What Happens to Your Body When You Diet? Explained By A Registered Dietitian

January is over and for a lot of people so are their fad diets. And this may be the BEST thing for your body!! Indeed, the diet industry has been on high speed in the last month and may try to convince you that you need to work on your willpower or try harder. However, statistics show that 95% of people who diet will regain their weight back within a year and sometime with interest (1, 2, 3)! Science is crystal clear: diets DO NOT work in the long-term. But why is that? What happen in our body that makes it so hard to keep the weight off? Let’s dive deep on what happens to your body on a physiological level.

What is the concept of energy balance? What is considered a calorie deficit?

To start off, we have to understand a few key concepts and one of them is energy balance. Basically, energy balance is the state where energy/calorie intake matches energy expenditure of the body, resulting in a stable weight. Energy expenditure includes basal metabolism (minimal need for cells to function properly), thermogenesis, such as thermic effect of food (yes, digestion in the body requires some energy!) thermal balance (when it’s -30 degrees outside, your body has to work to keep you warm!) and work and physical activity (3). However, when our energy intake is higher than our energy expenditure, we fall in the state of calorie surplus, which leads to weight gain. Our body will take the excess calories and store it as fat tissues for future use.

On the other side, a calorie deficit is when the amount of calorie we eat is inferior to the energy we expend. This is when weight loss happens. While this equation alone is pretty simple, the reality is a lot more complex. The calorie deficit that leads to weight loss also sends a signal to the brain to ”fight” against that disturbance in energy balance, which results in decrease energy expenditure and increased hunger (1). In the literature, this phenomenon is often called “energy gap” (1). Different mechanisms related to hormone changes will be discussed in the following lines.

How is energy balance regulated in the body?

While we might think that the food we eat (energy intake) is entirely within our control, there are actually several mechanisms regulating it behind the scenes. Regulation of our food intake occurs in the area of the brain called the hypothalamus by integration of central and peripheral messages, based on hedonic and homeostatic mechanisms (2, 3). For example, by the end of a meal, mechano-receptors in the gut detect gastric distention (i.e. your stomach expands) and chemo-receptors send signals of the presence of nutrients in the gut by the vagal nerve.

Hormones that play a role in energy balance

Then, a series of satiety hormones such as glucagon-like peptide-1 (GLP-1), cholecystokinin (CCK), peptide YY (PYY) and leptin are released in response to the presence of nutrients in the digestive system (2, 3). In the opposite situation, before a meal when you’re getting hungry, the concentration of these satiety hormones will be lower and ghrelin, the hunger hormone, will be released.

Leptin is a really important hormone that plays a role in hunger regulation and its mechanisms is a little bit different than the other hormones. It is actually produced mainly by the adipose tissue (fat tissues) and circulates in levels proportional to the amount of body fat, which indicates to the brain what are the body’s energy stores (2, 3) meaning the more body fat you have, the more leptin you will have. Leptin works by suppressing food intake by stimulating appetite-suppressing neurons and inhibiting orexigenic neurons in the brain (i.e. decreasing hunger signals).

Another important hormone in the regulation of the food intake is insulin, produced by the pancreatic beta cells. Insulin’s main role is the absorption of glucose, particularly from the blood into the liver, adipose tissue and skeletal muscle. Insulin also acts a satiety signals in the hypothalamus.

Furthermore, there is also hedonic regulation of food intake, which is based on the rewarding aspect of food and palatability. Even in the absence of an energy deficit, hedonic mechanisms can stimulate food intake by the action of endogenous opioids and cannabinoids (2). An easy example is when you just made muffins or cookies and they come out freshly cooked, warm and smelling terribly good from the oven. Even if you’re not hungry, you’re most likely to have one or two because they have a high level of palatability, they look delicious! The two graphics below give a good illustration and you also have a little table of the satiety/orexigenic hormones that will be addressed as a reminder.

What do all fad diets have in common?

Before going into the physiological and hormonal effects of energy restriction, I would like to bring your attention to one point that all fad diets have in common. While they will claim that their “awesome-new-diet” (or “lifestyle”) is non-restrictive, that you will not feel hunger, that this $29.99 supplement will help you burn fat or ”boost” your metabolism, they will ALL put you in some calorie deficit which ultimately leads to a quick weight loss. The diet industry is worth billions dollar (but not worth a second of your time!) and constantly tries to make us feel inadequate and bad in our body, so that we will try anything they offer us to make more money. There’s no magic solution and you will read in the next lines how these diets can have adverse effects on your health.

The Effects of Dieting on Hormones

Coming back to the heart of the topic, let’s talk about what happens to your body when you’re on a calorie deficit and after a weight loss. When the body falls in a calorie deficit states, this is perceiving as a ”threat” to the maintenance of energy balance and several changes in hormonal levels take place.

Ghrelin, Leptin & Insulin

As previously explained, leptin is considered the ”satiety” hormone, while ghrelin is considered a ”hunger” hormone. They both work with other mechanisms to regulate food intake. In the state of calorie deficit and when weight loss occurs, the body tries to fight against it by increasing ghrelin concentration, which enhances the feeling of hunger and the desire for cravings (1, 4). Furthermore, several studies have documented decreased levels of both leptin and insulin (1, 4). Insulin reacts to changes in body composition – insulin concentration increases as body fat increases. This logic also applies for the reserve, i.e. insulin concentration decreases with loss of body fat (4). Insulin passes across the blood brain barrier and stimulates neurons that decreases food intake. Basically, less body fat results in less insulin circulating resulting in enhanced hunger, cravings and a less satiety (1).

Now you might wonder, sure these hormonal changes lead to increase hunger, but how much? Does it make a real difference? Actually, some researchers had the same question and published a study in 2016 in which they found that weight loss leads to a proportional increase in appetite of about 100 kcal/day per kilos lost (5). So basically, if somebody lose 10 pounds (4.5 kg), their hunger could increase by 450 kcal per day, which is almost the hunger for a complete meal! Are you starting to see why so many people regain weight after attempting a weight loss? The main hormones that regulate food intake are completely going the other way so that the body goes back at the weight it was before! People that try to lose weight will often blame themselves for not keeping the weight off after, but there’s a lot more going on than just ”self-control for not eating more”. Furthermore, these alterations in hormone levels don’t come back to normal quickly. Several studies found that decreased level of insulin, leptin, peptide YY, cholecystokinin and higher level of ghrelin persist up to and even more than one year after weight loss (1, 2, 4, 6).

Cortisol

Cortisol is a steroid hormone that has several metabolic functions and we often relate it to stress. When we perceive a threat, our hypothalamus sends a signal to our body to release stress hormones including cortisol and adrenaline. Cortisol prepares the body by suppressing the action of different systems that are not useful to face the stressor, such as our immune, reproductive and digestive system. This process was very useful when our ancestors were faced with a tiger! This elevation in cortisol levels is normal and it returns to adequate levels once the threat is gone. However, chronic elevation and altered levels of cortisol are known to be harmful to our health (7) i.e., chronic stress! As previously mentioned, severe energy restriction in an attempt to reduce body weight can be perceived as a threat by the body. Some studies have reported an increased stress response following energy restriction and therefore, more elevated level of cortisol (7, 8). These higher levels can alter mood, memory, stimulate appetite and alter metabolism in favour of weight gain. A meta-analysis and systematic review from The International Journal of the Biology of Stress found that severe caloric restriction overall significantly increased serum cortisol (10). In animal models, it was found that the stress associated with calorie restriction can promote long-term alteration in genes and hormones critical for feeding and reward circuitry that influence energy intake (9). Mice that were stressed through energy restriction had more binge-eating behaviour than the control mice when exposed to high-fat food (9). Think about a time when you were stressed; how did it impact your energy intake? For more information, check out the blog post on stress and nutrition. This being said, more research is needed to better understand the long-term effect of energy restriction on stress hormones.

Thyroid hormones (T3 and T4) play a key role in body weight maintenance, primarily through regulation of energy expenditure (11). Alteration of thyroid metabolism occurs when energy intake and expenditure are not matching (4). After calorie restriction, a decrease in T3 concentration leads to a decrease in resting metabolic rate (the amount of energy your body uses at rest). Actually, variation in T3 concentrations have been shown to be independent predictors of change in energy expenditure, which is a determinant factor for weight regain after an attempt to lose weight (4). However, it is often challenging to assess the effect of thyroid hormones in relation to weight changes, because the causes of weight gain or weight loss are numerous and it’s hard to specifically quantify the effect of one factor. The POUND LOST study, a 2-year randomized clinical trial, tried to better understand the implication of thyroid hormones in relation to body weight (11). Their results showed that higher baseline levels of certain thyroid hormones (T3 and T4) predicted more weight loss induced by energy restricted diet, but not regain among participants with normal thyroid function (11). The effect of weight loss on thyroid hormone has mainly been studied in patient following bariatric surgery, which can hardly be generalized to the general population. In these patients, they found lower levels of thyroid-stimulating hormone (TSH) that has the function to stimulate the thyroid gland to release T3 and T4 (12). More data will allow to better understand the effect of weight loss on thyroid hormones.

Sex hormones are indeed known for their role within the reproductive system, but they are actually really important too for the regulation of appetite, eating behaviour and energy metabolism (13).

Just as sex hormones will have an impact on our nutrition, our energy intake can also influence levels of sex hormones. In women, severe energy restriction disrupts the secretion of GnRH (gonadotropin-releasing hormone) and reduces secretion of LH (luteinizing hormone), FSH (follicle-stimulating hormone) and therefore, also reduces estrogen levels (13). This is often seen in women with the Female Athlete Triad syndrome, a condition involving amenorrhea, osteoporosis and disordered eating pattern.  In men living with obesity, lower serum total testosterone levels have been consistently observed in several studies (15). Following bariatric surgery and very low-calorie diet, these levels increase and return to normal values (15). However, it is important to consider that these very low energy interventions are done under the supervision of research medical team and results apply to a very specific population. Moderate weight loss in mild-moderate subjects living with obesity are not well described (15). On the other hand, male athletes, especially in sports with an emphasize on leanness, reduced levels of testosterone and other sex hormones have been observed and men athletes with low BMI can experience similar adverse effects than women with the Female Athlete Triad (16). Unfortunately, there’s a lack of research on the effect of a similar syndrome on men.

Conclusion

To conclude, while it’s sadly too common that most people in our society have tried at least once to lose weight with a restrictive diet, there is strong scientific evidence showing that this is not without consequences on our body. The diet industry is making billions every year by promoting fad diets and products full of promises, but in the end, they’re all a sort of calorie restriction in disguise that can have a drastic effect on your hormones. Remember that health is achievable at every size and always consult your physician and a registered dietitian (look for RD, not a ”health coach”, ”fat loss expert”, etc.) when thinking of changing your diet.

I hope this was helpful – if you are still thinking of starting a diet check out this article on 7 reasons NOT to!

The Balanced Practice is a team of professionals specialized in eating disorder outpatient treatment. We strive to provide evidence based nutrition counselling to support you, or your loved one, in achieving full recovery. Schedule a connection call now.

Marie-Pier Pitre-D’Iorio, RD, B.Sc.Psychology
Lead Registered Dietitian at The Balanced Practice

A big thank you to Myriam Beaudry for this in depth article!

References:

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