BMI versus Body composition for weight management

If you read magazines, watch TV, see a doctor occasionally or have ever been on a diet, you’ll have heard of the BMI or body mass index. It is a number used to assess whether a person is underweight (BMI <25), of a healthy weight (25-29), overweight (30-35) or obese (>30). It is calculated by dividing body weight (in kilograms) by height (in metres) squared.

Although widely used and known by all, the BMI is not actually a very useful parameter to assess health or even weight. Why?

Take a chunky rugby player, six feet (183 cm) tall, weighing 16st (101kg). His BMI is 30.3, making him ‘overweight’. Yet he has a 32-inch waist, is all muscle and fighting fit. Now compare him to an armchair rugby watcher, also six feet tall, weighing 16st, but with a beer belly and a largely sedentary lifestyle. He would have the same BMI, but it doesn’t take an expert to see that the two men couldn’t be more different. For this reason, the BMI alone has limited significance in assessing good health because being ‘overweight’ or not according to BMI means nothing without knowing one’s body composition.

The BMI does not account for a person's muscle mass. In very muscular people, it suggests ‘overweight’ when they really are slim and healthy. In older people, a ‘healthy weight’ can be anything but, because muscle mass decreases with age, and their fat percentage is high for their weight. In addition, the BMI categories used in Western countries are less suitable for people of other ethnic backgrounds because they have a different stature than white people.

What is body composition, and why does it matter?

'Body composition' is the body's ratio of water, muscle, bones and fat. A body fat percentage of 8-25% is considered normal for men and 20-35% for women. Knowing where the body fat is situated also gives clues about our health. Studies have shown that fat deposited in the abdomen is more problematic than fat elsewhere because it is metabolically active, upsetting hormone levels and causing or exacerbating inflammation. It can also surround and even penetrate vital organs, impairing their function.

Muscle mass, on the other hand, is ‘expensive’ tissue. That means it burns more calories than fat. The basal metabolic rate (BMR) describes the minimum number of calories a body needs to function. Most people’s BMR lies between 1,000 and 2,000 kcal/day, depending on height, weight, sex and age. If that strikes you as too little, you’d be right. This is just the absolute minimum of calories you need to lie down and breathe. Once you consider activity levels, the calorie requirement goes right up. A low BMR means your body doesn’t burn many calories in a rested state, and you are likely to put on weight quickly, finding it hard to shed. Building muscle increases the BMR because even when not in use, muscle tissue requires energy, i.e it burns calories.

How can I measure my body composition?

Accurate body composition often reveals surprises. People with a favourable BMI may turn out to be TOFI: thin on the outside, fat on the inside.

Elaborate methods are used in research to determine body fat percentage as accurately as possible. In a medical or nutritional practice, a so-called bioelectrical impedance analysis (BIA) can help to estimate the body fat percentage. It also usually calculates the BMR. BIA devices measure the resistance in the body and thereby evaluate the body’s water content. From this, in turn, the fat content can be roughly estimated. However, it is not necessary for most people to determine the exact body fat percentage. There are also commercially available scales that make it possible to estimate the body fat content using BIA.

Another measurement you can use is the waist-to-hip ratio. It is calculated by dividing the waist circumference in centimetres by the hip circumference in centimetres. A value of more than 1 for men or more than 0.85 for women is considered unfavourable.

Your body fat is high. Now what?

Now that you know, you can do something about it. If your BMI is high, your doctor may already have advised you to lose weight. It may surprise you that cutting calories is not the answer.

If your body fat percentage is elevated, your BMR will likely be low. Many calories you eat are not burned, and any excess is deposited as – you guessed it – more fat. Eating less (fewer calories) seems to be the reasonable solution, but the body is not stupid. If less energy comes in, it will reduce energy expenditure (i. e. reduce its basal metabolic rate). You may feel cold and tired as a result. Muscle tissue may be burned for energy, with the added bonus that it is then gone and won’t cost the body any more of those precious calories. After a while, you’ll get fed up with being tired, cold and hungry all the time and start eating more again. But – surprise – you put any weight lost back on and then some because now that your body has turned down the dial, you are burning calories even less efficiently than you did to begin with. From now on, you are likely to put weight on even more quickly than before.

Increasing the BMR, however, is a lot more helpful; for this, you need to build muscle. More calories need to be burned to supply all this new muscle tissue with energy, and less gets deposited as fat. In fact, if you play your cards right, that pesky body fat can even be burned for energy. Your body composition improves as muscle tissue builds up and fatty tissue is reduced.

So, to improve your body composition, you must find a way to

· reduce your caloric intake without going hungry

· exercise to stimulate your muscles to build more lean body mass

· supply protein to enable muscle growth

Here are my tips to improve your body composition.

Low-carb eating for fat loss

Decreasing your intake of sugar and refined carbohydrates can have a beneficial effect on body composition. Numerous studies report improved body composition with a low-carb diet.

Every time we eat, our blood sugar - or rather blood glucose - level goes up. Whether that's by a little or a lot depends on what the meal consisted of. If it was high in sugar or refined carbohydrates, the blood sugar level rises high. If it was a meal with few carbohydrates and most of those complex carbs, it might just go up a little.

High blood sugar is hugely damaging to body cells. It ‘sugar coats’ cells, making them stiff and unresponsive. As that can be almost any kind of cell, the symptoms caused by long-term high blood sugar (aka diabetes) are highly varied and can affect the heart, the kidneys, the eyes, the nerves, including the brain, and more. In a bid to avoid damage, if blood sugar (glucose) levels are high, insulin is released to bring them back down to a healthy level again as fast as possible.

Insulin works by moving glucose inside cells, where it can be used to create energy. However, the cells can only take up so much glucose at any given time. What can’t be squeezed in will continue to circulate in the bloodstream, and that’s not an option. So what insulin does is convert this excess glucose into fat and store it in fat cells. It can be converted back into glucose if needed, though it usually never is. Therefore, the more glucose we get from sugary and starchy foods the more we add to the build-up of fat every day.

Another issue is that if glucose levels rise fast and high (as they do after eating sugary or starchy foods), a lot of insulin is released at a time to deal with the problem as quickly and efficiently as possible. As a result, blood sugar levels drop again, but now overshooting the mark and ending up too low. This is a problem, as a certain amount of glucose is required at all times, not least to support the brain. At this point with blood sugar low, you may feel tired, unable to concentrate, irritable and hungry. Willpower and reason go out of the window. We now need something fast, ideally something sugary as we know it will make us feel better quickly.

Fats do not affect blood sugar levels at all, and protein only minimally. Complex carbohydrates – the kind that releases glucose slowly – raise blood sugar levels gently and not as high. Less insulin is required to deal with them, and that, in turn, reduces blood sugar dips, too. Eating fewer refined carbs and more fat, protein and complex carbs means you stay feeling fuller for longer, and cravings become a thing of the past. Importantly for body composition you won’t be producing or storing excess glucose as fat.

Eat protein with every meal

Making sure that good quality protein is part of every meal you eat kills two birds with one stone. Because it barely affects blood glucose (except when eaten to excess), there will be no steep hikes or drops, and you’ll not end up feeling as ravenous as you would on the usual high-carb diet. Being hungry all the time is what makes most people fall off the wagon when dieting, so, clearly, avoiding hunger is a significant advantage.

To improve your body composition, you’ll also need to build muscle. You'll need to do resistance exercise (aka weights) to achieve that, but that new muscle must be made from something, which is where protein comes in. To create new muscle tissue, you need protein, so tuck in!

Good protein sources are meat, fish, seafood, eggs and dairy. The best vegan ones are nuts, seeds and pulses (eg beans, lentils, chickpeas – which also contain carbs). However, the latter are not 'complete' proteins, meaning they do not contain all eight of the essential amino acids. The only way around it is to also include grains in the diet – which are, of course, a source of starch. Grains and pulses don’t necessarily have to be consumed in the same meal, but both must be part of a vegan diet.

Sustainable calorie reduction

On the face of it, this seems to be the trickiest part. We have seen, after all, that eating too little only causes the body to reduce the basal metabolic rate, which is counterproductive, so we must find a way to preserve muscle despite a low calorie intake. Also, we know that low-calorie diets make us miserable, making it less likely for us to continue.

One gram of fat contains 9 kcal. The same amount of carbohydrate or protein contains just 4 kcal. It looks like a no-brainer: Cutting out fat will hugely reduce the calorie intake, and it is for that reason that conventional diets recommend you do that. Yet, as we have seen, metabolic processes matter much more than the actual number of calories. Low-fat inevitably means high-carb, and we’re en route to weight gain and misery (see above). As protein is just as low in calories as carbohydrates, I recommend increasing the share of protein in your diet.

How filling are starchy carbs? Does a bag of crisps or a packet of biscuits stave off hunger? Most people’s experience is that not long after eating starchy carbs, hunger starts niggling again and – surprise – there seems to be room for more. Not so if you increase your protein intake. You’ll soon find that you’re satisfied. There is only so much protein you can eat. That way, diets that are higher in protein naturally reduce calorie intake.

Time-restricted eating

Another way to effortlessly reduce calories is time-restricted eating. This means eating only during an 8- or 10-hour window and not eating during the remaining 16 or 14 hours of the day. That means just skipping one meal – either breakfast or dinner – and not snacking during the fasting period.

This way of eating may be much closer to the eating pattern we evolved to maintain. Hunter-gatherers don’t usually sit down for breakfast, lunch and dinner, with snacking in between. Humans evolved to eat when hungry and couldonly eat when the hunt or the foraging trip had been successful. Our bodies are well equipped to bridge the gap while we’re not eating. Body fat accumulated when food was available can now be converted back into glucose. This process cannot happen when glucose levels are up all the time. During fasting, with no new energy coming in, there is finally an opportunity to burn fat.

Not eating for 14 to 16 hours every day may seem daunting. You may find time-restricted eating difficult if your diet is based on carbohydrates. However, if you increase your protein intake and consume adequate healthy fat, you may be surprised to find that you are not feeling hungry and do not even miss that meal you’re skipping.

So, there you have it. The BMI is a less than useful measure of body weight. What really matters is body composition. If you need to lose fat and build muscle, implement the dietary changes described above and introduce regular resistance exercise into your life.

Increase protein and keep your carb intake low. Low-carb diets are much better at promoting fat loss than low-calorie diets. At the same time, they are more sustainable as you won’t feel as hungry. Once you get settled into your low-carb diet, up the ante by introducing time-restricted eating. However, take care not to restrict your calorie intake too much as this may cause loss of muscle mass.

When coaching my weight loss clients, I recently started using a professional Tanita scale to track their body composition and see how it changes over time with the above dietary changes combined with strength training. Weight loss can take time to show up on normal scales, which although clothing can start to feel looser can be disheartening for some so having scales that distinguish between fat loss and muscle gain is helpful. If you are interested in finding out if coaching could help you on your weight loss journey, you can check out my services here and book a free mini coaching call with me at your convenience here.