Healthy weight gain: what the NHS advises, how to track it, and when to see a GP

Are you underweight? NHS BMI ranges and their limits

Body mass index (BMI) is your weight in kilograms divided by your height in metres squared.⁴ The NHS gives an example: a person of 70 kg (around 11 stone) and 1.70 m (around 5 foot 7 inches) has a BMI of 24.2.⁴ The sum is 70 ÷ 1.70² = 24.2.

The NHS BMI calculator for adults places a result in one of four groups, one of which is “underweight”.⁴ The NHS malnutrition page says people with a BMI under 18.5 are at risk of being malnourished.³ The NHS BMI calculator names a group called “underweight”, but the pages it publishes as text do not give the number for that group, so 18.5 is quoted here from the malnutrition page and from NICE.

NICE, the body that writes guidance for the NHS, uses the same 18.5 line in a different way. Its guideline CG32 on nutrition support lists a BMI of less than 18.5 kg/m² among the situations in which nutrition support should be considered.⁹ That guideline is written for hospital and community care, so 18.5 is a screening marker used by professionals. It is not a personal diagnosis.

What 18.5 means in stones and pounds

A BMI of 18.5 corresponds to a different body weight at every height, because the sum is 18.5 × height². The table shows three heights. These are the weights at which the formula gives exactly 18.5, not targets.

Where you sit on this table matters less than what has been happening to your weight and why. The NHS says not to use its BMI tool if you have, or may have, an eating disorder; the other exclusions are on the BMI guide.⁴

For the formula and the lines in more detail, see BMI and healthy weight in stones and pounds. For where adults in England sit on average, see average weight in England. This page is about the other direction: weight that is low, or being lost.

Why weight loss or low weight needs a GP conversation

The NHS describes unintentional weight loss as losing weight without changing your diet or exercise. It says this can be a sign of stress or a serious illness, and that you should see a GP just in case.² On the page about gaining weight, the NHS also asks you to see a GP if you have lost weight suddenly, or without changing what you eat or how you exercise.¹

That advice comes before any plan for gaining weight. A plan to eat more does not address the reason weight is falling, and the NHS lists a range of possible reasons.²

• Mental health conditions.²
• Digestive problems, such as coeliac disease or inflammatory bowel disease.²
• Hormone-related conditions, including an overactive thyroid and type 1 diabetes.²
• Heart conditions.²
• Malnutrition or a loss of appetite.²
• Side effects of medicines.²
• Sometimes cancer. The NHS says weight loss and loss of appetite can sometimes be caused by cancer.²

At the appointment, the NHS says a GP will ask about your health and may arrange tests or scans.² For a big loss of weight over 3 to 6 months, the NHS malnutrition page says to see a GP, who may refer you to a healthcare professional such as a dietitian.³ The NHS gives 5% to 10% or more of weight lost over 3 to 6 months as one of the main signs of malnutrition.³

NICE adds a professional view. In CG32, screening in general practice takes place when someone registers and when there is clinical concern, and the concerns listed include unintentional weight loss, loose-fitting clothes and a loss of appetite.⁹ Screening looks at BMI and the percentage of unintentional weight loss, and the MUST tool can be used.⁹

If you think you may have an eating disorder, the NHS says to see a GP as soon as you can.⁸ It also asks you to see a GP if you use control over food to manage your emotions.¹

Gaining weight in a healthy way: energy, protein, strength work

The NHS page for underweight adults says to gain weight gradually by adding healthy calories.¹ It suggests that adults could try adding around 300 to 500 extra calories a day.¹ The page does not give a rate of gain in pounds a week, and the NHS does not convert that surplus into pounds of body weight, so this page gives no prediction.

Here is the arithmetic of that range. Over a week, 300 extra calories a day is 300 × 7 = 2,100, and 500 a day is 500 × 7 = 3,500. Over 12 weeks that is 25,200 to 42,000 extra calories in total. For comparison, the NHS gives average needs of 2,500 calories a day for a man and 2,000 for a woman. These are averages, not a target for one person.⁷

What the NHS says to eat

• A varied, balanced diet that includes fruit and vegetables, starchy foods, and dairy or dairy alternatives.¹
• More protein at meals, from beans, pulses, fish, eggs and lean meat.¹
• Not relying on chocolate, cakes and sugary drinks to gain weight.¹
• Not filling up on drinks before a meal.¹

Strength work

The NHS suggests building muscle with strength training or yoga, and says exercise can also improve your appetite.¹ For adults aged 19 to 64 the NHS recommends strengthening activities that work all the major muscle groups on at least 2 days a week.⁶ It also says to speak to your GP first if you have not exercised for some time or have a medical condition.⁶

The NHS does not say how much muscle a given amount of strength work will add, so this page makes no promise on that. If you are new to lifting, how to start strength training covers the basics.

Why the NHS cares

The NHS says that being underweight can mean you are missing vitamins and minerals, which could lead to health problems such as a weak immune system and bone fractures.¹ That is the reason a plan is aimed at steady, balanced gain rather than the quickest gain.

This page gives no meal plan, because a plan fitted to one person is a job for a GP or a dietitian. The British Dietetic Association fact sheet on malnutrition in older adults suggests “little and often”, with two nutrient-dense snacks a day, and says you can see a dietitian through a GP referral or choose a private dietitian.¹⁰ That fact sheet is written for older adults, so it is not general advice for everyone.

Lean mass versus fat: what the scale can and cannot show

A rise in weight does not say what has been gained. The NHS says BMI cannot tell the difference between muscle and fat.⁴⁵ A person with a lot of muscle can be classed as overweight or obese while carrying little fat.⁵ Weight is a total, and body composition is how that total divides between fat, muscle and everything else. What is body composition? explains the split, and the muscle mass guide covers muscle specifically.

A smart scale that uses bioelectrical impedance estimates the split. One study is a French comparison of three smart scales against DEXA, the reference scan, with about 50 patients for each scale.¹¹ The median error on body weight was small, 0 to 0.3 kg. The median errors on fat mass ranged from −2.2 to −4.4 kg, and the authors concluded that smart scales are not accurate for body composition.¹¹

The takeaway is a modest one. A scale is good at weight and the direction it moves. Its split into fat and muscle is an estimate that can be several kilograms out, so read it as a trend, not a measurement. How accurate are smart scales? goes through this in more detail.

Tracking progress: a weekly log and a worked example

A log turns a feeling into a pattern. The routine below is a suggestion built from the NHS advice to gain gradually.¹ The NHS sets no standard length for tracking, so choose a stretch that suits you, and talk to your GP if the numbers move the wrong way.

1. Pick one scale and one time. Weigh once a week on the same scale, at the same time of day, in similar clothing. Changing the scale or the time changes the number for reasons that have nothing to do with you.
2. Record in the unit you think in. Write the weight in stones and pounds, then the kilograms. One stone is 14 lb, and 1 lb is 0.45359237 kg.
3. Add two lines of notes. Note your energy and your appetite for the week. The NHS says strength work can improve your appetite,¹ and the log shows whether that happens for you.
4. Read two weeks at a time. Compare the average of the last two weighings with the average of the two before. One weighing can move with a meal or a drink, so a single result says little.
5. Stop and talk to your GP if weight falls. If your weight keeps falling when you are not trying to lose it, or your clothes become loose, that is the pattern the NHS and NICE list as a reason to seek advice.²⁹

A worked example

Here is an invented log to show the arithmetic. The numbers are an illustration of how to read a log, not a goal and not a forecast. The first row is 8 st 0 lb, which is 8 × 14 + 0 = 112 lb, or 50.8 kg. The last row is 8 st 7 lb, which is 8 × 14 + 7 = 119 lb, or 54.0 kg.

Reading it: the weight moved in the right direction across the 12 weeks, with small steps of 2 or 3 lb between rows. A log like this says nothing about whether the gain is muscle, fat or fluid. For that you would add a measure of body shape, such as your waist, and treat any scale estimate of body composition as a rough trend.¹¹

If you want a figure on energy intake, how many calories do I burn? sets out how the sums are done. It does not tell you how many calories are right for you.

When to see your GP or a dietitian

Seeing a GP is the first step, not a last resort. The NHS says to see a GP if you have lost weight suddenly, if you have lost weight without changing your diet or exercise, or if you use control over food to manage your emotions.¹ The NHS also advises seeing a GP if you have lost a lot of weight without trying over 3 to 6 months.³

A GP can ask about your health, arrange tests, and refer you to a specialist or a dietitian.²³ If you would like a dietitian, the British Dietetic Association fact sheet says you can be referred by your GP or choose a private dietitian.¹⁰

Before you start strength work, the NHS says to speak to your GP first if you have not exercised for some time or have a medical condition.⁶ If you are unsure whether your weight is low for you, take your log, your height and your notes with you.

Nothing here replaces a conversation with a health professional, and nothing on this page tells you what weight is right for you.

Frequently asked questions