In short: most women who cannot gain weight are eating less than they think, or burning more than they realise. A smaller number have a medical reason, and those reasons are worth checking. Start by tracking honestly for a week, then work through the checklist below.
- Under-eating without realising it is the most common reason. Small portions, skipped meals, and light days that feel like big days add up to far fewer calories than you think, and the only way to know is to track everything for a week.
- Low appetite is the second most common reason. Stress, poor sleep, and very filling low-calorie foods can leave you full on too few calories, even when you are eating three meals a day.
- An overactive thyroid, digestion problems, and some medicines can all make weight gain hard. An overactive thyroid affects about 1 in 100 people in the UK, and up to 90% of people diagnosed with it report weight loss.
- A very active lifestyle burns more than most people count. Walking all day, a physical job, and daily workouts can add hundreds of calories to your daily burn without feeling like exercise.
- See a doctor promptly if you are losing weight without trying, or if weight loss comes with a racing heartbeat, constant tiredness, diarrhoea, or feeling hot all the time. Those are red flags, not things to push through.
Introduction
You eat three meals a day, you snack, you have tried adding extra food, and the scale still will not move.
This post walks through the nine most common reasons women struggle to gain weight, with a simple checklist you can work through yourself, and a clear list of the red flags that mean you should see a doctor rather than keep trying on your own.
First, what counts as underweight. The World Health Organization classes a body mass index under 18.5 as underweight, but that is a screening number, not a verdict: some people are naturally slim and perfectly healthy at a BMI of 18. What matters more is the direction. If your weight is steady and you feel well, you may simply be naturally slim. If your weight keeps falling, or you feel unwell, that is worth looking into. Our ideal weight calculator gives you a healthy range for your height.
The 9 Reasons
1. You are eating less than you think
This is the most common reason, and the hardest to accept, because it feels wrong. Studies keep finding that people under-report what they eat, often by a large amount. The fix is boring and it works: track everything for seven days, weighing food where you can and including drinks, snacks, and cooking oil. Use the calories calculator to find your daily target, then compare it with what you actually ate. Many women find they are eating 300 to 500 calories less a day than they thought.
2. Low appetite
Some people simply do not feel hungry very often, and it is easy to mistake a quiet appetite for a small need. If you rarely feel hungry, you will eat less at every meal without deciding to. Low appetite is common when you are stressed, sleeping badly, unwell, or taking some medicines. The foods you choose matter too: a big salad, a bowl of soup, and a glass of water before a meal will fill your stomach on very few calories, leaving no room for the food that would have added the calories you need. If your appetite is low, eat little and often, and keep nuts, nut butter, cheese, and olive oil in easy reach.
3. Stress and anxiety
Stress changes appetite in both directions, and for many women it switches hunger off. When you are anxious, your body releases stress hormones that suppress appetite and speed up digestion, so food goes through faster and you feel full sooner. Set meal times, keep easy food ready, and eat with other people when you can, because most people eat more in company than alone.
4. Digestion problems
If your gut is not absorbing food properly, you can eat enough calories and still not gain weight, because some of them never reach your body. Coeliac disease, where gluten damages the lining of the small intestine, affects about 1 in 100 people and is a well-known cause of unexplained weight loss. Other gut problems can do the same thing. If you have ongoing stomach symptoms alongside weight that will not rise, take that combination to a doctor, because gut problems are diagnosable and treatable, and no amount of extra food fixes a gut that is not absorbing it.
5. An overactive thyroid
Your thyroid is a small gland in your neck that sets the speed of your metabolism. When it is overactive, a condition called hyperthyroidism, your body burns calories faster than it should, and weight loss is one of the most common symptoms. An overactive thyroid affects about 1 in 100 people in the UK and about 1% of people in the United States, and it is far more common in women than in men. The most common cause is Graves disease, which accounts for about 4 in 5 cases. The British Thyroid Foundation reports that up to 90% of people diagnosed with hyperthyroidism report weight loss, often despite eating normally or more. Other symptoms include a racing heartbeat, feeling hot, shaky hands, and poor sleep. A simple blood test can check it.
6. A very active lifestyle
Exercise is the obvious burner, but the bigger one is often everything else: walking all day, a physical job, chasing children, and fidgeting. All of that adds up to hundreds of calories a day, and none of it feels like a workout, so it never gets counted. A woman on her feet all day can easily burn 500 calories a day more than a desk worker. Count the movement honestly: our TDEE calculator lets you set your activity level and shows your daily burn, and the difference between lightly active and very active can be 500 calories or more.
7. Too much filling food, not enough calories
This is the quiet trap for women who eat healthily. Vegetables, salads, soups, fruit, and whole grains are wonderful foods, and they are also very low in calories for their volume. A plate piled high with salad and vegetables can look like a feast and contain fewer than 300 calories. If every meal looks like that, you can feel full all day and still be in a calorie deficit. The answer is not to stop eating vegetables. It is to add calorie-dense foods alongside them: a drizzle of olive oil on the salad, a handful of nuts with the fruit, cheese or avocado with the toast, full-fat yoghurt instead of fat-free. These additions barely change the volume of the meal, but they can add 300 to 500 calories across the day.
8. Poor sleep
Sleep and appetite are tightly linked. Short or broken sleep raises the hunger hormone ghrelin and lowers the fullness hormone leptin, which sounds like it should make you eat more, and for many people it does. But poor sleep also raises stress hormones, disrupts your routine, and leaves you too tired to cook or eat properly, and for many women the net effect is eating less, not more. Aim for 7 to 9 hours, keep a regular bedtime, and avoid large gaps between meals.
9. Medicines and other medical conditions
Some medicines suppress appetite or speed up metabolism, and weight loss is a listed side effect of several common ones, including some medicines for attention deficit hyperactivity disorder, some antidepressants, and some thyroid medicines when the dose is too high. If your weight started falling after you began a new medicine, tell the doctor who prescribed it rather than stopping the medicine yourself. Type 1 diabetes can also cause unexplained weight loss before diagnosis. None of these are things to diagnose yourself. They are reasons to see a doctor.
The Diagnostic Checklist
Work through this list in order. It takes a week, and it answers the question properly.
- Track for seven days. Write down everything, including drinks, snacks, and cooking oil. Compare your average with your target from the calories calculator. If you are eating less than your target, you have found the answer.
- Check your activity honestly. Count your steps for a week and put the real activity level into the TDEE calculator.
- Look at your appetite and your gut. Are you skipping meals because you are not hungry? Do you feel full after a few bites? Do you have bloating, diarrhoea, or stomach pain?
- Check the red flags below. If any apply, see a doctor now. If none apply and your tracking shows a genuine calorie gap, work on closing it for a month.
- Weigh yourself properly. Same scale, same time of day, once or twice a week. A slow gain of about a quarter to half a kilo a week is a sensible pace.
What to Do About It
If your checklist points to a calorie gap rather than a medical problem, the fix is straightforward: eat a little more than you burn, every day, and give your body a reason to build muscle with the extra calories. Aim for a surplus of about 300 to 500 calories a day. Protein matters, because you want the gain to be muscle: the site’s protein calculator uses 1.2 to 1.6 g per kg a day, and strength training two or three times a week gives that protein somewhere to go. Our protein intake calculator gives you a personal target.
Eat little and often if big meals defeat you: three meals plus two or three snacks is easier than three huge meals. Give it a month of honest tracking before you judge it. If a month of a genuine surplus produces no gain at all, take that to a doctor.
Red Flags That Need a Doctor
See a doctor promptly if any of these apply to you. They are not things to solve with extra snacks.
- You are losing weight without trying, or your weight keeps falling even though you are eating normally.
- Weight loss comes with a racing or irregular heartbeat, shaky hands, feeling hot all the time, or difficulty sleeping.
- You have ongoing diarrhoea, bloating, stomach pain, or blood in your stool.
- You feel very tired all the time, or you have lost your appetite completely.
- You have a new lump in your neck, or your eyes look more prominent than they used to.
- You are vomiting, or you cannot keep food down.
- You have a history of an eating disorder, or your thoughts about food and weight are causing you distress.
A doctor can test your thyroid, check for coeliac disease and diabetes, and review your medicines.
Frequently Asked Questions
Why can I not gain weight even though I eat a lot?
The most common reason is that you are eating less than you think. Portions are easy to overestimate, small meals are easy to forget, and light days cancel out big days. Track everything you eat for seven days, weigh food where you can, and compare your average with your target from a calories calculator. Many women find a gap of 300 to 500 calories a day, which is exactly enough to keep the scale still.
Can stress stop you from gaining weight?
Yes. Stress suppresses appetite in many people, disrupts sleep, and raises the hormones that speed up digestion, so food goes through faster and you feel full sooner. Stress also makes meals easy to skip. If stress is the main problem, fixing the routine around meals, regular times, easy food ready, eating with others, matters as much as the food itself.
Can an overactive thyroid stop you gaining weight?
Yes. An overactive thyroid speeds up your metabolism, so you burn calories faster than you should. It affects about 1 in 100 people in the UK, it is far more common in women, and up to 90% of people diagnosed with it report weight loss, often while eating normally or more than usual. A racing heartbeat, shaky hands, feeling hot, and difficulty sleeping are other common symptoms. A simple blood test can check it, so ask your doctor if those symptoms sound familiar.
How many calories should a woman eat to gain weight?
It depends on your size and activity, but a useful starting point is your maintenance calories plus about 300 to 500 a day. Use the calories calculator on this site to find your maintenance level, then add the surplus. A very active woman may need far more than a desk worker of the same weight, so set your activity level honestly. A slow gain of about a quarter to half a kilo a week is a sensible pace.
When should I see a doctor about not gaining weight?
See a doctor promptly if you are losing weight without trying, if weight loss comes with a racing heartbeat, constant tiredness, diarrhoea, or feeling hot all the time, or if you have ongoing stomach symptoms. Also see a doctor if a month of eating a genuine calorie surplus produces no gain at all. A doctor can test your thyroid, check for coeliac disease and diabetes, and review your medicines.
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Sources
- National Health Service (NHS). “Overactive thyroid (hyperthyroidism).” An overactive thyroid affects about 1 in 100 people in the UK, and unexplained weight loss is a key symptom. No stable public link was available, so no link is given rather than a dead one.
- Merck Manual, Consumer Version. “Hyperthyroidism.” Hyperthyroidism affects about 1% of people in the United States. https://www.merckmanuals.com/home/hormonal-and-metabolic-disorders/thyroid-gland-disorders/hyperthyroidism
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). “Graves’ Disease.” Graves disease is the most common cause of hyperthyroidism and accounts for about 4 in 5 cases. No stable public link was available, so no link is given rather than a dead one.
- British Thyroid Foundation. “Hyperthyroidism and weight changes.” Up to 90% of people diagnosed with hyperthyroidism report weight loss. No stable public link was available, so no link is given rather than a dead one.
- National Health Service (NHS). “Coeliac disease.” Coeliac disease affects about 1 in 100 people. No stable public link was available, so no link is given rather than a dead one.
- World Health Organization. Body mass index under 18.5 is classed as underweight. No stable public link was available, so no link is given rather than a dead one.
Medical Disclaimer
This article is general information only. It is not medical advice, it does not diagnose any condition, and it is not a substitute for advice from a qualified health professional. If you are losing weight without trying, or you have any red flag symptoms, see your doctor. Talk to your doctor, or a registered dietitian, before changing your diet, especially if you have a medical condition, are taking medication, are pregnant or breastfeeding, or have a history of disordered eating.