Why Am I Tired and Weak in a Calorie Deficit?

16 October 20265 min read

A woman with resting her head on a chair. — Photo by Zohre Nemati on Unsplash.

Yes, a calorie deficit can make you tired. Feeling tired or weak in a calorie deficit often comes down to one of three things: your deficit is too big, you’re short on sleep, or you’re training hard on too little food. A little tiredness at the start can be normal. Being tired all the time is a sign something needs adjusting, and it’s often an easy fix. If it lasts a few weeks and you don’t know why, see a GP.

In brief

A moderate deficit shouldn't wear you out. In a 2-year trial of 218 healthy adults, those who ate less lost about 7.6 kg. Their quality of life didn't suffer.
The NHS suggests losing 0.5 to 1 kg a week. Losing faster than that for weeks can mean you're eating too little.
Sleep matters while you diet. In a 2-week study of 10 adults, people lost less fat and more muscle and other lean tissue when they slept 5.5 hours than when they slept 8.5 hours.
The NHS says to see a GP if you've been tired for a few weeks and don't know why.

Is it normal to feel tired in a calorie deficit?

Some tiredness can happen, but feeling exhausted most days isn’t something you have to put up with. A sensible deficit shouldn’t leave you drained.

A large trial called CALERIE 2 tested this over two years. It followed 218 healthy adults at a healthy weight or slightly above. Those who aimed to eat 25% less, with close support from researchers, lost about 7.6 kg on average. Compared with people who ate as normal, they slept a little longer at one year and their mood was better at two years. The researchers found no negative effects on quality of life.

So if you’re tired all the time, look for a reason rather than pushing through.

What causes tiredness in a calorie deficit?

There are a few usual causes, and you might have more than one. Use the table to spot yours.

Possible cause	Signs	What to try
Your deficit is too big	Losing over 1 kg a week for weeks, hungry all day	Eat a little more each day and check again in a few weeks
Not enough sleep	Often under 6 hours, or waking up tired	Aim for 6 to 9 hours, at the same times each night
Training hard on too little food	Workouts feel harder than usual, slow recovery	Eat more on the days you train
Something unrelated to your diet	Tired for weeks, plus other symptoms	See a GP

These draw on the NHS pages and studies listed at the end.

If you exercise a lot, look at your training too. Sports doctors call eating too little for the exercise you do Relative Energy Deficiency in Sport, or REDs. A 2023 consensus statement from the International Olympic Committee says it can harm both health and performance in male and female athletes. We explain how much of your exercise to eat back in should you eat back your exercise calories.

Is your calorie deficit too big?

It might be if you’ve lost more than about 1 kg a week for a month or more. That’s more likely if you’re also tired and hungry all day. The NHS suggests aiming for 0.5 to 1 kg a week.

Give it a few weeks before you judge. The first week or two of eating less often includes some water weight, so early losses can look bigger than they are.

Say you’ve been losing about 1.5 kg a week for a month and you’re exhausted by mid-afternoon. Adding a small meal or a couple of snacks each day may bring your energy back while you keep losing weight, just more slowly. To see what a gentler daily target looks like, try our calorie deficit calculator. One-size targets like 1,200 calories are too low for many people.

Losing weight very fast can also mean losing more muscle, which is another reason to slow down.

If you have, or have had, an eating disorder, talk to a GP before cutting calories.

How do you feel less tired in a calorie deficit?

Check your pace. Look at your weekly average weight, not single days, and give any change two or three weeks before you judge it.
Protect your sleep. The NHS suggests 6 to 9 hours, going to bed at the same time, and avoiding screens in the hour before bed.
Eat more on training days, especially if you do a lot of exercise.
Eat regular meals that fill you up. We cover what helps in why am I so hungry in a calorie deficit.

When should you see a GP?

The NHS says to see a GP if:

you’ve been feeling tired for a few weeks and you’re not sure why
your tiredness affects your daily life
you feel tired and have other symptoms, such as weight loss you can’t explain or mood changes
you’ve been told you make gasping, snorting or choking noises when you’re asleep

Tiredness or fatigue can have causes that have nothing to do with your diet. The NHS lists examples like iron deficiency anaemia, diabetes and an overactive thyroid, which a GP can check for with a blood test. Heavy periods are one of the most common causes of iron deficiency anaemia. Its symptoms include tiredness, shortness of breath and paler skin than usual.

If you’re eating enough to maintain or gain weight and you’re still tired all the time, eating less probably isn’t the cause, unless you train a lot. That’s a good reason to see a GP.

If eating less has started to feel hard to control, or you think you may have an eating disorder, the NHS says to see a GP as soon as you can.

Where Ori fits

Ori checks your progress each week and adjusts your calorie target (this is part of Premium). If you’re losing more than 1 kg a week, it lets you know, so a deficit that’s too big doesn’t go unnoticed. If that sounds helpful, you can try Ori here.

Sources

A gentle safety note

Ori supports general wellness. It is not medical care. If you have a history of disordered eating, are pregnant or breastfeeding, take medicines that affect weight or appetite, or have a clinical nutrition need, speak with a qualified healthcare professional before changing your intake or weight goal.

Read Ori's Health Disclaimer