Fasting Tracker

Quick answers

Feeling cold while fasting: is it normal?

Common on longer fasts, little studied, and sometimes a sign to eat more

What the research shows, and what it doesn't

We couldn't find a trial that measured how cold people feel on a daily plan such as 16:8. What exists is a few small studies of longer fasts and of eating less over months. They point in the same direction, but they're a long way from proof for an everyday fast.

Key study

A 48-hour fast changed how the body handled cold

  • Laboratory study
  • Small groups of young women
  • 48 hours or 7 days

Healthy young women either fasted for 48 hours or ate far less than usual for a week, then wore a cooling suit. After the 48-hour fast, their bodies no longer made extra heat in response to the cold, and their temperature fell instead of rising. A week of underfeeding didn't have this effect.

Mansell et al., Clin Sci 1989

Over longer periods, eating less seems to turn the thermostat down a little. In a 6-month trial, adults with overweight who cut their calories by a quarter had a lower core body temperature at the end, and burned a little less energy than their weight loss alone would explain 2.

MYTH
Feeling cold on a fast means your metabolism has crashed.
FACT
In the first days of a fast, resting metabolism actually rose slightly, by about 14% by day 3, along with noradrenaline 3. Feeling cold isn't a sign of a slowed metabolism on a short fast.

So where does the feeling come from? Nobody has pinned it down. Fasting Tracker's stage notes say that feeling weak and cold is common at 48 to 72 hours, and our guide to extended fasts lists feeling cold among the common complaints of a longer fast. Dehydration and tiredness can also make you feel chillier.

What helps

Staying warm while you fast

  1. Have a warm drink: herbal tea, plain tea or black coffee keep you fasting.
  2. Dress in layers and keep your hands and feet warm.
  3. Move: a short walk warms you up.
  4. Drink enough water through the day 4.
  5. Make sure your eating window gives you enough food, with protein at every meal 5.
  6. If you're cold on longer fasts, keep to shorter ones.

When cold means eating too little

A daily fasting window with enough food in it is very different from eating too little. If you feel cold most of the time, especially with some of the signs below, your body may be short of energy. That's a reason to eat more, not to push on. Our weight goal calculator can help you choose a gentle pace.

  • You're tired all the time, or feel weak or dizzy.
  • You're losing weight quickly, or are already underweight.
  • Your periods become irregular or stop.
  • Your hair is shedding more than usual.
  • You're thinking about food constantly, or feel anxious about eating.

When to see a doctor

See a doctor if you feel cold all the time even when you eat normally, if it comes with tiredness, weight changes, hair loss or missed periods, or if your fingers or toes turn white or blue in the cold. A simple check-up and blood test can look for common causes, such as your thyroid or iron levels.

Sources

  1. Mansell PI, Macdonald IA. Effects of underfeeding and of starvation on thermoregulatory responses to cooling in women. Clin Sci (Lond). 1989;77(3):245-52. doi.org/10.1042/cs0770245
  2. Heilbronn LK, de Jonge L, Frisard MI, et al. Effect of 6-month calorie restriction on biomarkers of longevity, metabolic adaptation, and oxidative stress in overweight individuals: a randomized controlled trial. JAMA. 2006;295(13):1539-48. doi.org/10.1001/jama.295.13.1539
  3. Zauner C, Schneeweiss B, Kranz A, et al. Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. Am J Clin Nutr. 2000;71(6):1511-5. doi.org/10.1093/ajcn/71.6.1511
  4. Armstrong LE, Ganio MS, Casa DJ, et al. Mild dehydration affects mood in healthy young women. J Nutr. 2012;142(2):382-8. doi.org/10.3945/jn.111.142000
  5. Leidy HJ, Clifton PM, Astrup A, et al. The role of protein in weight loss and maintenance. Am J Clin Nutr. 2015;101(6):1320S-1329S. doi.org/10.3945/ajcn.114.084038
  6. Grajower MM, Horne BD. Clinical Management of Intermittent Fasting in Patients with Diabetes Mellitus. Nutrients. 2019;11(4):873. doi.org/10.3390/nu11040873

A note on health

Fasting Tracker is designed for general wellness and is not a medical device. Fasting is not right for everyone. If you are pregnant, diabetic, have a history of eating disorders or take medication, talk to your doctor first, and before any extended fast.