This is based on the premise if you can stand with your legs together and see a gap between your thighs–‘a thigh gap’– you're slim.For some it is seen as a desirable shape and something to aim for, but it is not necessarily healthy or normal. The plus side:

People carry fat in different parts of their bodies. Some people carry extra body fat around their middle. This is more of a health risk than carrying weight on your hips and thighs. The BMI calculation divides an adult's weight in kilograms (kg) by their height in metres (m) squared.Wrap a tape measure around the waist midpoint between the bottom of your ribs and top of your hips. For most people this is just above the belly button. A child's BMI is given as a "centile". The centile number shows how their BMI compares with other children of the same age and sex as a percentage. Underweight (BMI of less that 18.5) - Normal (BMI of 18.5 - 250 -Overweight (BMI of 25 - 30) -Obese (BMI of 30 and above) World Health Organization 2004 guidance on BMI thresholds Many of these diseases can be prevented by having a healthy weight and a healthy lifestyle. This includes eating a well-balanced diet and doing regular physical activity. Underwater weighing measures your density, which is then used to estimate body fat. Fat is less dense than bone and muscle, so a person with a higher percentage of fat will weigh less underwater, relative to the amount of water they displace, and be more buoyant.

See a GP if you're concerned about your child's weight. They may be able to refer you to your local healthy lifestyle programme for children, young people and families. Limitations of the BMI Muscle mass

A waist to height ratio of 0.5 or higher means you may have increased health risks such as heart disease, type 2 diabetes and stroke.

Third, exercise, especially of the resistance training type. As Katch et al. write [2], you can eat more yet weigh less and have a lower body fat percentage through regular exercise. Diligent and persistent exercise allows a person to maintain a lower percentage of body fat even despite age-related tendency toward weight gain. All you need is a tape measure and a simple calculation: waist measurement divided by hip measurement. You can use any units as it is only the ratio that is important. High risk is defined as a waist-hip ratio above 0.90 for males and above 0.85 for females. The down side: DEXA is more commonly used to assess bone density, but can also be used to measure body composition. You lie still on a table while a machine arm passes over your entire body, which emits a high- and a low-energy X-ray beam. Using at least three spots on your body – chest, abs and thigh are often used – pinch the skin, pulling the muscle away from the fat and measure the fold with the calipers.Smart scales don't just give your weight, but also a host of body composition stats including your body fat percentage. They can look like normal scales with foot plates, or have additional hand plates. This means muscular adults and athletes may be classed as overweight or obese even though their body fat is low. BMI results Underweight By measuring the absorption of each beam into parts of the body, technicians can get readings for bone mineral density, lean body mass and fat mass. The plus side: For accurate readings you need to know where to place the measuring tape. It is only an estimate of body fat.