If you’re overweight or obese, it can increase your risk of developing a whole range of health problems. These include conditions such as:
- coronary heart disease
- high blood pressure
- type 2 diabetes
- some types of cancer
- liver disease
- sleep apnoea
- fertility problems
On the other hand, being underweight and not having an adequate diet is associated with:
- osteoporosis (if you don’t get enough calcium in your diet)
- poor muscle strength
- reduced immune function – so you're more likely to get infections
- increased risk of heart problems
- fertility problems
Keeping to a healthy weight will reduce your chance of developing these health problems.
The most common way of measuring if you're a healthy weight for your height is to calculate your body mass index (BMI). It can also be useful to look at your waist circumference, which gives a better idea of where you store fat on your body.
BMI (body mass index)
BMI is used to estimate if you're a healthy weight for your height.
You can work out your BMI using our BMI calculator.
BMI is classified as in the table below.
BMI Classification Less than 18.5 Underweight 18.5–24.9 Healthy weight 25–29.9 Overweight 30– 39.9 Obese Over 40 Morbidly obese
As your BMI increases, so does your risk of various diseases.
Limitations of BMI
BMI isn't always very accurate in certain groups of people or because of other factors that you need to take into account.
- Children. Children’s weight changes a lot as they grow. The BMIs of children are usually compared to others in the same age group so as to get an idea of whether they are higher or lower than average.
- Having a lot of muscle. You may have a very high BMI if you have a lot of muscle but have very little body fat.
- Certain ethnic groups including people of Black African, African–Caribbean and Asian descent. In these groups, your risk of health problems such as type 2 diabetes is greater at a lower BMI. You risk is classed as increased at a BMI of 23, and high at a BMI of 27.5.
Body shape and waist circumference
Where you store fat on your body is also an important indicator of whether or not your weight is a risk to your health. Storing fat around your abdomen ('apple-shaped') is thought to be worse for your health than storing it around your thighs and bottom ('pear-shaped').
Your waist circumference gives you a good indication of how much fat you store around your abdomen.
Here’s how to measure your waist circumference properly.
- While you’re standing, put a tape measure around your middle – place it midway between your hipbones and the bottom of your rib cage (usually about level with your belly button).
- Make sure the tape measure is horizontal and around your waist.
- Keep the tape snug around your waist but don’t pull it in.
- Breathe out and measure your waist.
Check the table below to see if your waist circumference is putting you at risk of health problems.
Risk of health problems due to weight Waist circumference Men Women Low less than 94cm (37 inches) less than 80cm (31 inches) High 94–102cm (37–40 inches). For Asian men, this is 90cm (35 inches) or more) 80–88cm (31.5–35 inches) Very high more than 102cm (40 inches) more than 88cm (35 inches)
If you’re not sure whether you’re a healthy weight, check with your practice nurse at your GP surgery.
How much you weigh is mainly determined by the balance between what you eat and drink, and how active you are. The energy that food provides and which you use up walking, running or even sitting still is measured in calories. Put simply:
- you'll gain weight if you take in more calories than you use up
- you'll lose weight if you use up more calories than you take in
- you'll maintain your weight if you balance the calories you take in with the calories you use up
Other factors can also make a difference – for instance, your genetic make-up can sometimes make you more likely to put weight on. And some medical conditions can also affect your weight. If you're currently over or underweight, you'll need to make some changes to get to your ideal weight.
If you’re overweight or obese
If you're overweight, it will really benefit your health if you try to lose the excess weight and maintain a healthy weight. To lose weight, you'll need to burn off more calories through physical activity than you take in from food and drink. This means reducing how much you eat, as well as increasing how much activity you do.
It's worth thinking about the following points.
- What you eat. Eating foods high in fat and sugar will make you more likely to put on weight.
- How much you eat. Many people eat much larger portion sizes than they need.
- How much activity you do. The recommended target to maintain your weight is to do 30 minutes of moderate-intensity at least five times a week. You may need to do more than this to lose weight; particularly if you don't reduce the amount you eat enough.
Read our article Losing weight safely for more tips.
If you’re underweight
There are lots of reasons why people might be underweight. There could be an underlying medical reason or you may just find it hard taking the time to make healthy, nutritious meals. Being underweight can also be the result of a mental health conditions such as depression and anxiety.
Being underweight means you might not be getting all the nutrients, vitamins and minerals that your body needs to be healthy. Try to increase your calorie intake through eating a balanced and nutritious diet. You might need to eat nutritious snacks in between meals, and go for higher fat options (such as full-fat milk) until you reach your ideal weight.
Unexplained weight loss, or inability to put on weight, can sometimes be a symptom of an underlying health problem. If you have concerns about weight loss, contact your GP.
- Health matters: obesity and the food environment. Public Health England. www.gov.uk, published 31 March 2017
- Preventing excess weight gain. National Institute for Health and Care Excellence (NICE), March 2015. www.nice.org.uk
- Obesity. NICE Clinical Knowledge Summaries. cks.nice.org.uk, last revised June 2015
- Healthy living. Oxford handbook of general practice (online). Oxford Medicine Online. oxfordmedicine.com, published April 2014
- Nutrition support. Oxford handbook of nutrition and dietetics (online). Oxford Medicine Online. oxfordmedicine.com, published December 2015
- Rheumatology, dermatology and bone health. Oxford handbook of nutrition and dietetics (online). Oxford Medicine Online. oxfordmedicine.com, published December 2015
- Nutrition assessment. Oxford handbook of nutrition and dietetics (online). Oxford Medicine Online. oxfordmedicine.com, published December 2015
- Obesity: identification, assessment and management. National Institute for Health and Care Excellence (NICE), November 2014. www.nice.org.uk
- Body Mass Index – BMI. World Health Organization. www.euro.who.int, accessed 28 November 2017
- BMI: preventing ill health and premature death in black, Asian and other minority ethnic groups. National Institute for Health and Care Excellence(NICE), July 2013. www.nice.org.uk
- Waist circumference and waist–hip ratio. Report of a WHO expert consultation. World Health Organization, 2008. apps.who.int
- Energy intake and expenditure. British Nutrition Foundation. www.nutrition.org.uk, accessed 30 November 2017
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Reviewed by Pippa Coulter, Freelance Health Editor, January 2018
Expert reviewer, Mr Paul McArdle, Registered Dietitian
Next review due January 2021
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