You're deep into a fast, it's 77°F outside, and you're reaching for a hoodie. Cold hands, icy feet. People often say it's because you've "run out of reserves." But your body fat didn't vanish in one morning. The explanation is somewhere else.
Your body turns down the thermostat
Your thyroid makes T3, the hormone that largely sets how fast your metabolism runs, and so how much heat you produce. When food gets scarce, the body cuts back on active T3. It's a pretty logical energy-saving move: less coming in, less going out.
A classic 1976 study (Spaulding and colleagues) measured this in people with obesity who fasted completely for 7 to 18 days. Their T3 dropped 53%, while the inactive form of the hormone (reverse T3) rose 58%. The most interesting part: on an 800-calorie diet with no carbs, T3 also fell, by about 47%. But when those 800 calories included at least 50 g of carbs, the numbers didn't move in any significant way.
And no, it's not a broken thyroid. A 2024 review in Frontiers in Endocrinology points out that during extended fasting, T3 drops without the brain hormones that drive the thyroid (TSH and TRH) rising to compensate. In other words, the body chooses to slow down. It's a controlled power-saving mode.
The extreme example is the Minnesota semi-starvation experiment in the 1940s. After months of severe restriction, participants handled cold very poorly. They piled on blankets in the middle of summer, and their hands ran about 2°C colder than those of the control group.
What about 16:8?
Here the effect is much smaller. In a 2016 trial (Moro and colleagues), 34 resistance-trained men did 16:8 for 8 weeks while eating the same calories as the control group. Their T3 dropped about 11%. But their TSH and resting energy expenditure stayed stable. So a real effect, but a modest one. Nothing like the drops seen after several days without food.
The other heat source you switch off
Digesting food produces heat. On a normal diet, digestion accounts for roughly 5 to 15% of the energy you burn each day (Westerterp, 2004). During your fasting window, that little space heater is off. That helps explain why you can feel a bit colder on an empty stomach in the morning than in the afternoon after a meal.
What we don't know yet
You'll often read that cold hands and feet while fasting come from reduced circulation. Truth is, it's not that clear. A study on a 72-hour fast (Webber and Macdonald, 1994) actually measured an increase in forearm blood flow. For short fasts, the exact reasons behind cold extremities are still poorly documented. We'd rather tell you that than make up an explanation.
In practice
Feeling a bit cold during a fast is usually your body saving energy, not an alarm bell. Move: your muscles are your best heater. Dress for it in the morning. If the cold really bothers you during longer fasts, keeping some carbs when you eat seems to limit the T3 drop, based on that 1976 study.
On the other hand, if you're cold all the time, even on days when you eat normally, along with fatigue or other symptoms, get your thyroid checked by your doctor. Fasting doesn't explain everything.
We started Fasted Rebels to explain what's going on in your body, no shortcuts. Feed the mission. Not the hunger.
This article is for information only and isn't a substitute for advice from a health professional.
Sources: Spaulding SW et al., "Effect of caloric restriction and dietary composition on serum T3 and reverse T3 in man," Journal of Clinical Endocrinology & Metabolism, 1976; Sui X et al., "The influence of extended fasting on thyroid hormone: local and differentiated regulatory mechanisms," Frontiers in Endocrinology, 2024; Keys A et al., The Biology of Human Starvation, 1950, as summarized in Smith LL, Frontiers in Behavioral Neuroscience, 2023; Moro T et al., "Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males," Journal of Translational Medicine, 2016; Westerterp KR, "Diet induced thermogenesis," Nutrition & Metabolism, 2004; Webber J, Macdonald IA, "Cardiovascular, metabolic and hormonal changes accompanying acute starvation in men and women," British Journal of Nutrition, 1994.
French version: Pourquoi tu gèles en plein jeûne
Also read, the fasting and electrolytes series: Why You're Cramping Mid-Fast · More Water While Fasted Isn't Always Safer · Why You Get Dizzy Standing Up Mid-Fast · Why Your Heart Pounds Mid-Fast · Why You Twitch and Sleep Like Garbage Mid-Fast · Why You Get a Headache Mid-Fast · Why Your Uric Acid Spikes Mid-Fast · Why Your Creatinine Spikes on a Blood Test · Do You Need Sugar to Get Creatine Into Your Muscles?