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Body composition explained

By Haddi Browne

Smart bathroom scales measure your body composition and can tell you whether changes to your lifestyle and diet are helping.

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In this guide: 

It’s become common knowledge that being healthier is about much more than simply losing weight. You may have heard of terms like ‘body composition’, ‘body mass index’ and ‘body fat percent', but what do they actually mean?.

If you’re trying to lose weight, gain muscle or generally become healthier, understanding your body composition can be key to making sure you get on the right track and stay there. 

Many of us have jumped on a scale or calculated our body mass index (BMI) when thinking of losing weight and/or gaining muscle to improve our health. The problem is neither of these tells us anything about how healthy we are, they just compare how heavy you are to a generic standard that isn’t tailored to you. 

Why measure body composition? 

Obesity is a major health concern in the UK: more than half of adults in England are overweight or obese. In 2018, a record high of 20% of children aged 9-11 were classed as obese. Overweight children are more likely to be overweight as adults and are at an increased risk of a various health conditions.

Knowing your body composition gives you a complete picture of your body health and provides insights into areas you need to focus on. Body composition scales are used by hospitals, gyms, sports clubs and slimming groups to provide benefits to overall health, such as: 

  • Motivation to achieve your health goals
  • Reporting how much body fat you have
  • Helping you set an accurate and realistic calorie limit for your body to stay healthy
  • Helping you to discover which exercises work best for you by telling you how much of your body is muscle
  • Whether you’re dehydrated or overhydrated by measuring your body water levels
  • Enabling you to track changes in body fat and muscle over time

If you measure your body composition and find that you have high levels of fat, you could start exercising while measuring body fat percentage and muscle growth. A traditional scale might show the demotivating sight of no change to your weight, even though you’re actually gaining muscle and losing fat. Using a body composition scale makes actual progress crystal clear.

Body composition and smart scales 

Bathroom scales are much smarter and more powerful than they once were. Smart bathroom scales go beyond basic weight measurements and can give you reports on everything from your body fat percentage, muscles mass and bone density to, water mass, heart rate and even our metabolic age. Measuring changes in your body composition over a period of time is the key to determining whether lifestyle changes are working.

The problem with weighing yourself on traditional bathroom scales is that it won’t tell you how much of your body is made of fat, water or muscle. You just get one number staring back at you telling you that you’ve either lost weight or gained it. You could be exercising five times a week and having salad for dinner just to step on the scale and see that nothing’s changed. This discouraging sight causes many to give up and cancel their gym membership.

If you’re weighing yourself every day with a normal scale, the number you see is likely to be distorted by fluctuations in weight because of factors like being dehydrated or having a big lunch. For a good idea of changes occurring over time, weigh yourself once a week under similar conditions (e.g. first thing in the morning).

Not all smart scales are accurate so, for a reliable, in-depth understanding of your body composition, try one of our smart bathroom scale reviews that got a good body-fat analysis score in our tests.

Body composition explained 

Body Mass Index (BMI)

  • Simply measured by dividing a person’s weight in kilograms by the square of their height in metres
  • A healthy BMI is in the region of 18.5-24.9 kg/m2
  • Doesn’t take into account an individual’s body shape
  • BMI above the healthy weight range can increase your risk of serious health problems
  • Can't tell the difference between excess fat, muscle, or bone
  • For adults, BMI doesn't take into account age, gender or muscle mass

Body Fat Percentage

  • Separates body composition into two categories – the amount of body fat you have and everything else (bones, muscle, hair and water)
  • Tells you exactly how much of your body weight is made of fat
  • Provides an insight into your body composition, but is not enough to use it as a standalone reading
  • Isn’t based on weight – a slim person with a low weight may have a higher body fat percentage than a muscular person who weighs more
  • Healthy body fat percentage for men is 8-20% for men and 15-31% for women
  • Too much fat can increase the risk of high blood pressure, heart disease, Type 2 diabetes and certain cancers
  • Too little fat can lead to osteoporosis in later life, irregular periods in women and possible infertility

Total Body Water

  • Shows how hydrated the body is
  • Useful for transporting waste, helping organs to function, regulating body temperature and digestion
  • Healthy total body water for men is between 60-65%; for women it is between 45-60%
  • Check out reviews of scales that measure total body water

Basal Metabolic Rate (BMR)

  • The amount of calories your body needs to function
  • Typically around 1662 calories for men and 1393 calories for women
  • Helps you set an accurate, realistic calorie limit for your body
  • Not simply based on generic average male/female/age statistics you would find online
  • Based on how many calories your body would need if it was resting for 24 hours and only needed to support vital functions, such as breathing
  • Browse our reviews of smart scales that measure BMR

Fat Free Mass

  • Everything that isn’t body fat
  • Bone, water, muscle, organs and tissues
  • Metabolically active, meaning they burn calories for energy
  • Take a look at our reviews of bathroom scales that measure muscle mass

The problem with relying on BMI 

Measuring BMI is the most common way of determining whether a person is overweight or not. However, while it can tell you if you are carrying excess weight, it can't tell you if you have excess fat. So an avid weight-lifter could be the same age, height and weight as an individual who’s devoted to their sofa and TV. 

They’d both have the same BMI but the former’s weight consists mainly of muscle while the latter’s is mostly fat mass. This means they have distinctly different exercise needs, nutritional needs, health risks and fitness goals.

Using BMI alone very muscular, athletic adults could be classed "overweight" or "obese" even though their body fat percentage is low. Similarly, adults who lose muscle as they get older may be carrying excess fat but still fall into the "healthy weight" category. This is why BMI should be considered alongside fat mass and body fat percentage to determine overall health.

Men, women and athletes 

There is no universally agreed ideal body fat percentage due to factors such as height, gender and genetics all playing a part. The below table of body fat percentages by the American Council on Exercise (Ace) is a frequently used reference point.

According to the World Health Organization (WHO) the healthy body fat range for men aged 20 to 40 is anything between 8% and 19% body fat. The normal range for men older than 40 is between 11% and 25%. To be considered obese, a person would have a body fat percentage of more than 30%.

Women generally need a higher body fat percentage than men because it’s necessary for childbearing and oestrogen production. The WHO defines a healthy body fat range for women aged 20 to 40 as being between 21% and 33%, while women over 40 need 23% to 36% body fat. Women with more than 40% body fat are considered obese.

The essential body fat required for maintaining minimum health levels is 5% for men and 8% for women.

Athletes typically have lower body fat levels than the non-athletic population because excess fat may affect their performance. Plus they tend to eat sufficient amounts of carbs and protein while engaging in intense exercise that burns fat. Professional male runners typically have body fat between 4% and 10% while female runners have 7% to 13% body fat. 

Achieving a Healthy Body Fat 

Your body is made up of two types of mass: body fat and fat-free mass. ‘Essential fat’ is necessary to stay healthy – it helps protect our internal organs, stores fuel for energy and regulates important body hormones. However, the problem comes when we have an excessive storage of non-essential body fat. You should ideally aim to achieve a healthy amount of body fat for your age group and have a higher amount of muscle than fat.

Body fat percentage is a body composition measurement. Bear in mind that not all fat is visible so you could have more of it than you think. Visceral fat is stored within the abdominal area and surrounds important organs, including the liver, stomach, and intestines. It can also build up in the arteries and increases the risk of serious health problems. Because visceral fat isn’t easily seen, even the slimmest people can have high levels of it. We’ve tested bathroom scales that can measure the amount of visceral fat your body has.

We all know that eating a healthy, balanced diet is an important part of maintaining good health. You can achieve and maintain a healthy body weight by eating the right amount of food and drink. It’s also important to eat a wide variety of foods in the right proportions. Contact your GP for help and advice.

Having a high percentage of body fat can mean that you’re overweight and/or your fitness levels are too low. The best way to lose weight is through a combination of diet and exercise. To lose weight at a safe weight and have a greater chance of maintaining a lower weight, cut 500 to 1000 calories from your diet each day as this should help you lose one to two pounds a week.

Dangers of being overweight or obese 

  • Type 2 diabetes
  • Coronary heart disease
  • Linked to 13 types of cancer, such as breast cancer, womb cancer and bowel cancer
  • Stroke
  • High blood pressure and cholesterol
  • Metabolic syndrome (a combination of diabetes, high blood pressure and obesity)
  • Gastro-oesophageal reflux disease (GORD) (where stomach acid leaks out of the stomach and into the gullet)
  • Reduced fertility
  • Osteoarthritis (a condition involving pain and stiffness in your joints)
  • Sleep apnoea (a condition that causes interrupted breathing during sleep)
  • Liver disease and kidney disease
  • Pregnancy complications, such as gestational diabetes or pre-eclampsia (when a woman experiences a potentially dangerous rise in blood pressure during pregnancy)
  • Stress incontinence (leaking urine when you are, for example, laughing, coughing, etc
*Information sources: NHS, Patient

Being underweight could mean you're not eating enough or that you may be ill. Adjusting your calorie intake will help you increase or decrease your body fat levels. To gain weight at a safe rate, increase your calorie intake by 250 to 500 per day.

Dangers of being underweight 

  • Getting more infections
  • Taking longer to recover from illness
  • Slow wound healing (eg, after surgery)
  • Fragile / thinning of bones
  • Irregular heart rhythms
  • Higher risk of heart attack
  • Lack of periods, and infertility
  • Feeling drained and tired
  • Osteoporosis from too little vitamin D and calcium
*Information sources: NHS, Patient

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