Have you ever noticed that your hands and feet feel like ice when you’re fasting, even when everyone else seems perfectly comfortable? You’re not imagining it. Some people become noticeably more sensitive to cold when they go without food, particularly during longer fasts. 

Several physiological changes may explain why fasting can make you shiver. Let’s look at what’s happening inside your body and what you can do about it. 

Your body may be producing less heat

That seems obvious, right? But here’s why. Your body uses energy for everything it does, from breathing and circulating blood to digesting food. Some of that energy is released as heat. 

When you eat, your body uses energy to break down food, absorb nutrients, and process them. This is called the thermic effect of food. It is a normal part of daily energy use, and it means eating a meal temporarily increases the energy your body uses. 

When you’re fasting, you’re not digesting new food, so this source of heat production decreases. 

Your body may also adjust its energy use when  you eat less over time. Research on calorie restriction has found changes in energy expenditure and metabolism, although the size of these changes varies from person to person. 

In simple terms, your body may be producing less heat than it would after a meal, which can leave you feeling chilly. 

Lower insulin and changes in circulation

Insulin is a hormone that helps your body manage blood sugar. After you eat, insulin rises to help move glucose into cells and store energy. During fasting, insulin levels generally fall, allowing your body to release stored energy.

This shift is an important part of the fasting process, and it can affect circulation. Insulin acts as a mild vasodilator, meaning that an insulin response to food will push blood flow closer to the skin. Without that happening, you can feel colder than normal. 

Changing fuel sources

Your body doesn’t rely on just one fuel source. It can use glucose from carbohydrates, stored glycogen, fatty acids, and ketones to meet its energy needs. 

When you begin fasting, your liver releases glucose from stored glycogen to help maintain blood sugar. As the fast continues, these stores decline, and your body increasingly relies on stored fat and produces more ketones. 

This is a normal metabolic adjustment. A review published in The New England Journal of Medicine explains how fasting changes the body’s use of glucose, fatty acids, and ketones. 

So, where does feeling cold fit in?

It’s important to note that this adjustment doesn’t always automatically make you feel cold. These changes in fuel use happen alongside other adjustments in energy and hormone activity. They may contribute to how you feel during a fast, but there is no simple rule that says switching to fat for fuel will make you cold. 

Thyroid hormone and your internal thermostat

This is a big one.

Your thyroid helps regulate metabolism, energy use, and body temperature. One thyroid hormone, called triiodothyronine (T3), plays an important role in how the body uses energy and produces heat. 

Research has found that prolonged fasting and calorie restriction can lower T3 levels in some people. This may be one way the body adapts to having less energy available. So, a lower T3 level does not automatically mean you have a thyroid disorder. It can just be part of the body’s response to reduced energy intake. 

That being said, feeling cold all the time isn’t something you should just simply blame on fasting. An underactive thyroid can also cause cold intolerance, fatigue, constipation, and other symptoms. 

If you regularly feel unusually cold, especially outside your fasting window, consider discussing it with your healthcare provider. 

Does feeling cold mean you’re burning fat?

You’ve probably seen the claim that feeling cold during a fast is a sign that your body has switched into fat-burning mode. While it can be a good sign that the body is doing what it’s supposed to do, that doesn’t mean feeling cold always equals burning fat. And it doesn’t mean that if you don’t feel cold you’re doing something wrong. 

Your body can burn fat without making you feel cold. Likewise, you can be chilly for reasons that have little do with fat use, such as a cool room, inadequate food intake when not fasting, anemia, or a thyroid problem. 

Feeling cold doesn’t really prove that you’re in ketosis either, although the two can go hand in hand sometimes. Ketosis can be measured through ketone levels, but temperature alone can’t tell you whether you’re there. 

Rather than treating coldness as a sign of success, pay attention to how you feel overall and whether your fasting routine is sustainable and giving you the desired results. 

Tips to help warm up

If you feel a little chilly during a fast, a few simple steps may help.

• Dress in layers. Warm socks, slippers, and an extra sweater can make a difference.

• Try a warm drink. Unsweetened tea or warm water can help you feel more comfortable. 

• Move around. Some light movement can help generate heat.

• Review your eating habits. If you’re consistently eating too little during your eating window, you may not be getting enough energy to support your needs.

• Consider your fasting schedule. If you regularly feel miserable, weak, or unusually cold, a shorter fasting window may be more appropriate. 

If coldness comes with other symptoms, such as confusion, weakness, or fainting, stop fasting and seek medical attention. 

Bottom line

Everyone responds differently to fasting. Some may find themselves shivering and hiding under blankets,  while others feel no difference at all. The important thing is to know that it can happen, but to also pay attention to your body and recognize when feeling cold could mean something else. 

Research Sources:

Calcagno M, Kahleova H, Alwarith J, Burgess NN, Flores RA, Busta ML, Barnard ND. The Thermic Effect of Food: A Review. J Am Coll Nutr. 2019 Aug;38(6):547-551. doi: 10.1080/07315724.2018.1552544. Epub 2019 Apr 25. PMID: 31021710.

de Cabo, R., & Mattson, M. P. (2019). Effects of intermittent fasting on health, aging, and disease. The New England Journal of Medicine, 381(26), 2541–2551. https://doi.org/10.1056/NEJMra1905136

Redman LM, Ravussin E. Caloric restriction in humans: impact on physiological, psychological, and behavioral outcomes. Antioxid Redox Signal. 2011 Jan 15;14(2):275-87. doi: 10.1089/ars.2010.3253. Epub 2010 Aug 28. PMID: 20518700; PMCID: PMC3014770.