Metabolic Adaptations to Weight Loss

Christian Coulson
Updated: June 10, 2020
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Metabolic Adaptation is a term used to describe your body’s response to a change in calorie intake.

In this article, we will discuss the different metabolic adaptations that can happen during weight loss.

Why Do Metabolic Adaptations Happen?

Your metabolism reduces energy expenditure and slows the rate of weight loss in response to lower calorie intake as a method of survival.

Metabolic adaptations happen to bring your body back to a state of internal equilibrium where it feels comfortable and functions optimally.

Long periods of calorie deficit and low levels of body fat will result in reduced energy expenditure, increased metabolic efficiency, and hormonal alterations. (1, 2)

What Happens During Metabolic Adaptations?

Reduced Energy Expenditure

Being in a calorie deficit and losing body weight serve as signals of low energy availability. Your body will adjust accordingly and attempt to restore balance by reducing energy expenditure.

The total amount of calories you burn in a day is known as Total Daily Energy Expenditure (TDEE) and can be divided into the following components:

  • Resting Energy Expenditure (REE)
    • Basal Metabolic Rate (BMR)
  • Non-Resting Energy Expenditure (NREE)
    • Exercise Activity Thermogenesis (EAT)
    • Non-Exercise Activity Thermogenesis (NEAT)
    • Thermic Effect of Food (TEF)

Since all of these components make up your total daily energy expenditure, a decrease in any one of them will cause a decrease in TDEE.

Graph showing TDEE components and percentages.
  • Basal Metabolic Rate: the rate at which your body burns calories while at rest to sustain essential functions, such as breathing, digestion, and circulation.

BMR accounts for ~70% of your TDEE and decreases during weight loss due to the loss of metabolically active tissue (3) – tissues that burn calories even at rest, such as the heart, muscles, and intestine.

  • Exercise Activity Thermogenesis: calories you burn during exercise.

Accounts for ~5% of your TDEE and decreases during weight loss due to the muscular and cardiovascular systems not having to work as much as they do a higher body fat levels.

For instance, it takes less energy to move a 150 lb person than it does a 250 lb one.

However, studies show that EAT remains low even when external weight is added to match the person’s initial body weight. (17) It’s thought that this happens due to the decrease in leptin and hormones released by the thyroid gland. (18)

  • Non-Exercise Activity Thermogenesis: calories burned during non-exercise activities, such as housework and involuntary movements like fidgeting.

Accounts for ~15% of your TDEE and decreases during weight loss for reasons similar to that of EAT. (4)

  • Thermic Effect of Food: also known as Diet-Induced Thermogenesis (DIT), are the calories used for ingesting, digesting, and metabolizing nutrients from foods.

Accounts for ~10% of your TDEE with values changing based on your macronutrient distribution.

Each macronutrient has a different TEF value, with protein having the highest, followed by carbs and lastly, fats.

  • ~15-30% of calories are burned through processing protein.
  • ~5-10% through processing carbs.
  • ~0-3% processing fats. (5)

For instance, if you ate 200 calories worth of protein, you could burn anywhere from 30 to 60 calories from digestion.

A decrease in TEF is also expected as food intake is typically reduced during weight loss.

Explore More: Calculating your TDEE and Macros

Increased Metabolic Efficiency

ATP is the primary energy molecule found in our cells. We derive ATP from nutrient oxidation (food), however, for this to happen, a series of chemical reactions need to take place.

Uncoupling Protein Thermogenin Graph.

ATP Synthase produces ATP during the transportation of protons across the inner mitochondrial membrane.

During this process, some protons can leak through uncoupling protein (UCP) and still promote nutrient oxidation, however, this leak via UCP happens very fast and ATP synthesis does not happen. Instead, energy is dissipated as heat. (6)

An increase in metabolic efficiency means that your body has improved its ability to extract calories from foods while minimizing the amount of energy lost in the form of heat.

Hormonal Alterations 

A variety of hormones help regulate energy expenditure, metabolic rate, and body composition.

Changes in circulating hormones can threaten muscle mass retention, cause mood swings, and lower satiety levels while increasing hunger levels.

  • Leptin ↓

Responsible for sending signals of energy availability and telling your central nervous system to adjust energy expenditure and food intake accordingly. (7)

Since leptin comes from fat cells, the smaller your fat cells become, the lower your leptin levels will be. Lower levels of leptin will serve as a signal of low energy availability and cause an increase in hunger.

  • Ghrelin ↑

This hormone acts in opposition to leptin, it increases during periods of fasting and signals your brain that you’re hungry.

  • Thyroid ↓

The thyroid gland releases hormones that help regulate your heart rate and how quickly you process food.

The decrease in thyroid hormones during weight loss may cause constipation and a slower than usual heart rate. (8) For female athletes, a decrease in these hormones can cause menstrual irregularities, such as amenorrhea. (9)

  • Insulin ↓

This hormone also regulates macronutrient metabolism and signals the brain about energy availability.

Insulin increases the speed at which some amino acids are transported into the tissues and moves glucose from the bloodstream into storage.

It also decreases the rate of muscle protein breakdown, glycogen breakdown, and fat oxidation in the muscle and liver. (10)

Insulin levels, however, decrease during weight loss, which can hinder muscle retention and increase hunger levels.

  • Testosterone ↓

Testosterone helps increase muscle protein synthesis and muscle mass while reducing fat mass; however, this hormone tends to decrease during low levels of body fat, which can cause a loss in strength and muscle mass. (11)

  • Cortisol ↑

Cortisol is a hormone mainly released during stressful situations. It helps your body metabolize macronutrients as well as control blood pressure and reduce inflammation.

Cortisol levels tend to increase during a calorie deficit. An increase in cortisol levels can cause muscle protein breakdown as well other issues, such as deregulated sleep.

How Often Do Metabolic Adaptations Happen?

  • It usually takes more than two weeks for metabolic adaptations to happen. (12)

Most people can expect adaptations to happen within 3 – 4 weeks; however, this is something that will vary between individuals.

I’ve had clients go 8 weeks without any signs of adaptations. Me, on the other hand, adapt reasonably quickly and have found myself making changes as soon as 1 week after the previous change.

Rapid Weight Regain

Hormonal changes due to long dieting periods and low body fat levels promote overeating and weight regain.

Your body will try to gain weight as quickly as possible to get to a range of body fat where it feels comfortable. This range is usually referred to as “Body Fat Set Point.”

Image showing how cells react when losing weight too fast and gaining it back. (hyperplasia)

Increased Size of Fat Cells

Weight loss causes your fat cells to shrink. On the other hand, weight regain causes an increase in their size (adipocyte hypertrophy) due to the accumulation of energy.

It’s thought that the body promotes weight regain as a way to relieve the stress imposed on the cells during the changes in size. (13)

Increased Number of Fat Cells

Metabolic conditions during early weight regain may promote the creation of new fat cells (adipocyte hyperplasia). (14)

Studies suggest that the creation of new fat cells is most likely permanent and that it could be one of the reasons why some people tend to regain more weight than they had before they started dieting. (15)

However, if the creation of new fat cells does happen, is likely limited to individuals who have a genetic susceptibility to obesity. (16)

Reverse Dieting

Hormonal imbalances and lowered metabolic rate are reasons why some think it’s ideal to add a period of “recovery” after long calorie deficits.

During this period, you look to slowly increase calories to restore hormone levels and increase both metabolic rate and energy expenditure, while minimizing fat gain.

This period is often referred to as “Recovery Diet” or “Reverse Dieting.”

An increment in calories is highly recommended at the end of a weight loss period; however, the rate of weight gain during the “Recovery Diet or Reverse Diet” will likely be dependent on the athlete’s competitive schedule.

I hope you loved this article about metabolic adaptations to weight loss!

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