How to lower your BMI and lose weight healthily

What is body mass index (BMI)?

People naturally come in different shapes and sizes, and there is a range of weight which is believed to be healthy. Body mass index (BMI) is one way to work out if you're a healthy size. It's calculated by dividing your weight in kilograms by your height in metres, squared.

Put your measurements into our BMI Calculator to find out what your BMI is.

• A healthy BMI is between 18.5 and 25.

• A BMI between 25 and 30 is classed as overweight and the increase in risk is moderate.

• A BMI over 30 means you have obesity, and need to lose weight to lower your BMI.

Some people naturally have a larger frame than others - they may be taller, or you may be able to see that their bones are larger than average by the size of their head, their wrists, or their ankles. But this causes quite a small variation in weight and is accounted for in the range of healthy BMI.

Research has highlighted that BMI may be best used alongside other measures to diagnose unhealthy weight. This is because it doesn't take into account factors such as muscle mass, age, and sex¹. To help predict your risk of weight-related illnesses, waist-to-hip ratio may be preferred².

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Obesity and weight loss

If you are obese or overweight, you have an increased risk of developing various health problems, including cancer, diabetes and heart disease. Even a modest amount of weight loss can help to reduce your increased health risks. The best chance of losing weight and keeping the weight off, is to be committed to a change in lifestyle. This includes eating a healthy diet and doing some regular physical activity.

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What is waist-to-hip ratio?

Waist-to-hip ratio is another way of finding out if losing weight would reduce your risk of serious illness. Measure the waist around your narrowest part, or an inch above the belly button. Measure your hips around the widest part of your bottom.

Abdominal obesity is defined as a waist-to-hip ratio of above 0.9 for men and above 0.85 for women. The risk of heart disease and type 2 diabetes is higher in people who are more apple-shaped, carrying more weight around their middle, than in those who are more pear-shaped, widest around their hips²..

Measuring the percentage of body fat you have can also tell you a lot about your health. This can be estimated by measuring the skinfold thickness at certain points on the body with special calipers. But it has not been shown to be a useful predictor of disease risk.

How much weight should I lose?

If your BMI is under 25, you may want to lose some weight, but you must be aware that being underweight (BMI under 18.5) carries health risks too. Ideally, you should aim to reduce your BMI to under 25 if it's currently over that. But if you are very overweight then setting a target of a BMI under 25 may seem unachievable.

Losing 10% of your body weight may be a more acheivable target. For example, if you weighed 100 kg (15 stone 10 pounds) and you lost 10 kg, that would benefit your health. The smaller your weight loss target, the easier it may be to stay motivated by exercising and eating well - and you would reduce your risk of serious illness. Even a 5% weight reduction produces measurable health benefits in terms of your risk of developing type 2 diabetes and heart disease³.

Every small amount of weight you lose is a step in the right direction. Those who lose weight slowly and steadily are generally more likely to keep it off.

Which diet will help me lose weight?

There are lots of different ways you can adjust your eating habits to lose weight.

Some people manage it by reducing portion sizes and cutting down on treats and more unhealthy foods such as biscuits and cakes.

Many people find it easier to follow a plan which will tell them what to eat. There are lots of different weight management plans that you can find online, or which are recommended by different weight loss organisations. There are several popular types of diet plans, and they tend to fall into the following categories:

Low-fat diets to lose weight

For a long time, eating a low-fat diet was the most popular weight loss plan. It is based on the idea that eating too much fat leads to unhealthy weight gain. One gram of fat contains twice as many calories as one gram of protein. So if you reduce the fat in your diet, you can still eat enough food to make you feel full, but you will be consuming fewer calories.

Low-fat diets, which include the plans suggested by popular slimming organisations such as Weight Watchers, Slimming World and Rosemary Conley, encourage people to eat plenty of fruits and vegetables, as these tend to be low in fat, and to try to eat lower-fat meat such as chicken, along with plenty of complex carbohydrates such as brown rice and sweet potatoes, which will help you not to feel hungry.

However, some people find these kinds of healthy eating plans hard to stick to, and there are some people for whom it just doesn't seem to work.

Low-carb diets to lose weight

In low-carb weight loss plans, fat is not limited, but carbohydrates are. Not just sugar, which is a simple carbohydrate, but starchy foods also known as complex carbohydrates, such as bread, pasta, rice and potatoes. The most well known low-carb diets are the Atkins diet and the Dukan diet, but reducing carbs has become very popular, and there are lots of recipes, books and websites for those who want to eat low-carb.

Low-carb diets work on the principle that eating carbohydrates triggers a rise in blood sugar which leads to a rise in insulin. The insulin can lead to the carbohydrates being converted into fat. A low-carb diet does not cause this insulin rise and a very low-carb diet can push the body into a state called ketosis, which can cause weight loss because fat is converted into energy for the body to use. However, inducing a state of ketosis can cause weakness, bowel disturbance and bad breath.

When low-carb diets first became popular some years ago, there were concerns that the high protein and fat levels in the diet might adversely affect the kidneys and liver, and raise cholesterol levels. Studies done so far have not provided clear answers to these concerns, so it is likely that the effects, if any, are not very large.

Mediterranean diet - healthy fats and carbs

This style of eating is neither low-fat nor low-carb, but the parts of the world that eat a Mediterranean diet are home to some of the healthiest and longest living people on Earth. These populations have significantly lower rates of obesity and weight-related health problems.

Here, it's the quality of the fats and carbs you eat that counts. Mediterranean foods contain mostly healthy fats, like those found in olive oil, fish, and nuts. Carbs are also healthy as they're fibre-rich and unrefined, like those found in beans, sweet potatoes, and whole wheat foods. There are no - or very little - processed foods or refined sugars, which can drive up unhealthy weight gain, blood sugar levels, and cholesterol. The Mediterranean diet is also rich in foods that help keep these health markers stable, such as fruits, vegetables, nuts, and seeds.

Intermittent fasting to lose weight

Intermittent fasting diets, such as the 5:2 diet, have become popular. On the 5:2 diet plan, you eat a normal balanced diet on five days of the week, and then have a very small intake on the other two days, of perhaps 500 kcal per day. Another way to intermittantly fast is to limit your eating to an eight-hour period and to fast for the other 16 hours - called the 16/8 method.

The Fast 800 diet combines intermittant fasting with the Mediterranean diet. It tackles weight loss in stages - starting with The Very Fast 800 to kick-start significant weight loss with 800 kcal every day. For more gradual weight loss, The New 5:2 stage is the orginal 5:2 diet with 800 kcal permitted on fasting days. For long-term healthy weight maintenance, the Way of Life stage shifts focus away from fasting days to eating a moderately low-carb Mediterranean diet each day.

Intermittant fasting is thought to work in a similar way to a low-carb diet, by reducing insulin secretion, using up sugar stores, and burning body fat during the fasting periods. There is a growing body of evidence that this diet can benefit many groups, including those with obesity and those with a high chance of developing type 2 diabetes.

Lots of people find fasting very difficult, and worry that they will feel faint if they don't eat, but the people who recommend this kind of diet often find that it makes them feel better, not worse.

If you want to try intermittent fasting,you should speak to your doctor first. They will be able to provide you with advice to ensure you are safe and healthy.

Special diet food plans to lose weight

Some companies market meal replacement foods which are calorie counted to help you lose weight. SlimFast and Celebrity Slim are sold over the counter in the supermarket or chemist, whereas LighterLife sell meal replacements in conjunction with a support group offering lifestyle advice.

The advantages of a programme like this are that you don't have to think about what you will eat or what nutrients are in it; you just have the meal replacements as instructed. Usually you will eat one normal meal a day, perhaps in the evening, and have a shake or a bar for breakfast and lunch.

Some people find that a meal replacement bar or shake is not as satisfying as real food, so they may be tempted to top up with other foods. Another criticism of these plans is that they do not help you to make changes to a more healthy diet for the future.

Can I be fat and fit?

Becoming more physically active will help you in losing weight. Even a small increase in activity, such as taking the stairs rather than the lift, or having a daily walk, will help. For it to help your fitness, you need to aim for 10 minute bursts of aerobic exercise at a time that gets your heart pumping faster, such as brisk walking. Just 10 minutes a day of this will help.

Many people do much higher levels of exercise, such as workouts at the gym, running or cycling. Almost everyone will benefit from doing regular vigorous exercise of this type, and there are benefits for the heart and lungs as well as for emotional wellbeing.

However, one study found that those who exercise but are still have obesity do have a higher rate of heart disease than those who are not overweight⁴. Another study found that the benefits of exercise were reduced in people with obesity and that early death was more likely in fit people with obesity than in unfit normal weight people⁵. In short, if you're overweight or have obesity, it's important to lose weight alongside regular exercise⁵.

Further reading

1. Khanna et al: Body Mass Index (BMI): a screening tool analysis.

2. Freigang et al: Waist-hip ratio more appropriate than Body Mass Index.

3. Magkos et al: Effects of moderate and subsequent progressive weight loss on metabolic function and adipose tissue biology in humans with obesity.

4. Lassale et al: Separate and combined associations of obesity and metabolic health with coronary heart disease: a pan-European case-cohort analysis.

5. Högström et al: Aerobic fitness in late adolescence and the risk of early death: a prospective cohort study of 1.3 million Swedish men.