Is Metabolic Damage Real? What Happens When You Diet for Too Long

Written by Robbie Clark, BExSc (Nutrition), M (Nutr+Diet), Accredited Practising Dietitian (APD) and Exercise Scientist · Last updated 28 October 2026 · 9 references

If you've been dieting for a long time and progress has stalled, you may have heard of "metabolic damage" or "starvation mode". As a dietitian, I hear this worry a lot. Here's what's really going on, and what you can do about it.

What is metabolic damage?

"Metabolic damage" and "starvation mode" are popular names for something scientists call adaptive thermogenesis. When you eat less than you use for a while, your body uses a little less energy at rest, on top of what you'd expect from being smaller. It's a normal survival response, not a sign that something is broken.

Your body also pushes back in other ways. In one study, a year after adults lost about 13 kg, their hunger hormones were still changed in ways that encourage eating, and they still felt hungrier than before they started.² This, more than a slower metabolism, is why keeping weight off is hard.

Is starvation mode real?

Partly. Your metabolism does slow a little, but not enough to stop weight loss or cause weight gain while you're still eating less than you need.

In a study of adults on a very low-energy diet, resting energy use fell by about 400 kJ (about 90 calories) a day more than expected while they were losing weight. Once their weight had been stable for four weeks, that gap roughly halved, and it didn't predict who regained weight a year later.¹ For comparison, that's about the energy in a banana.

If weight loss has stalled, it's usually because eating has drifted back up or activity has dropped, not because your metabolism has stopped working. See why weight loss stops and how to break a plateau.

What happens if you eat too little for too long?

Where real harm can happen is when you eat too little for the amount of exercise you do, over weeks or months. This is called low energy availability, and it can lead to Relative Energy Deficiency in Sport (REDs). It affects men and women, and you don't have to be an elite athlete.^5,6

Possible signs include:

• Irregular or missed periods
• Low sex drive
• Ongoing tiredness and poorer performance
• Frequent injuries, including bone stress injuries
• Getting sick often
• Mood changes or unintended weight loss

These signs can have many other causes, so don't self-diagnose. Treatment usually means eating more, sometimes with less training, and is best managed with your practitioner.⁶

How do you protect your metabolism while dieting?

• Lift weights. In a trial of women losing weight, those who did resistance training kept their muscle and resting energy use, while those who did aerobic training or no exercise lost some of both.³ Aim for muscle-strengthening activity on at least two days a week.⁸
• Eat enough protein. Protein helps keep you full and uses slightly more energy to digest than carbohydrate or fat.
• Don't cut too hard. A moderate deficit is easier to stick to and less likely to cost you muscle than a very low-energy diet.
• Consider a diet break. In a trial in men with obesity, alternating two weeks of dieting with two weeks of eating at maintenance led to more fat loss than dieting continuously.⁴ Diet breaks may help some people, ideally planned with your practitioner.
• Protect your sleep and recovery. Most adults need 7–9 hours of sleep, and rest days matter if you train hard.⁹

How do you recover after long-term dieting?

If you've been dieting for a long time and feel flat, hungry and stuck, a planned period of eating at maintenance can help you reset. Keep strength training, ease back on very long or very intense cardio if you're training most days, and focus on regular balanced meals. A practitioner can help you work out how much you need to eat and when it's sensible to try losing weight again.

Frequently asked questions

Can dieting permanently damage your metabolism?

There's no good evidence that it does. Resting energy use falls a little while you're dieting, and the gap shrinks once your weight is stable.¹

Can you gain weight from eating too little?

Not while you're still eating less than you use. Very low-energy diets do make you hungrier, which often leads to regain once the diet ends.²

How do I know if I'm eating too little?

Ongoing tiredness, missed periods, frequent injuries or illness and poorer performance can be signs. See your practitioner if these sound familiar.⁶

Do diet breaks work?

One trial in men found that two-week breaks at maintenance led to more fat loss than continuous dieting, but more research is needed.⁴

What's the best exercise to protect your metabolism?

Resistance training, because it helps you keep muscle and resting energy use while you lose weight.³

References

Research (PubMed)

1. Martins C, Roekenes J, Salamati S, Gower BA, Hunter GR. Metabolic adaptation is an illusion, only present when participants are in negative energy balance. Am J Clin Nutr. 2020;112(5):1212–1218. PubMed 32844188
2. Sumithran P, Prendergast LA, Delbridge E, et al. Long-term persistence of hormonal adaptations to weight loss. N Engl J Med. 2011;365(17):1597–1604. PubMed 22029981
3. Hunter GR, Byrne NM, Sirikul B, et al. Resistance training conserves fat-free mass and resting energy expenditure following weight loss. Obesity. 2008;16(5):1045–1051. PubMed 18356845
4. Byrne NM, Sainsbury A, King NA, Hills AP, Wood RE. Intermittent energy restriction improves weight loss efficiency in obese men: the MATADOR study. Int J Obes. 2018;42(2):129–138. PubMed 28925405
5. Mountjoy M, Ackerman KE, Bailey DM, et al. 2023 International Olympic Committee's (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). Br J Sports Med. 2023;57(17):1073–1097. PubMed 37752011

Australian guidelines

6. Australian Institute of Sport. Energy availability. View source
7. Butterfly Foundation. Helpline. View source
8. Australian Government Department of Health. Physical activity and exercise guidelines for adults. View source
9. Sleep Health Foundation. How much sleep do you really need? View source

This article is general information, not medical advice. Speak with your practitioner about your own situation before changing your diet or training.