The concept of “metabolic damage” or “starvation mode” often stirs concern in the fitness world. It’s the idea that aggressive calorie restriction can slow your metabolism so much that weight loss becomes nearly impossible. While this sounds plausible, scientific evidence reveals a more nuanced picture. Let’s understand first how our body’s energy expenditure works:
Total energy expenditure (TEE) is the total amount of energy your body uses in 24 hours. It’s made up of four main components:
Basal Metabolic Rate (BMR)
Measuring true basal metabolic rate is challenging and typically requires an overnight lab stay for precise conditions. Instead, most studies use resting metabolic rate (RMR) as a substitute, measuring energy expenditure after an overnight fast and a period of rest.
Weight loss, which involves losing both fat mass and fat-free mass, naturally lowers RMR since fat-free mass primarily drives resting energy expenditure. However, studies show that RMR often drops more than expected from tissue loss alone, indicating a small energy-conserving adaptation. Research, including work by Leibel et al, confirms this phenomenon, with findings supported by meta-analyses and case studies on competitive athletes. These studies reveal that RMR in weight-reduced individuals tends to be slightly lower than predicted, likely due to energy restriction and fat loss. Overall, weight loss induces modest but measurable reductions in resting energy expenditure in both general and athletic populations.
The thermic effect of food (TEF)
The thermic effect of food naturally decreases during weight loss because consuming less food reduces the energy required for digestion and metabolism. While some studies suggest minor adaptive changes in how efficiently the body processes food during weight loss, the evidence shows these changes are negligible. For example, one study found only a 3-calorie difference in TEF after participants lost ~7.3 kg. Another study observed no significant changes in TEF after weight loss compared to baseline. Overall, the decrease in TEF during dieting is due to reduced food intake and less activation of the sympathetic nervous system, with minimal evidence of meaningful biological adaptation.
Exercise Activity Thermogenesis (EAT)
It refers to the energy burned during intentional exercise, and it can be influenced by various factors, including weight loss. As you lose weight, you naturally expend less energy during activities like walking, running, or cycling because your body mass decreases, requiring less effort to move. Even non-locomotive exercises like stationary cycling become slightly more efficient as limb weight decreases.
Additionally, regular cardio can improve your skill and efficiency over time. For instance, as you become a better runner, you may burn fewer calories for the same distance due to improved running economy. While beneficial for physical performance, this adaptation also reduces calorie expenditure.
Studies from Columbia University confirm that weight loss increases muscle work efficiency, meaning you burn less energy to perform the same physical tasks, even when accounting for reduced body or limb weight. This metabolic adaptation further contributes to decreased EAT during weight loss. However, these reductions can be mitigated by increasing exercise volume, though such an approach may have its challenges.
Non-Exercise Activity Thermogenesis (NEAT)
When someone loses weight, their total daily energy expenditure (TDEE) decreases, which includes energy burned through all activities like exercise and even basic movements. Studies show that after losing 10% or more of body weight, a person’s TDEE can drop by 20-25%. While some of this drop is due to losing body mass, a significant portion (10-15%) is due to an adaptive response where the body becomes more efficient at using energy.
A large part of this drop is due to a reduction in NEAT (Non-Exercise Activity Thermogenesis), which includes everyday activities like walking, fidgeting, or even moving around during daily tasks. For example, in a study where participants lost 23% of their weight, their total energy expenditure was about 24% lower than expected, but their resting metabolic rate (the energy used while resting) was only slightly lower (about 2-3%). This suggests that most of the reduction in energy expenditure came from less movement in daily activities rather than changes in basic metabolic functions.
Example: Imagine someone who regularly walks around 10,000 steps per day but, after losing weight, becomes less active and starts walking only 7,000 steps a day. That decrease in daily movement, even if they’re not consciously trying to cut back, could significantly lower their overall calorie burn, contributing to the weight loss plateau many people experience after dieting. This reduction in NEAT can be a major factor in why weight loss slows down over time.
The data suggests that weight loss causes a disproportionate drop in total energy expenditure, with NEAT being the main factor behind this effect. As people lose weight, they naturally move less, leading to a significant decrease in calories burned through daily activities. This reduction in NEAT is a major reason why many people hit a weight loss plateau, as the body adapts to burning fewer calories over time. Understanding the role of NEAT can help explain why maintaining weight loss can be challenging and why small changes in daily activity levels can have a big impact on long-term weight management.
One of the most insightful studies on this topic is the Minnesota Starvation Experiment, conducted during World War II. In this study, 36 volunteers ate around 1,500 calories daily—half their normal caloric needs—and performed physical labour for six months.
The results were striking. Participants lost about 25% of their body weight, and their metabolic rates dropped by only 20% compared to predictions based on their new, lower weights. Importantly, once they transitioned to a recovery diet, their metabolisms began to bounce back. Within 12 weeks, many participants’ metabolic rates were nearly normal, and eventually, all fully recovered
Scientific research consistently shows that weight loss leads to a decrease in energy expenditure, accompanied by various physiological changes. However, some discussions around this topic, such as claims of gaining fat despite low-calorie intake or the idea of “starvation mode,” can be misleading. While our calorie needs for weight loss may vary, everyone has a specific number of calories they need to consume. The key takeaway is that metabolic adaptation may slow down fat loss, but it doesn’t stop it entirely. No one fails to lose fat simply because they’re eating too few calories.
Miles CW, Wong NP, Rumpler WV, Conway J. Effect of circadian variation in energy expenditure, within-subject variation and weight reduction on thermic effect of food. Eur J Clin Nutr. 1993 Apr;47(4):274-84. PMID: 8491165.
Leibel RL, Rosenbaum M, Hirsch J. Changes in energy expenditure resulting from altered body weight. N Engl J Med. 1995 Mar 9;332(10):621-8. doi: 10.1056/NEJM199503093321001. Erratum in: N Engl J Med 1995 Aug 10;333(6):399. PMID: 7632212.
Astrup A, Gøtzsche PC, van de Werken K, Ranneries C, Toubro S, Raben A, Buemann B. Meta-analysis of resting metabolic rate in formerly obese subjects. Am J Clin Nutr. 1999 Jun;69(6):1117-22. doi: 10.1093/ajcn/69.6.1117. PMID: 10357728.
Rosenbaum M, Vandenborne K, Goldsmith R, Simoneau JA, Heymsfield S, Joanisse DR, Hirsch J, Murphy E, Matthews D, Segal KR, Leibel RL. Effects of experimental weight perturbation on skeletal muscle work efficiency in human subjects. Am J Physiol Regul Integr Comp Physiol. 2003 Jul;285(1):R183-92. doi: 10.1152/ajpregu.00474.2002. Epub 2003 Feb 27. PMID: 12609816.
Dulloo AG. Physiology of weight regain: Lessons from the classic Minnesota Starvation Experiment on human body composition regulation. Obes Rev. 2021 Mar;22 Suppl 2:e13189. doi: 10.1111/obr.13189. Epub 2021 Feb 5. PMID: 33543573.
Trexler, E. (n.d.). Metabolic adaptation: The key to understanding weight loss plateaus and rebounds. Stronger By Science. Retrieved December 11, 2024, from https://www.strongerbyscience.com/metabolic-adaptation/
Matthews, M. (n.d.). Metabolic damage: What it is and how to fix it. Legion Athletics. Retrieved December 11, 2024, from https://legionathletics.com/metabolic-damage/
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