Athletes control their weight for many reasons.
Some aim to improve their strength-to-mass ratio (generally more advantageous to weigh less but able to lift proportionately more compared to your body weight), for some they want to improve their locomotive efficiency, and some do it for purely aesthetic/appearance reasons.
Being in a caloric deficit (eating fewer calories than your body uses in a day) is accompanied by changes in hormones, mitochondrial efficiency, and energy expenditure that are actually trying to minimize that deficit, prevent weight loss, and promote weight regain. Your body craves homeostasis – it’s where it runs best. This is true for athletes as well as weekend warriors or couch potatoes.
For athletes, however, the stakes are considerably higher. Missing a weigh-in and getting bumped up to the next higher weight class, center of gravity changes that can throw off your triple salchow, or the difference between center callouts and third callouts at a national level bodybuilding show.
Being at a low level of body fat while retaining as much lean body mass as possible is considered advantageous in a lot of competitive arenas. However, few scientific studies are ever done on this population because it makes up such a small part of the overall population. When 2/3 of the United States is overweight or obese, time is arguably better spent on finding ways to help those who are not athletic than those who are.
It has been well documented that weight loss and energy restriction result in many metabolic adaptations to bring the body back into a state of homeostasis – decreased energy expenditure (BMR and TDEE), improved metabolic efficiency, and increased cues for energy intake (ex. hunger).
One effect of this increased efficiency is that further weight loss becomes increasingly difficult despite lower calorie intakes and higher training volumes.
Why does this happen?
Several hormones play important roles in the regulation of body composition, energy intake, and energy expenditure.
Thyroid hormones, especially T3, are known to play an important and direct role in regulating metabolic rate. Increased levels of circulating thyroid hormones are associated with an increase in metabolic rate; lower levels are seen in decreased thermogenesis and overall metabolic rate.
Leptin, produced by fat cells, serves as the body’s indicator of short- and long-term energy storage. Short-term energy restriction and lower levels of body fat are associated with decreases in leptin levels – this can result in higher levels of hunger. Higher levels are associated with increased satiety and energy expenditure.
Insulin, which people usually think of as the hormone released in response to carbohydrate intake and diabetes, also plays an important role in inhibiting muscle protein breakdown. Similar to leptin, it is considered another “adiposity signal”. High levels of insulin tell the body that there is energy available for use and have a satiating effect.
Ghrelin is also referred to as the “hunger hormone” in that higher levels stimulate appetite and food intake. Levels increase with fasting and decrease quickly after eating.
Testosterone, usually known for its role in increasing muscle protein synthesis and muscle mass, may also have a role in regulating adiposity. More research is needed, but decreases in body fat levels have been shown to result in decreased levels of testosterone as well and higher levels may repress fat formation.
Cortisol, the “stress hormone”, is a glucocorticoid that influences metabolism and shown to induce muscle protein breakdown. Higher cortisol levels are correlated with elevated blood sugar. This is advantageous in an emergency situation – such as if you need to run from a bear. The elevated blood sugar can be used by your body’s cells to make a quick exit. However, in today’s society, cortisol can be chronically elevated and result in adverse health outcomes – including blunting lean body mass formation and retention.
In general, calorie restriction leads to hormonal responses that promote increased hunger, reduce metabolic rate, and threatens maintenance of lean mass (which is metabolically more “expensive”) – decreases in leptin, insulin, testosterone, and thyroid hormones with increases in ghrelin and cortisol. These changes even seem to persist for some time after the dieting period ends as the person tries to maintain their new, lower body weight.
Low calorie intake and low body fat levels are signs to the body that energy may be unavailable in the environment. Your body does not care about your athletic ventures or your desire for six-pack abs – it just wants to keep you alive. In order to do that, hormone levels become aimed at conserving energy and promoting increased food intake.
Resistance training and sufficient levels of dietary protein intake can help preserve lean body mass during periods of energy restriction, but some degree of loss is inevitable (especially among “naturals” or non-anabolic steroid drug users).
Weight Loss and Metabolic Rate
Someone’s total daily energy expenditure (TDEE) is made up of resting energy expenditure/basal metabolic rate (REE/BMR), thermic effect of food (TEF), exercise activity thermogenesis (EAT), and non-exercise activity thermogenesis (NEAT).
Metabolic rate is not constant.
TDEE has consistently been shown to decrease during periods of weight loss. BMR, the largest component of TDEE, decreases. Interestingly, this decrease in TDEE often is greater than what might be predicted by loss of body mass – possibly due to what is termed “adaptive thermogenesis”. This adaptive thermogenesis may partially explain why it become increasingly difficult to lose additional weight when weight loss plateaus despite lower calorie intakes and the tendency to quickly regain weight after a period of weight loss.
Roughly 10% of TDEE is attributed to TEF – the energy needed to ingest, absorb, metabolize, and store nutrients from food. Fewer calories consumed = less energy needed to process that food.
NEAT, the energy expended during normal daily activities and fidgeting, also decreases. EAT also decreases in response to weight loss.
Due to lowered TDEE, ever lower energy intake adjustments are required in order to achieve further weight loss.
Adaptations in Mitochondrial Efficiency
In order to transform the food you eat in to useable energy, a series of chemical reactions must take place to create ATP.
In aerobic metabolism, where most of our energy is derived, this involves the movement of protons across the inner mitochondrial membrane and ATP is produced. Protons may also leak across the inner membrane in a process that uses energy but does not produce ATP (“uncoupled respiration”) – this accounts for ~20-30% of BMR (in rats, not determined in humans).
Uncoupling proteins (UCPs) of brown adipose tissue (BAT) and skeletal muscle are of interest due to their potentially significant roles in energy expenditure and thermogenesis. Skeletal muscle we know contributes greatly to energy expenditure (more muscle = more calories burned during activity and at rest).
BAT, while it is “fat tissue”, actually has the characteristic of taking stored energy and generating heat, such as when it is cold outside, and so is often referred to a “good” fat. A person who is overweight has proportionately less BAT than someone of a lower weight. Note: there is no known way to increase how much BAT your body has.
Energy restriction has been shown to decrease BAT activation and UCP expression – indicating an increase in metabolic efficiency (and therefore lower calorie burn). Along with UCP expression, thyroid hormone and leptin affect the magnitude of uncoupled respiration in BAT. Above it was noted that thyroid hormone and leptin are decreased under conditions of energy restriction. Lowered hormones = less BAT activation = less energy used.
During periods of calorie restriction, proton leak is reduced. “Diet resistant” subjects show decreased proton leak compared to “diet responsive” subjects during maintenance of a reduced body weight.
It has also been reported that females have more BAT than males (this makes sense because females tend to have more adipose tissue than males and higher levels of essential body fat) and energy-restricted females see greater decreases in BAT mass and UCPs than males (in rats). This shows that there may be a gender-related different in uncoupled respiration during weight loss.
Mitochondrial research and metabolism is a growing area of interest, but evidence is still lacking. Current evidence suggests that increased mitochondrial efficiency and a decline in uncoupled respiration might serve to decrease the energy deficit in calorie-restricted conditions which would make weight maintenance and further weight loss more difficult.
It is likely that the magnitude of these metabolic adaptation that serve to prevent further weight loss and conserve energy are proportional to the size of the energy deficit. Because of this, it is recommended to use the smallest possible deficit while still achieving appreciable weight loss. This may decrease the rate of weight loss but prevent more unfavorable adaptations.
When weight loss plateaus (when intake = expenditure), energy intake or expenditure can be adjusted to re-create an energy deficit. Large deficits are also more likely to induce greater losses of lean body mass and compromise performance and recovery.
Sufficient protein intake and structured resistance training programs can help prevent lean body mass losses. Additionally, high protein diets are associated with increased satiety and increased thermogenesis.
Periodic “refeeding” has become common during periods of extended dieting in p. A refeed consists of a brief overfeeding periods in which caloric intake is raised to maintenance (or slightly above) levels and is predominately achieved by increasing carbohydrate intake, typically one or two days per week.
The proposed reason for these refeeds is to temporarily increase leptin levels and stimulate metabolic rate. There is evidence that leptin is acutely responsive to short-term overfeeding and is highly correlated with carbohydrate intake. The reported effect on metabolic rate has shown to be modest at best in studies done to date – possibly a 5-10% increase in TDEE which can partially be attributed to an increase in TEF from more food consumption.
More research is needed to determine if these refeeds are an effective strategy for improving weight loss success during prolonged dieting periods. For many, the success behind having weekly refeeds is more psychological. It is “easier” to stick to a more restrictive diet if the person knows that they have a day where they will be eating more in the near future.
In the time shortly after ending a diet, body weight often increases toward pre-diet values. This weight is preferentially gained as fat (“post-starvation obesity”) due to the fact that any of the metabolic adaptations to weight loss persist. It is common for subjects to “overshoot” their baseline level of body fat – the individual may increase body fat beyond baseline levels yet retain a lower metabolic rate than when they were at the same weight prior to dieting.
There is evidence that repeated weight loss and weight regain cycles by athletes in sports with weight classes are associated with long-term weight gain – a phenomenon also seen in chronic yo-yo dieters. Therefore, athletes who aggressively diet for a competitive season and rapidly regain weight may find it more challenging to reach their desired body composition in subsequent seasons.
To avoid this rapid fat regain, “reverse dieting” has become a popular term among physique athletes. This process involves slowly increasing caloric intake, the theory being that small caloric surpluses over time might help restore hormone levels and energy expenditure toward pre-diet levels while closely matching energy intake to the recovering metabolic rate in an effort to reduce fat gain. Research is still needed in this area as mostly anecdotal evidence is available at this time to evaluate its efficiency.
- A higher strength-to-mass ratio, improved aesthetic presentation, or more efficient movement is advantageous to a wide variety of athletes.
- Weight loss efforts will be counteracted by numerous metabolic adaptations as more time is spent in an energy deficit and lower body fat levels are achieved.
- These adaptations include changes in energy expenditure, mitochondrial efficiency, and hormone production.
- It is recommended to approach weight loss slowly by utilizing small energy deficits over time to ensure a slow rate of weight loss rather than large energy deficits.
- Following a structured resistance training program and consuming adequate amounts of protein are also important for optimal body composition achievement.
- More research is needed on the effectiveness of periodic refeeds and reverse dieting in supporting prolonged weight loss and attenuating post-diet fat gain.
Trexler et al. “Metabolic adaptation to weight loss: implications for the athlete”. JISSN 2014;11:7. http://www.jissn/content/11/1/7