What is the Body Mass Index (BMI)?

BMI is a calculation which combines the height and weight of a person. The calculation is an adult's weight in kilograms divided by their height in metres squared.

Why does the BMI matter?

The BMI gives an indication of whether a person is in a healthy weight range or if their weight may lead to health risks. An ideal adult BMI is between 18.5 and 25 kg/m2. However, BMI is not a perfect tool, as muscle mass, either very muscular (e.g. athletes) or very little muscle mass (e.g. muscular dystrophy), distorts the figures. The standard BMI charts were originally developed in white, North American and European, men, and are therefore less applicable to many ethnic minority populations. In many populations the health risk BMI range needs to be adapted and is lower than the original BMI standard.

What do overweight and obesity mean?

Overweight and obesity are medical terms, defined by Body Mass Index, relating to the health risks from having too large fat stores. When used in this medical way a person is described as 'having' overweight or obesity much like having any other condition, not as 'being' overweight or obese. Dr Fox uses this standard through the weight loss pages.

Standard weight categories used by the World Health Organisation:

Weight BMI kg/m2
Underweight < 18.5
Healthy weight 18.5-25
Overweight 25-30
Obese 30-40
Severely obese > 40

Which ethnic groups have a greater health risk at a lower BMI?

All non-white ethnic groups have a greater health risk at lower BMI, than white European and North Americans. NICE (the National Institute for Health and Care Excellence) have previously suggested that increased weight related health risk should be defined for ethnic minorities at BMI over 27.5kg/m2, rather than 3kg/m2 in the white population.

Research published in the Lancet in 2021, looked at more than a million people's medical records in England and compared BMI, ethnicity, and diagnosis of type 2 diabetes, which is strongly linked to overweight and obesity. They showed that all minor ethnic populations had a higher risk of diabetes at lower BMI than white Europeans. The risk of diabetes associated with BMI can be seen as a marker for all diseases linked to overweight and obesity and indicates that health risks do increase at a lower BMI, for non-white ethnicity groups in England.

The study results are summarised below.

Ethnicity BMI with increased Health Risk kg/m2
Bangladeshi > 21
Tamil & Sri Lankan > 23
Indian, Nepali, Pakistani > 24
All South Asian combined > 24
Arab & Chinese > 27
Black Caribbean > 26
Black African > 29
Black British > 29-30
Black other > 27
All Black combined > 28
White > 30

Why does my waist measurement matter?

Waist measurement reflects how much fat there actually is around your middle, and that is directly related to the amount of fat collected around and inside your internal organs. Fat around and inside organs will reduce the function of those organs and contribute to poorer health. Waist measurement is especially helpful when the BMI is between 25 and 35. Again, ethnicity plays its part.

What are the health risks of having overweight or obesity?

People with obesity or overweight die younger from a variety of causes.

Many chronic (long term) health conditions and also many cancers are directly linked to having overweight or obesity.

These include:

  • Coronary heart disease.
  • Type 2 diabetes.
  • High blood pressure.
  • High cholesterol.
  • Stroke.
  • Dementia.
  • Breast cancer.
  • Bowel cancer.
  • Non-alcoholic fatty liver disease.
  • Respiratory problems and asthma.
  • Gallbladder disease.
  • Gastro-oesophageal reflux disease.
  • Kidney disease.
  • Osteoarthritis.
  • Sleep apnoea.
  • Depression and anxiety.

What causes obesity and overweight?

This is a difficult question to answer, and there is lots of ongoing research.

At the most basic level taking in more calories than the body needs to function leads to storing the extra as fat, and eventually to overweight and obesity.

However, the reason why someone eats more than is needed, is a complex interplay between the way the body is genetically programmed, environmental factors, cultural attitudes to food, lifestyle, and probably many other factors!

Stopping smoking can be associated with weight gain, often as a combination of improved sense of taste, eating to prevent cravings, and an actual slowing of the metabolism.

Some medications are linked with weight gain including beta blockers, some diabetic treatments, some antidepressants and antipsychotic drugs. However contrary to popular belief the contraceptive pill and HRT have not been shown to increase weight.

There are some medical conditions, such as low thyroid function and polycystic ovarian syndrome, which can cause weight gain. It is recommended that anyone with overweight or obesity has a medical check up, both to check for conditions causing - and health problems caused by - the excess weight.

What are calories and kilocalories?

A calorie is a measure of energy, like miles and kilometres are measures of distance. In terms of weight and diet, calories are used to measure the amount of energy we take in from food and also the amount of energy our bodies use to function and exercise.

One calorie is scientifically defined as the amount of energy required to raise the temperature of 1g of water by 1°C. A kilocalorie (kcal) is 1000 calories.

When discussing 'calories' in relation to weight and exercise, the 'kilo' tends to be forgotten and so the word 'calorie' in general use is really 1000 'scientific' calories! Scientists also use the metric measurements of joules and kilojoules to measure energy.

How many calories should I eat per day to lose weight?

To lose weight you need to take in fewer calories than your body needs to function. A good target is 500kcal less each day than that recommended to maintain your weight, for your height, age, sex, and activity level. There are many charts and calculators to choose from showing normal and weight loss calorie needs!

A table is also available in the alli patient information leaflet on page 44.

See also NHS - What should my daily intake of calories be? and BHF - How many calories should I eat in a day?

What is weight cycling?

Weight cycling describes the cycle of losing weight but then regaining it again over time, followed by further dieting and weight loss, and then again regaining weight. It is also sometimes called yo-yo dieting. This cycle can be very depressing, and there is no easy solution.

What is 'hidden fat'?

In the UK, foods that are prepackaged usually have nutritional information on the packaging. Sometimes there is also a 'traffic light' system with red, amber, green - green is the healthiest. The amount of fat is recorded in grams. This can be confusing as the actual energy provided by that fat is measured in calories not grams. There are twice as many calories in fat than in the equivalent weight (grams) of protein or carbohydrates! So fat is often 'hidden' in foods.

The fat content of a meal taken with Xenical or alli should be less than 65-70 total grams, or less than 30% of the total calories. Calorie and fat counters and diet apps can be used to help to work this out.

Labels also often show a fat 'percentage figure' which is also confusing! This figure is the percentage of the adult recommended allowances NOT the percentage of the total product (calories or grams) that is fat in the product!

Food label example

How long can I take Xenical for?

Xenical should only be continued if you have lost 5% of your body weight after 3 months use. After that, as long as you are not gaining weight, and your BMI is still over 30, Xenical can be taken indefinitely, though studies have only been done for up to 4 years. You can swap to alli when the BMI drops below 30 until the BMI is below 28.

How long can I take alli for?

The manufacturers recommend that alli should be stopped after 3 months if there has been no weight loss, at all. Alli can be bought without a prescription, so there are tighter controls with the manufacturers not recommending alli to be used for longer than 6 months in total. However, as with Xenical, alli can actually be safely used indefinitely, but use of alli beyond 6 months is 'off label' where doctors take responsibility for prescribing it.

Alli should be stopped once your BMI is below 28.

What type of diet is best?

A healthy balanced diet is always recommended. This includes:

  • Five portions of fruit and vegetables per day.
  • High fibre, starchy foods.
  • Low fat milk and dairy.
  • Small to moderate portions of protein - meat, fish, eggs, beans, lentils.

Also important:

  • Cut down/cut out high fat foods, sugar, and salt.
  • Don't fry meals - grill or steam instead.
  • Cut down alcohol intake (high in calories).
  • No snacks containing fat between meals - NO biscuits, cakes, savoury snacks, crisps.

Does stopping smoking increase weight?

Yes. Stopping smoking can often be associated with increasing weight. On stopping smoking: metabolism slows; there is an improved sense of taste; and many people eat more, or eat sweets to prevent cravings, which all contribute to weight gain.

Do medicines increase weight?

Yes. Unfortunately some medicines can be associated with an increase in weight. These include beta blockers, some diabetic treatments, high dose steroid tablets, some epilepsy medications, some antidepressants and medication for schizophrenic illness. Contrary to popular belief contraceptive pills and HRT have not been shown in studies to cause weight gain.

Should I take vitamin supplements whilst taking Xenical or alli?

Whilst taking Xenical and alli, there is a theoretical risk of inadequate absorption of vitamins A, D, E, K and beta-carotene, which are fat soluble vitamins. In reality, if the diet contains adequate fruit and vegetables, this is not usually a problem. A multivitamin supplement taken at bedtime, would minimise any risk.

Is there anything else can I do to help lose weight?

Weight loss, and maintenance at the new weight, involves lifestyle change, in particular permanent changes to diet and exercise.

Our relationship with food is complex. Thinking about this will often help with making sustainable diet changes. There are many small steps which can be taken to support behaviour changes.

Relearning your personal relationship with food can be helped by Cognitive Behaviour Therapy (CBT), mindfulness techniques, or hypnotherapy.

Some people respond well to losing weight alongside others, with clubs and organisations such as Slimming World and Weight Watchers. There are many phone apps, websites, personal trainers etc., all aiming to provide more motivation to keep going.

Do Xenical and alli contain lactose or gelatin?

Neither Xenical or alli contain lactose but both Xenical and alli capsules contain gelatin.

Weight loss treatment

Dr Amanda Wood

Authored 12 December 2022 by Dr A. Wood
MB ChB Manchester University 1984. NHS GP in Bristol. GMC no. 2855422

Reviewed by Dr C. Pugh, Dr B. Babor
Last reviewed 12 December 2022
Last updated 19 January 2024