Body Mass Index (BMI)

What it is

Body Mass Index, or BMI, is a simple way to work out if your weight is healthy for your height. It’s calculated using a formula that takes into account both your weight in kilograms and your height in metres. The result is a number that helps categorise your weight.

The calculation is: BMI = weight (kg) / height (m)^2

For example, if someone weighs 70kg and is 1.75m tall, their BMI would be 70 / (1.75 * 1.75) = 22.86.

In the UK, the generally accepted ranges are:

• Underweight: Below 18.5
• Healthy weight: 18.5 to 24.9
• Overweight: 25 to 29.9
• Obese: 30 and above

It's important to remember that BMI is a screening tool, not a diagnostic one. It doesn't directly measure body fat, and it doesn't take into account factors like muscle mass, bone density, or where fat is distributed on the body.

Why it matters

Your BMI can offer a general indication of whether your weight might be putting your health at risk. Being in the overweight or obese categories is associated with an increased risk of several health conditions. These can include:

• Heart disease and stroke: Higher BMI can be linked to higher blood pressure and cholesterol levels, which are risk factors for these conditions.
• Type 2 diabetes: Obesity is a significant risk factor for developing type 2 diabetes.
• Certain types of cancer: Studies have shown a link between higher BMI and an increased risk of some cancers.
• Joint problems: Carrying extra weight can put more strain on your joints, particularly your knees and hips, potentially leading to conditions like osteoarthritis.
• Sleep apnoea: This is a condition where breathing repeatedly stops and starts during sleep, and it's more common in people who are overweight.

Conversely, being underweight can also be associated with certain health concerns, such as:

• Nutritional deficiencies: This can occur if calorie intake is too low to meet the body's needs.
• Weakened immune system: Malnutrition can impact the body's ability to fight off infections.
• Osteoporosis: Low body weight can sometimes be a risk factor for weaker bones.
• Fertility issues: In some cases, very low body weight can affect reproductive health.

What can cause it to be high or low

A high BMI is typically associated with consuming more calories than your body uses over a sustained period. This energy imbalance can lead to the storage of excess energy as body fat. Factors contributing to this can include:

• Dietary habits: A diet high in processed foods, sugary drinks, and unhealthy fats, and low in fruits, vegetables, and whole grains, can contribute to weight gain.
• Physical inactivity: A sedentary lifestyle, with little regular exercise, means fewer calories are burned.
• Genetics: Some individuals may have a genetic predisposition that influences their metabolism or how their body stores fat.
• Medical conditions: Certain health conditions, such as an underactive thyroid (hypothyroidism), can slow down metabolism and contribute to weight gain.
• Medications: Some medications, including certain antidepressants, steroids, and medications for diabetes or epilepsy, can have weight gain as a side effect.
• Age: Metabolism can naturally slow down with age, making it easier to gain weight.
• Sleep patterns: Poor or insufficient sleep can affect hormones that regulate appetite, potentially leading to increased food intake.

A low BMI can be associated with:

• Insufficient calorie intake: Not eating enough food to meet the body's energy requirements. This could be due to a lack of appetite, restrictive eating patterns, or difficulty absorbing nutrients.
• High energy expenditure: Engaging in very high levels of physical activity without adequate calorie intake.
• Medical conditions: Certain conditions can interfere with nutrient absorption or increase metabolism, leading to weight loss. Examples include hyperthyroidism (an overactive thyroid), inflammatory bowel disease, coeliac disease, or chronic infections.
• Medications: Some medications can cause weight loss as a side effect.
• Stress and mental health: Significant emotional stress or conditions like eating disorders can impact appetite and body weight.

Lifestyle factors that may help

Making certain lifestyle adjustments can be helpful for managing weight and improving overall health, regardless of your current BMI. These are areas that might be beneficial to discuss with your GP:

• Balanced Diet: Focusing on a diet rich in fruits, vegetables, lean proteins, and whole grains, while limiting processed foods, sugary drinks, and excessive saturated fats.
• Regular Physical Activity: Incorporating a mix of aerobic exercise (like brisk walking, cycling, or swimming) and strength training into your routine. Aiming for at least 150 minutes of moderate-intensity aerobic activity per week is often recommended.
• Adequate Sleep: Prioritising 7-9 hours of quality sleep per night, as this can play a role in hormone regulation and appetite control.
• Stress Management: Finding healthy ways to cope with stress, such as mindfulness, meditation, yoga, or engaging in hobbies.
• Hydration: Drinking plenty of water throughout the day can help with satiety and overall bodily functions.

Questions to discuss with your GP

If you have concerns about your BMI or your weight, your GP is the best person to talk to. Here are some questions you might consider discussing:

• Given my current BMI, what are the potential health implications for me specifically?
• Are there other measurements or tests that would give a clearer picture of my body composition or health risks beyond BMI?
• What are some personalised strategies for achieving and maintaining a healthy weight that would suit my lifestyle and any existing health conditions?
• Are there any underlying medical conditions or medications that might be contributing to my current weight?
• What are the most effective and sustainable ways to incorporate dietary changes and physical activity into my daily life?
• How often should I be monitoring my weight and BMI, and what are the signs that I should seek further medical advice?