Does a Calorie Deficit Make You Tired? What Is Normal

Not necessarily. Does a calorie deficit make you tired? That depends on how deep the cut is and what else changes. Tiredness that lasts for weeks, or comes with weight loss you can’t explain, is a reason to see a GP. In the two-year CALERIE 2 trial, healthy adults on a prescribed 25% calorie cut reported better mood and, at 12 months, longer sleep.

Does a calorie deficit make you tired?

CALERIE 2 randomized 220 adults with a BMI of 22 to 28 to two years of 25% calorie restriction or their usual eating. The restricted group lost an average of 7.6 kg (17 lb) against 0.4 kg for the others. Compared with the control group, they reported improved mood, less tension and better general health at 24 months, and longer sleep at 12 months. The mood difference was small, 0.76 points on a 0–63 depression score where lower is better. Greater weight loss went with more vigor and less mood disturbance. The authors concluded that restriction had some positive effects and no negative effects on quality of life.

Don’t stretch that result too far. These were healthy adults on a structured research program, not a self-improvised diet, and 152 of the 218 analyzed were women. The trial measured mood, sleep and general health, not tiredness itself. It can’t rule out tiredness in one dieter, but it does show that a 25% cut doesn’t produce it by default. If you feel flat on a diet, check the causes in the next section first, because the cut alone rarely explains it.

What makes you more tired in a deficit?

These are the first things I check, and none of them takes more than a day.

Suspect	Check	What to change
Cut too deep	Is your intake under 1,200 kcal (women) or 1,500 kcal (men), or is your cut over 750 kcal, the top of the guideline’s options?	Eat back up to your floor, then cut in 100–200 kcal steps
Too little sleep	Do you get 6 to 9 hours?	Fix sleep before you cut further
Training on too little	Are you lifting or running hard on a small budget?	Cut the deficit, not the training
Something else	Tired for a few weeks, with no clear reason?	See a GP

A 1,200 kcal day is a 600 kcal cut for one person and a 1,200 kcal cut for another. A woman with the NHS average maintenance of 2,000 kcal who eats 1,200 kcal is cutting 800 kcal, which is more than the 500–750 kcal the AHA/ACC/TOS guideline offers.

Maintenance	Eating 1,200 kcal	Cut as a share of maintenance
1,800 kcal	cut of 600 kcal	33%
2,000 kcal	cut of 800 kcal	40%
2,400 kcal	cut of 1,200 kcal	50%

Half your usual food is not a moderate deficit, and a floor-level intake is the wrong starting point if your maintenance is 2,400 kcal. If the arithmetic behind a deficit is new to you, what a calorie deficit is covers it from scratch, and starting a calorie deficit as a beginner takes a much smaller cut than 1,200 kcal.

To find your own maintenance, enter age, height, weight and activity in the calorie calculator. Its NASEM 2023 equations never set a target under 1,200 kcal for women or 1,500 kcal for men.

Sleep is the one I’d fix before touching calories. The NHS says short sleep may make you more likely to choose high-calorie foods and too tired to exercise or cook. A short list of calorie deficit foods that fill you up makes those days easier.

The International Olympic Committee’s 2023 consensus links Relative Energy Deficiency in Sport to low energy availability: too little food for the energy spent. If you train hard and feel flat for weeks, eat more, then see a doctor.

Iron deficiency anaemia, diabetes and an overactive thyroid are other causes of tiredness on the NHS list. Blood tests can check for them, so don’t guess at the cause yourself.

Does a calorie deficit cause headaches?

It can, mostly through long gaps without food. Fasting headache is a recognized type. In most cases it begins after at least 16 hours without eating and resolves within 72 hours after you eat again. A Danish population study found a lifetime prevalence of 4.1%. The review says low blood sugar and caffeine withdrawal have been especially implicated.

Irregular meals and too little fluid add to it. The NHS lists not eating regular meals and not drinking enough fluids among common causes of headache, and tells you not to skip meals. A period can also trigger one. A fixed morning meal helps, and calorie deficit breakfast ideas lists some with the grams.

Dropping coffee can do it too.

How much caffeine are you cutting?

If your deficit includes dropping coffee or energy drinks, your headache may be caffeine rather than calories. A critical review of caffeine withdrawal found that headache occurred in 50% of people in experimental studies. Symptoms typically started 12 to 24 hours after stopping, peaked between 20 and 51 hours and lasted 2 to 9 days. Even stopping doses as low as 100 mg a day produced symptoms.

How big is 100 mg? I worked it out from USDA records, using the weight of the serving and mg of caffeine per 100 g:

Drink	Serving	Caffeine
Brewed coffee	1 cup, 237 g	95 mg
Espresso	1 fl oz, 29.6 g	63 mg
Red Bull	1 can, 258 g	75 mg
Brewed black tea	1 cup, 237 g	47 mg

One cup of brewed coffee, at about 95 mg, is right at that line. The sum for the coffee is 40 mg per 100 g × 237 g = 95 mg. Someone who goes from three cups to none loses around 285 mg overnight, far past the point where the review saw symptoms.

My rule is to change one thing at a time. Keep your caffeine steady for the first two weeks of a deficit. If you want less of it after that, step down slowly: from three cups (285 mg) to two (190 mg) for a week, then to one (95 mg). Never cut calories and caffeine on the same day. Change both on Monday and by Wednesday you can’t tell which one gave you the headache. Swapping a cup of coffee (95 mg) for a cup of black tea (47 mg) halves the dose without dropping it to zero.

When is tiredness or a headache not normal?

When it lasts, gets worse or arrives with other symptoms. NHS advice is to see a GP if you’ve been tired for a few weeks and aren’t sure why, or if you’re tired and have other symptoms such as weight loss or mood changes. For headaches, see a GP if self-care doesn’t help or you get them regularly. Call 999 or go to A&E for a headache that started suddenly and is extremely painful, or comes with numbness, weakness, trouble speaking or loss of vision.

A doctor or midwife sets the plan if you are pregnant, breastfeeding, under 18 or living with a medical condition. For everyone else, the first moves are to eat back up to your floor and keep caffeine steady for a fortnight.

If counting calories has started to feel hard to stop, or eating feels out of control, talk to someone: ANAD in the US (1-888-375-7767), Beat in England (0808 801 0677), NEDIC in Canada (1-866-633-4220) or the Butterfly Foundation in Australia (1800 33 4673). If you’re in danger right now, call your local emergency number. Other UK numbers and crisis lines are on our crisis support page.