How much should I weigh?
Shedding pounds to improve your health and reach your ideal weight can be a challenge, but how do you know when you get there? Registered nutritionist, Kerry Torrens explains
Why is weight used as a measure of health?
The reason we’re so obsessed with weight is not simply because of its aesthetic appeal – we’re told that being under or overweight can also put us at increased risk of a number of health conditions. Carrying extra pounds is said to increase our risk of heart disease and diabetes, whereas carrying too little may increase our chances of developing osteoporosis.
Knowing your weight in proportion to your height is said to help gauge whether it’s appropriate for you. Although a useful indicator, weight alone is by no means the only measure by which to assess health.
What is considered a healthy weight?
The most common way of assessing whether you’re a healthy weight is to calculate your body mass index (BMI). This is a measure that uses your height and weight to evaluate whether your weight falls within a ‘healthy’ range. The figure for an adult is calculated by dividing weight in kilograms by height in metres squared. You can calculate your BMI here.
For most adults, an ideal range is considered to be a BMI between 18.5-24.9. Age doesn’t factor in the BMI calculation for adults, but it does for children.
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What does my BMI mean?
If your BMI is:
- Below 18.5: you're in the underweight range
- Between 18.5-24.9: you're in the healthy weight range
- Between 25-29.9: you're in the overweight range
- 30 or over: you're in the obese range
What is the downside of the BMI calculation?
BMI as a tool has been in existence for almost two centuries, originally developed to assess how 'at risk' an individual was of premature illness or death. It became used as an international measure for obesity assessment in the 1980s.
However, science has moved on since the BMI was first developed, and we now know that it's not just body weight that is a risk factor when it comes to longevity. How weight is carried and in which tissues is also important, but not covered by the BMI score. For example, muscle is much denser than fat, so very muscular people, such as weight trainers and athletes, may be a healthy weight even though their BMI falls within the ‘obese’ range. The BMI calculation also can’t distinguish between excess fat or bone mass, nor does it provide an indication of the proportion or distribution of body fat.
Add to this that the existing BMI definitions are based largely on white populations, although body composition, including body fat or amount of muscle mass, can vary by ethnic group, means using BMI as a predictor of health may be less accurate for non-white groups.
What are the alternatives to BMI?
There are numerous ways to measure body composition, but most rely on more complex technologies than BMI, making them less accessible. Skinfold thickness, bioelectrical impedance, underwater weighing and dual energy x-ray absorption (DEXA) are examples, and are thought to be more accurate. This is because they provide a picture of total body composition, including fat, bone, lean tissue and water. However, many of these methods are expensive, intrusive or simply not available to the majority of us.
What can be useful, however, is to take simple waist and hip measurements – these can provide an assessment of your fat distribution. Even if your BMI is in the moderate range, if more of your weight is carried in this mid-section, you may be at higher risk of heart disease or diabetes.
Is BMI a useful measure for everyone?
No, BMI is not appropriate for all people. This is because the resulting figure can be distorted by pregnancy and high muscle mass, and it may be unreliable as a measure for children or the elderly. If you are pregnant and want to understand if your weight gain is healthy, seek advice from your GP or midwife.
For children and young people (aged two to 18 years), the BMI calculation takes account of age and gender, as well as height and weight. This is because children’s body composition varies as they age, and between girls and boys.
Despite its failings, BMI can be a useful starting point, but you need to recognise that other factors, such as fat distribution, genetics, age and fitness level all contribute to your overall health status.
It’s worth remembering that weight alone is not a determinant of health, and many other factors, including eating a balanced diet, getting adequate sleep and exercise, as well as measures such as your blood pressure, cholesterol and blood sugar levels, liver and kidney function and red and white blood cell counts contribute to your overall health picture.
Although your BMI can be a useful starting point, it doesn’t take account of your race, age, gender, genetics, amount of body fat versus muscle mass or your lifestyle.
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