Metabolic Adaptation, or What Happens to Our Body When We Lose Weight?

Weight loss is often considered as a simple ratio of calories in and calories out – if our body uses more calories than we take in, we lose weight.

This is basically what happens.

However.

The human body and its processes are much more complex, and weight loss is a complex process that involves not only calorie deficit and physical activity but also the body’s physiological and hormonal reactions. One of the main challenges in weight loss is metabolic adaptation — the body’s ability to adapt to reduced energy availability.

During this adaptation, a series of hormonal, neurological and metabolic changes take place.

These changes, sometimes called ‘metabolic slowdown’ or ‘adaptive thermogenesis’, explain why weight loss plateaus occur and why many people find it so difficult to maintain their new, reduced weight.

In this article, we look at:

• What is metabolic adaptation?
• What changes occur in our bodies during weight loss?
• Common myths about metabolic slowdown.
• How to prevent metabolic slowdown.

What is metabolic adaptation?

Metabolic adaptation (also known as adaptive thermogenesis) is a physiological response in which the body reduces energy expenditure in response to calorie restriction and weight loss. This adaptation involves a reduction in basal metabolic rate (the amount of energy needed to maintain life processes at rest), hormonal changes and other energy-saving mechanisms, reducing the amount of calories the body needs and can therefore slow or even stop weight loss.

For example:

• Your body spends on average 1600 Kcal/day.
• You follow a 15% calorie deficit diet (1600-240=1360 Kcal/day).
• Your body reduces its energy consumption to 1300 Kcal.
• And you are no longer in a calorie deficit – your weight does not decrease or even start to increase slowly.

How does it work?

The main mechanisms of metabolic adaptation are:

1. BMR decreases as the body adapts to reduced energy availability and lower body weight (the lower the body weight, the less energy is needed to maintain it).
2. Hormonal changes, for example:
   1. A decrease in leptin levels, which reduces satiety and increases appetite.
   2. Increases in ghrelin, which stimulates hunger.
   3. Decreases in thyroid hormone (T₃), which slow down metabolism.
   4. Cortisol levels may increase, contributing to fat storage.
3. Improved insulin sensitivity – although weight loss usually improves insulin sensitivity, a reduction in insulin levels can increase hunger, making it more difficult to maintain weight.
4. Decrease in physical activity. People often unconsciously reduce their physical activity when losing weight, thus reducing their overall energy expenditure. This includes both conscious and unconscious reductions in movement, e.g. walking less, gesticulating less, etc.
5. Muscle efficiency improves – muscles burn fewer calories during the activity.

Reduction in 24-hour energy expenditure at a 20%-40% calorie deficit.

Metabolic adaptation also affects our psyche and behaviour:

• Appetite and cravings for high-calorie foods increase.
• Motivation to continue the diet or to exercise decreases.
• Stress levels increase, and emotional eating becomes more frequent.

In other words, calorie deficit leads to changes in both our physiology and our psyche. Moreover, we are not able to detect these changes ourselves – we feel that everything is the same as before.

And.

We wonder why the calorie deficit doesn’t work?

The impact of metabolic adaptation on weight loss

Metabolic adaptation can have a significant impact on the rate and amount of weight loss:

1. Plateau in weight loss – after initial weight loss, there is often a period when weight no longer decreases. Further caloric restriction and/or increased physical activity do not help.
2. Weight regains – Research shows that most people regain the weight they lost within a few years. After dieting (especially after crash/fad diets or prolonged calorie deficits), as a result of metabolic adaptation, the body requires fewer calories, and once the person returns to previous eating habits and portion sizes, they eat many more extra calories (calories the body is not using) than before the diet. And the weight comes back on quickly.

Myths about metabolic adaptation

1. There is no such thing as metabolic adaptation – it’s just an excuse for failed diets.

If this myth makes someone’s life easier, stick with it.

However.

Many scientific studies confirm that metabolic rate decreases with weight loss more than it should at a person’s newly acquired weight. For example, the Minnesota Fasting Experiment back in 1944 showed that prolonged calorie restriction can reduce metabolic rate (the amount of calories needed by the body) by up to 40%.

2. “Starvation mode” (calorie deficit above 30-40% for ~3 days or longer) means you stop losing weight completely.

Although metabolic slowdown reduces energy expenditure, it does not stop fat loss, it only slows it down.

In other words, fat loss during fasting decreases not because the body stops burning fat, but because it significantly reduces overall energy expenditure and switches to energy-saving mechanisms and easier ways to obtain energy, such as breaking down muscle (it is easier to convert protein into glucose than fat). As a result, the metabolic rate drops even further as muscle mass decreases (organs consume 60-70% (including brain ~20%, liver ~27%), muscle 18-22%, fat 4-5% of basal metabolism).

3. To speed up your metabolism, you should eat more often.

Eating more often has little effect on 24-hour energy expenditure – changing the order you count does not change the total.

The study comparing 1 and 3 meals a day found no difference in metabolic rate.

I also recommend that my clients and participants in my weight loss challenges eat at least 4 times a day.

But.

This recommendation is not aimed at boosting metabolism but at distributing calorie intake more evenly, thus reducing hunger and the risk of overeating.

4. Metabolic adaptation only applies to very overweight people.

Nothing comes of nothing – if you take in fewer calories than your body needs, it will try to save (survive on what it gets).

In other words, with a prolonged calorie deficit, the body will try to adapt to it – the criterion here is the “fuel” deficit, not body weight.

5. Metabolic adaptation is irreversible.

If this were true, it would be terrible 😊.

However, it is not. Reversible dieting (gradually increasing calories) can restore the metabolic rate.

How to reduce metabolic adaptation?

1. Increasing calorie intake periodically, for a few days or weeks, to a weight maintenance level can help restore hormonal balance and prevent metabolic adaptation.
2. Regular physical activity, especially strength training, helps to maintain muscle mass and a higher basal metabolic rate.
3. Adequate sleep and effective stress management help maintain hormonal balance, crucial for weight loss and maintenance.
4. Avoiding crash/fad diets (1000 Kcal/day or less for women) and focusing on long-term lifestyle changes helps to reduce the risk of metabolic adaptation and promotes sustainable weight loss.
5. High-protein diets help reduce muscle loss and hunger and increase the thermic effect of food (TEF). Aim for 1.6-2.2 g protein per kg body weight during slimming.

Key takeaways

Metabolic adaptation is our body’s natural response to prolonged calorie deficit and reduced weight.

Our body’s “weight thermostat” is our brain, or more precisely, the part called the hypothalamus.

When fat stores decrease, the brain:

• Reduces energy expenditure (adaptive thermogenesis).
• Increases hunger signals to restore lost energy reserves (fat).

This mechanism has historically evolved to help us survive, but now it works against our weight loss efforts.

However.

It is also not tragic or insurmountable – adaptive thermogenesis is not irreversible (although it can persist for years).

And.

Although we cannot consciously influence our brain function, we can significantly reduce/slow down metabolic adaptation during weight loss.

Some evidence also suggests that maintaining weight for 12 months or more can help the brain adapt to the new, lower weight.

Eat delicious, eat a balanced, move and – be healthy!