Energy Balance 101: TDEE, BMR, TEF & PAL
Before diving into hormones and diet breaks, it's key to understand how your body burns energy.
TDEE (Total Daily Energy Expenditure): the total calories you burn in a day, a sum of the next three.
BMR (Basal Metabolic Rate): the energy your body needs at rest to basically keep you alive.
TEF (Thermic Effect of Food): the energy used to digest and metabolize what you eat; some macronutrients require more than others, with protein needing the highest amount to be broken down, and fats the least amount of energy.
PAL (Physical Activity Level): the calories burned through movement and exercise.
When you’re in a caloric deficit — eating fewer calories than your TDEE — your body starts adapting in ways that might not be ideal for long-term fat loss, depending on how far you took the restriction.
Metabolic Adaptation: More Than Just Fat Loss
While calorie deficits are essential for fat loss, the body responds by trying to prevent weight loss — a survival mechanism. We do not have to go hunting for our next meal anymore, and the abundance of food is not something that our bodies have adapted to yet. Over time, you may experience:
Decreased resting metabolic rate (RMR);
Increased hunger;
Decreased satiety;
Changes in hormone levels like leptin, ghrelin, thyroid hormones, and insulin, which are the primary appetite regulators.
Studies show that these changes can persist for months — even years — after a diet ends, especially if the deficit was extreme or prolonged.
Hormones: The Silent Drivers Behind Metabolic Slowdown
Thyroid hormones are responsible for up to 30% of your RMR, making them a major determinant of the speed of your metabolism. Moreover, in conditions such as hypothyroidism, when the function of the thyroid gland is abnormally low, patients experience a gain in fat mass and a decrease in fat-free mass (or simply put, muscle mass). Let’s look at a few of the main players responsible for these changes:
Leptin is produced in your fat cells and is known as the satiety hormone. More leptin = reduced appetite, but during severe caloric restriction leptin levels drop, blunting your satiety, which could lead to overeating.
Ghrelin has the opposite role and is the one that drives hunger, sending you signals during fasting periods to initiate your next meal. To make things harder for you, ghrelin levels rise significantly in a caloric deficit, driving cravings.
Insulin is similar to leptin in its satiety regulation, but it is also essential for efficient muscle recovery, especially for athletes. After a good workout, the muscle glycogen stores are depleted, and insulin is the one that mediates their replenishment with glucose. Therefore, lower levels of insulin prevent adequate muscle recovery and negatively impact performance.
What About Testosterone?
Interestingly, moderate calorie restriction doesn’t always reduce testosterone in overweight men. But for leaner individuals — particularly athletes — reductions in testosterone during dieting may be more pronounced and problematic for muscle maintenance.
Why Women Lose Their Periods
In women, very low calorie intake can interfere with the hypothalamic-pituitary-gonadal (HPG) axis, the hormone system responsible for regulating the menstrual cycle. Disruption here can lower luteinizing hormone (LH) and follicle-stimulating hormone (FSH), affecting ovulation, which is why you have heard that extreme diets can result in a loss of ones period.
The Fat Cell Trap: Why Weight Rebound Happens
When body fat is lost, fat cells shrink — but their number doesn't change. These smaller fat cells are “hungrier” for storage and better at utilizing extra glucose. Unfortunately, these are less likely to produce leptin and help your appetite regulation.
In fact, research suggests that in early weight regain, fat cell hyperplasia (increase in fat cell number) may occur, setting the stage for faster fat rebound. This is why some people tend to gain even more weight than they managed to lose after going off a diet.
The Case for Diet Breaks & Refeeds
So, how do we fight back?
One proposed strategy is using intermittent periods of energy balance, often referred to as:
Diet breaks (mostly done for a week or more);
Refeed days;
Intermittent energy restriction (IER).
These are short breaks where calories are brought back up to maintenance, particularly from carbohydrates, to help reduce the effects of metabolic adaptation and hormonal suppression. Additionally, they could also provide a period of mental refreshment, improving the overall adherence to the diet.
What the Research Says
In a study on resistance-trained females comparing six weeks of continuous dieting with a 25% energy restriction vs. intermittent dieting:
No significant difference was found in fat loss or RMR between groups.
However, diet breaks reduced disinhibition, which may lead to better long-term adherence.
Another study — the MATADOR trial — found that obese men using two-week diet breaks:
Lost more fat;
Maintained more metabolic rate;
Had better weight maintenance 6 months later.
However, the caloric restriction in the second trial was more aggressive- the patients were put on a 33% caloric deficit. These observations drive the theory that the diet breaks strategy might be beneficial for those who severely cut down their calories and suffer from metabolic adaptation. On the other side, if the caloric deficit does not result in a largely slowed down metabolism in the first place, no positive effects compared to continuous dieting are observed.
Refeeds, Leptin, and Carbs
A study in female athletes found that carbohydrate overfeeding temporarily boosts leptin levels more than fat. For athletes, higher carb intake during refeeds may also help restore performance and glycogen, since carbs are of highest importance for endurance sports such as running or swimming. Moreover, this strategy might be even more beneficial for overweight and obese individuals due to their susceptibility to leptin resistance, but it is yet to be tested.
The Role of Protein in Preventing Muscle Loss
While the battle around carbs vs fats is still ongoing, protein remains especially important during a deficit:
It helps preserve fat-free mass;
Has the highest thermic effect (up to 20% of the energy is used for its breakdown);
Promotes satiety.
Many experts suggest increasing protein intake during a diet to combat hunger and prevent muscle loss. Moreover, muscle mass is much more metabolically demanding than fat, requiring more energy to be maintained. More muscle mass = faster metabolism.
So, Should You Take a Diet Break?
It depends.
If your calorie deficit is aggressive or prolonged, and you're seeing signs of:
Constant fatigue
Poor performance
High hunger
Slow recovery
… then yes, a diet break might help reset hormones and improve your relationship with food.
But if your deficit is modest, you're feeling good, and you're still making progress — a diet break might not be necessary.
The Bottom Line
Long-term calorie restriction can reduce metabolic rate and disrupt hormonal signals.
Adaptive thermogenesis is real — and it may persist after weight loss.
Diet breaks or refeeds may help reduce these effects, especially in athletes.
Carbohydrates are key for performance, leptin, and overall energy.
Prioritize high protein intake and avoid extreme, unsustainable deficits.
Sources:
Gripeteg L, Torgerson J, Karlsson J, Lindroos AK. Prolonged refeeding improves weight maintenance after weight loss with very-low-energy diets. Br J Nutr. 2010 Jan;103(1):141-8. doi: 10.1017/S0007114509991474. Epub 2009 Aug 7. PMID: 19664301; PMCID: PMC2923049.
Martínez-Gómez MG, Roberts BM. Metabolic Adaptations to Weight Loss: A Brief Review. J Strength Cond Res. 2022 Oct 1;36(10):2970-2981. doi: 10.1519/JSC.0000000000003991. Epub 2021 Mar 3. PMID: 33677461.
Peos, J. J., Norton, L. E., Helms, E. R., Galpin, A. J., & Fournier, P. (2019). Intermittent Dieting: Theoretical Considerations for the Athlete. Sports, 7(1), 22. https://doi.org/10.3390/sports7010022
Siedler MR, Lewis MH, Trexler ET, Lamadrid P, Waddell BJ, Bishop SF, SanFilippo G, Callahan K, Mathas D, Mastrofini GF, Henselmans M, Vårvik FT, Campbell BI. The Effects of Intermittent Diet Breaks during 25% Energy Restriction on Body Composition and Resting Metabolic Rate in Resistance-Trained Females: A Randomized Controlled Trial. J Hum Kinet. 2023 Jan 20;86:117-132. doi: 10.5114/jhk/159960. PMID: 37181269; PMCID: PMC10170537.
