease populations 4, 5
The European Society of Cardiology recommends against extended fasting for patients with acute coronary syndrome, advanced heart failure, or recent cardiovascular procedures 5, 7
Metabolic Recovery and Long-Term Effects
Reversibility with Exercise
Resting metabolic rate is depressed in previously sedentary obese individuals on a very low calorie diet, but it quickly returns to the predieting level when exercise of sufficient frequency, intensity and duration is undertaken while dieting 1
Exercise of adequate intensity and duration may enhance resting metabolic rate, counteracting the metabolic suppression from caloric restriction 1
Loss of Benefits After Refeeding
3-4 months after prolonged fasting was completed, all metabolic benefits were no longer observed, even when weight loss was maintained 3
This suggests that the metabolic adaptations and benefits are temporary and do not persist without continued intervention 3
Physiological Mechanisms
Adaptive Responses to Energy Deficit
The body metabolically adapts to reduced energy availability through changes in reproductive capacity, somatic maintenance, and hormone levels, whether the deficit comes from calorie restriction or increased activity 2
Fasting leads to reduced uptake of circulating triacylglycerols by adipocytes and stimulates degradation of stored triacylglycerols, representing coordinated metabolic changes 8
Key features of the adaptive response include breakdown of liver glycogen and muscle protein to produce glucose, as well as gradual depletion of fat stores 8
Clinical Recommendations
Optimal Approach to Avoid Metabolic Slowdown
Use time-restricted eating with an 8-12 hour eating window rather than severe caloric restriction or prolonged fasting to minimize metabolic adaptation while achieving weight loss 4, 5
Incorporate exercise of sufficient frequency, intensity and duration during any caloric restriction to prevent or reverse the decrease in resting metabolic rate 1
Avoid very low calorie diets and prolonged fasting periods, as these produce the most significant metabolic suppression and excessive lean mass loss 3, 1