What Happens to Metabolism When You Skip Meals? | Hardworkout Magazine

Key takeaways

- Resting metabolic rate stays roughly unchanged when you skip a single meal; what disappears is the temporary rise in energy expenditure from digesting food.

- The thermic effect of food usually accounts for around 10 per cent of daily energy expenditure, and three large meals require about as much digestive energy as six smaller meals with the same content.

- When insulin falls, glucagon prompts the liver to break down glycogen and release glucose, while fat cells release fatty acids through lipolysis; during longer fasts the liver produces ketone bodies the brain can use.

- Using a greater proportion of fat for fuel does not increase total energy expenditure, so skipping breakfast only aids fat loss if it lowers overall calorie intake.

- A prolonged calorie deficit can lower energy expenditure more than weight loss alone would predict, through adaptive thermogenesis, falling leptin and T3, and reduced NEAT.

Some eat breakfast shortly after getting up, while others wait until lunch for their first meal of the day. How often we eat varies with appetite, activity level, and daily routines.

When there is a longer time between meals, several changes occur in how the body regulates and uses energy.

Does your metabolism slow down when you skip a meal?

You burn energy all day long, even when you are resting or sleeping.

Most of the energy goes to keeping the brain, heart, liver, and other organs functioning. This is called basal metabolic rate and constitutes the largest part of daily energy expenditure. In addition, you use energy for physical activity and digestion.

After a meal, energy expenditure temporarily increases because the food needs to be digested and the nutrients absorbed and metabolized. This is called the thermic effect of food and usually accounts for around 10 percent of daily energy expenditure.

When you skip a meal, this increase in energy expenditure is absent.

Resting metabolism, on the other hand, remains approximately unchanged. During short-term fasting, levels of norepinephrine may increase. This hormone helps release stored energy and can provide a small, temporary increase in energy expenditure.

How much energy you use for digestion primarily depends on how much and what you eat. Three larger meals therefore require about the same amount of energy to digest as six smaller meals with equivalent content.

What does the body use for fuel between meals?

After a meal with carbohydrates, blood sugar rises, and the pancreas releases insulin.

Insulin helps cells take up glucose, which is either used as energy or stored as glycogen in the liver and muscles.

When several hours pass without food, insulin levels drop, while the hormone glucagon becomes more significant. Glucagon stimulates the liver to break down stored glycogen and release glucose into the blood. This helps keep blood sugar stable.

At the same time, the use of fat as an energy source increases. Fat cells release fatty acids through a process called lipolysis. The fatty acids are transported by the blood and can be used as energy in, among other places, the muscles.

During longer fasting periods, the liver's production of ketone bodies from fatty acids increases. These can also be used as energy by the brain, reducing the need for glucose gradually.

Does burning more fat mean you lose more fat?

The energy sources you use and how many calories you burn are two different things.

Even though a larger proportion of energy comes from fat when you go without food, it does not mean that the total energy expenditure increases.

To reduce fat stores over time, you must consume fewer calories than you burn. If you skip breakfast and thus consume fewer calories overall, it can contribute to weight loss.

However, if you eat correspondingly more later in the day, the period without food will not lead to greater fat loss.

When does metabolism actually slow down?

There is a difference between skipping certain meals and eating too little over a longer period.

With a prolonged calorie deficit, energy expenditure can gradually decrease. When you lose weight, you need less energy both at rest and during physical activity.

Energy expenditure can also decrease more than what weight loss alone would suggest. This is called adaptive thermogenesis and is associated with hormonal changes.

The level of leptin, which affects appetite and energy balance, can decrease. The same applies to the metabolism hormone T3, which helps regulate metabolism.

Also, NEAT (Non-Exercise Activity Thermogenesis) can be reduced. This is the energy you use for everyday activities outside of exercise, such as walking, standing, and moving throughout the day. With low energy intake, you may unconsciously become less active, which reduces total energy expenditure.

How much energy expenditure decreases varies from person to person and is influenced by factors such as the size of the calorie deficit, how long it lasts, and how much weight you lose.


Sources:

Does time-restricted eating add benefits to calorie restriction? A systematic review – [Wiley Online Library](https://onlinelibrary.wiley.com/doi/10.1002/oby.23984)

Effects of Intermittent Fasting on Cardiometabolic Health: An Energy Metabolism Perspective – [Nutrients](https://www.mdpi.com/2072-6643/14/3/489)