What’s the best diet to lose weight?
The one you can stick to. In trials, people on low-carb and low-fat diets lost about the same weight over a year. Want it fast? One NHS number tells you how fast is safe.
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What is the best diet to lose weight?
Any diet that has you eating fewer calories than you use. NICE says you can do that with a low-fat or low-carb diet, or another way of cutting calories. So pick one you can keep up.
A review looked at 48 trials in 7,286 adults with a BMI (a weight-for-height score) of 25 or more. It found the gaps between named diets were small. Its authors back any diet a person will stick to.
| Diet | What you do | What the evidence says |
|---|---|---|
| Low-fat | Eat less fat, which has more calories, gram for gram, than any other nutrient. | About 7.27 kg more lost than with no diet after a year (review of 48 trials). |
| Low-carb | Eat under 130 g of carbohydrate (carbs, such as bread, rice, pasta and sugar) a day. | About 7.25 kg more lost than with no diet after a year (same review). |
| Mediterranean | Mostly plant foods and healthier fats, with little or no red meat or processed food. | An NHS London guide for people with heart or diabetes risks says it can help reduce weight. |
| 5:2 | Eat healthily 5 days a week. On 2 days, not in a row, have a quarter of the calories your body needs. | NICE makes no recommendation yet and has asked for more research. |
| Very low calorie | Under 800 calories a day, from food, meal-replacement drinks or both. | NICE: only in specialist services, for no more than 12 weeks. |
Sources: Johnston 2014; NHS London; NICE NG246
Whichever you pick, keep it balanced. NICE advises against strict diets that leave out nutrients your body needs, because they don’t work long term and can be harmful.
Looking for the best diet for women? No guideline we checked gives women a different one. Women do use fewer calories: around 2,000 a day on average, against 2,500 for men.
Going through the menopause, or the years leading up to it? The NHS says you may find it harder to stay a healthy weight. See menopause weight gain.
Low carb or low fat: which works better?
Neither, in the trials. In the DIETFITS trial, 609 adults were put on a healthy low-fat or a healthy low-carb diet for a year.
Source: Gardner 2018 (DIETFITS)
They were aged 18 to 50, without diabetes, with a BMI of 28 to 40, and 57% were women. Neither a gene test nor how much insulin they made predicted which diet worked better. Insulin is the hormone that controls blood sugar.
A diet is like a gym membership. The best one is the one you use. So ask which diet you’d still be on in a year.
Five checks before you choose a diet
- Pick a way of eating that fits the foods you like, including your culture’s dishes.
- Check it fits your home life and your budget.
- Tell your GP or dietitian about any health condition, such as type 1 diabetes or an eating disorder, so the plan fits you.
- Keep plenty of fruit and vegetables in, and cut down on high-fat or sugary foods.
- Plan for some weight coming back, as NICE says it may in many cases.
Sources: NICE NG246; NHS
Want ideas you can cook? See Mediterranean-style meals.
How fast is it safe to lose weight?
The NHS says to aim for 0.5 to 1 kg a week, or 1 to 2 lbs. That’s the one number to remember. The London guide calls this pace safe and achievable.
To get there, it says to eat around 600 calories less a day than your body needs. It says this is more likely to work long term than a strict plan you can’t keep up.
- Below 0.5 kg a weekSlower than the NHS aim.
- 0.5 up to 1 kg a weekThe NHS aim.
- Above 1 kg a weekFaster than the NHS aim.
Sources: NHS; NHS London
Going much faster may carry risks. The NHS says you may be more likely to get gallstones if you’ve lost weight quickly, for example through weight loss surgery.
Gallstones are small stones in the gallbladder, a small organ in the top of your tummy that helps you digest food. Most people with them get no symptoms.
Scales stuck? NICE says a better diet can still help. It may mean healthier blood fats and a lower risk of type 2 diabetes and heart and blood vessel disease.
Check your pace by weighing yourself regularly, as the NHS suggests. To keep your strength as you lose, see protecting muscle during weight loss.
What about 5:2 and intermittent fasting?
On the 5:2, you eat a healthy diet 5 days a week. On the other 2, you have a quarter of the calories your body needs. The London guide puts that at 600 for men and 500 for women.
Don’t put the 2 fasting days in a row. Spread them across the week.
NICE makes no recommendation on intermittent fasting yet. It has asked for more research instead.
Have diabetes? Talk to your diabetes care team before you try 5:2 or low-carb. This matters most if you take insulin or medicine that can cause a hypo, when blood sugar drops too low.
Never stop or change a prescribed medicine without your prescriber.
When are very low calorie diets used?
Only under specialist care. The London guide says not to try one without medical supervision. NICE says to consider diets under 800 calories a day only in specialist weight services.
They’re for people living with obesity who have a need to lose weight fast that a doctor has checked, for example to make surgery safer.
Low-calorie diets of 800 to 1,200 a day need long-term support in a specialist weight service, or a service for conditions like type 2 diabetes. NICE considers them for people living with obesity, or overweight with type 2 diabetes.
What NICE says should happen on a low or very low calorie diet
- 1
Checks first
Help if you may have an eating disorder or other mental health issue, and a review of your medicines.
- 2
Up to 12 weeks
Has all the nutrients you need, with ongoing clinical support and supervision, and access to a registered dietitian or nutritionist.
- 3
Food comes back
Advice on bringing back a wider range of foods.
- 4
For life
Calorie control and activity after it ends. Regain is likely.
Source: NICE NG246
NICE lists the risks as weight cycling, regain and possible side effects. Weight cycling means losing and regaining, again and again. Very low calorie diets also carry a risk of constipation, tiredness and hair loss.
If weight comes back, NICE says it’s not because you failed. Think one could help you? Ask your GP whether a specialist weight service is right for you.
When to speak to a GP
The NHS says to see a GP if your BMI is in the overweight or obese range and:
- you want help to lose weight
- your waist is more than half your height (see waist-to-height ratio)
- you have pre-diabetes (blood sugar higher than normal, but not diabetes), diabetes, high blood pressure, joint pain, high cholesterol, sleep apnoea (breathing that stops and starts while you sleep) or heart and blood vessel disease
- obesity runs in your family
- you have, or think you may have, an eating disorder, such as binge eating
Have type 2 diabetes? The NHS says don’t switch to a very strict, low-calorie diet without talking to a health professional first. It may not suit you if you take insulin or other diabetes medicine.
Gallstones can cause tummy pain. Ask for an urgent GP appointment or call 111 if you have pain in your tummy that lasts longer than 30 minutes.
Call 999 or go to A&E if:
- you have sudden, severe pain in your tummy
- you have pain that spreads from your tummy to your back, and you’re being sick
- your temperature is very high, or you feel hot, cold or shivery
- your skin or the whites of your eyes look yellow