Collectively, chronic elevation of cortisol can promote loss of lean tissue, make weight loss appear to stall by increasing water retention, exacerbate the effects of low leptin levels, and potentially increase urges to overeat. As weight loss causes increases in ghrelin and cortisol with reductions in leptin and insulin, this four-pronged attack on your hunger regulation yields a very unhappy dieter. Hormonal adaptations to weight loss also have the capacity to interfere with reproductive function, thereby amplifying the unhappiness.
Intuitively, this makes a great deal of sense from an evolutionary perspective. Low energy availability is perceived as a sign of insufficient resources in the environment, which is only exacerbated by the energy cost of gestation and further division of resources to accommodate a population increase via reproduction. Aside from the obvious threat to muscle mass retention, lack of libido is a very common complaint of male physique athletes late in the contest preparation process. Similarly, female athletes typically experience a drop in estradiol in response to contest prep, along with menstrual cycle disruption.
The severity of this disruption may depend upon a variety of factors, including acute energy availability, magnitude of overall weight loss, body fat percentage, and genetic predisposition. In less severe cases, menstruation may simply become more irregular; in more severe cases, menstruation may cease altogether.
A case study of a female figure competitor revealed that menstruation became disrupted early in the contest preparation process, had ceased by the end of prep, and did not return until a full 71 weeks after competition.
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These reproductive side effects are not to be taken lightly; in addition to being generally unpleasant, there are long-term consequences to maintaining a prolonged state of sex hormone suppression, such as impaired bone health and fertility. The primary hormonal adaptations to weight loss are summarized in Figure 3.
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As the previous sections would suggest, I prefer not to think of metabolic adaptation as just a drop in metabolic rate. Rather, I prefer to conceptualize it as a multifaceted set of adaptations that involves regulation of energy expenditure, appetite, reproductive function, and the balance between anabolism and catabolism. The biological goal is to constrain energy use during times of underfeeding, but the effects are widespread, and to discuss them in isolation from one another is a bit short-sighted.
Despite this wide variety of adaptations to weight loss and energy deficiency, the drop in energy expenditure, also known as adaptive thermogenesis, is the one that garners much of the attention in the fitness world. We have talked about proton leak, brown adipose tissue activation, and hormones related to ATP consumption and thermogenesis. But physique athletes and other dieters rarely utter a single one of those words in day-to-day conversation about dieting. But before getting into specific aspects of metabolic rate adaptations, we need a quick primer on energy expenditure in general.
Every day we consume energy and we burn energy; the net balance of this equation determines whether we gain or lose weight. We know that brown adipose tissue increases thermogenesis, particularly in response to cold exposure, thyroid hormone, leptin, and insulin; BAT thermogenesis would therefore primarily affect basal metabolic rate and the thermic effect of feeding.
We noted that thyroid hormone and leptin increase mitochondrial uncoupling and muscle thermogenesis; these broad effects would have the potential to affect essentially all components of energy expenditure. So, all of the mitochondrial and thermogenesis-related concepts previously discussed fit somewhere within our four components of energy expenditure, and sometimes span multiple components. Unfortunately, when we embark on a weight loss diet, all four components are influenced to some extent. It remains unclear whether or not we have an adaptive shift in the thermic effect of feeding that is, a disproportionate increase in the efficiency with which we handle the energy cost of feeding.
While there are some isolated studies suggesting that a small adaptation favoring a relatively lower thermic effect of feeding does occur, the collective evidence would suggest that any such adaptation is negligible in magnitude. Leibel et al carried out an ambitious study in which some subjects were over-fed to induce weight gain, and others were under-fed to induce weight loss. Their results showed that weight gain from intentional overfeeding did reduce the energy efficiency of feeding, leading to a relatively greater thermic effect of feeding.
In contrast, weight loss from intentional under-feeding did not increase this efficiency, and the thermic effect of feeding was not significantly different when comparing baseline values to the values obtained after weight loss. Overall, the body of evidence suggests that the thermic effect of feeding is lower during weight loss dieting, but this is caused by lower overall food intake and less sympathetic nervous system activation in response to caloric intake.
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If there is any adaptation that makes us more efficient with regard to this thermic effect, it is very small in magnitude. It requires an overnight laboratory visit to allow for metabolic rate assessment directly upon waking, in the absence of food intake or substantial movement. As a result, labs that are not equipped to house overnight guests often use resting metabolic rate as a surrogate measurement. They still require subjects to fast overnight, but they let the subject come to the lab in the morning. To account for the movement associated with transportation to the lab, they have the subjects lay down and rest for a while before the actual measurement occurs.
Under these conditions, resting metabolic rate can be used as a pretty solid representation of basal metabolic rate, and the terms are often used fairly interchangeably in the literature. Weight loss typically involves the loss of both fat mass and fat-free mass; fat mass is a metabolically active tissue, and fat-free mass is the primary determinant of energy expenditure at rest.
As a result, we fully expect resting metabolic rate to drop as we lose weight. In many cases, the drop in resting energy expenditure is greater than we would expect based purely on the loss of tissue, which suggests that an energy-conserving adaptation has taken place. The study by Leibel et al that evaluated the thermic effect of feeding also evaluated resting energy expenditure; their results indicated that resting metabolic rate was reduced following weight loss, even when accounting for the amount of fat-free mass lost. While there are some studies showing no effect of weight loss on resting metabolic rate, a meta-analysis on the topic supports the findings of Leibel et al and suggests that resting metabolic rate is indeed slightly lower in weight-reduced individuals compared to controls.
These data match up quite favorably with data obtained from competitive physique athletes. For example, case studies have reported drops in resting metabolic rate that far exceed a magnitude attributable to lean mass loss and have shown resting metabolic rates to be consistently lower than predicted, even after accounting for the amount of lean mass. These low values in physique athletes are almost certainly related to the acute effects of energy restriction at the time of measurement, but the loss of fat mass is most likely a contributing factor.
Collectively, the evidence would suggest that weight loss typically induces a small but not negligible adaptive effect that reduces resting energy expenditure, and that this is observed in both non-athletic populations and competitive physique athletes. Exercise activity thermogenesis is a tough one, because we directly control it. You decide how much weight training and cardio to do, so you have the capacity to influence it substantially.
For any kind of locomotive cardio, such as walking, running, or cycling, the task itself involves generating enough force to transport the mass of your body. As you lose weight, the mass is reduced, and you naturally require less energy to transport your body a given distance. Even for some non-locomotive tasks like stationary cycling, weight of the limbs is reduced, so slightly less energy is required to perform the same amount of cycling work.
Another factor that is rarely discussed relates to the fact that a lot of people, myself included, do absolutely no cardio unless required to. Many forms of cardio are trainable skills that we become substantially more proficient at as we do them more frequently throughout a weight loss attempt. Using running as an example, there are about 22 factors give or take that relate to running economy. Many of them are trainable, meaning that a novice runner can probably make substantial improvements to their running economy over the course of a few months.
Aside from these predictable reductions in exercise activity thermogenesis in response to a weight loss attempt, there are also metabolic adaptations that increase energy efficiency and reduce energy expenditure during exercise. As the researchers hypothesized, muscle work efficiency at lower exercise workloads was increased following weight loss and decreased following weight gain.
This means that weight gain caused the participants to use more energy to produce the same amount of physical work, whereas weight loss caused the participants to use less energy to produce the same amount of work.
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Several years later, the same lab group replicated their finding, observing that muscle work efficiency was increased by weight loss and decreased by weight gain. Fortunately, the researchers used little weights in one of these studies to replace the leg weight that was lost; while the magnitude of the effect was smaller after weight replacement, a significant effect remained. This relative decrease can certainly be offset by increasing the absolute amount of exercise. Non-exercise activity thermogenesis NEAT is the final component to discuss, and it is unequivocally the most influential when it comes to adaptive thermogenesis.
Part of its impact relates to its capacity to be so incredibly variable. If I know your body size and composition, I can give a pretty good estimate of your resting metabolic rate. If I know your macronutrient intakes, I can give a pretty good estimate of your thermic effect of feeding. But two individuals of similar body size can have a daily difference of up to 2, Calories when it comes to NEAT. This variability is huge but not surprising when you consider the wide range of factors that can affect NEAT.
The hypothalamus plays a primary role in regulating NEAT, in conjunction with components of the hindbrain and mesolimbic system.
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The degree of NEAT is likely to vary based on occupation, sex, age, and season, and the exact cues that feed into these regulatory centers include temperature, activity level, food intake, body composition, and a large number of neuroendocrine inputs. Taken together, these observations would suggest that weight loss induced a fairly substantial reduction of energy expenditure, and this reduction was largely attributable to reductions in NEAT.
The problem with NEAT is that it can be a bit difficult to measure in an accurate and affordable manner. For example, Weigle et al made use of controlled feeding protocols, daily weight changes, and linear regression to make inferences about NEAT.
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One of the classic studies regarding weight loss and NEAT was not actually intended to study either topic and can be objectively categorized as a very fortuitous mistake. A small group of people were placed in a 3-acre dome, which was intended to be a completely closed, self-contained world with a variety of biomes, much like a dome we could theoretically set up on a moon or a different planet.
You could argue that the study fared a bit better than the film, but it was not without some stumbling blocks.
Most notably, the Biosphere inhabitants were supposed to grow all of their own food, but they were unable to successfully grow enough calories to support their energy needs. As a result, the study became an unintentional weight loss study that lasted a full two years. A week after leaving the dome, energy expenditure was assessed via a hour confinement in a respiratory chamber and compared to control subjects. Total energy expenditure was suppressed in the Biosphere residents compared to controls, and this difference persisted after adjusting for relevant characteristics age, sex, fat mass, and fat-free mass.
When in the respiratory chamber, control subjects had nearly twice as much spontaneous physical activity fairly synonymous with NEAT than the Biosphere residents.