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Five things you need to know about BMI, weight, body composition and exercise

Five things you need to know about BMI, weight, body composition and exercise


It’s the ‘silly’ season when it comes to dieting, and most magazines and article are full of the latest trends in dieting and fitness. The golden rule? Not everything you read or hear about weight loss is true and a quick dieting ‘blast’ to help you lose a few kgs before you hit the beach is not necessarily the healthiest or the most permanent way to control your weight………
As always, it is so much more important to put the focus on eating for your health rather than to lose weight. You might also need a sense of humour and a strong sense of scepticism – there is absolutely no magic potion for weight loss so please never believe the hype! Weight loss is not a competition or a race. For you to succeed, you need to find a way of eating (and a form of physical activity) which you can follow on a permanent basis and allows you to stay fit and healthy.

These are 5 key facts that you need to be aware of if you are monitoring your weight and exercising.

Your fat composition is a more accurate marker of health than your BMI.

The Body Mass Index (BMI) is calculated by dividing the weight in kgs by the square of the height in m. It based on research by the World Health Organisation (WHO) which indicates that for the majority of people, the healthy range where people have the least health issues is between 18.5 – 25. A figure of over 25 is considered overweight, while a figure of over 30 is considered obese.

It’s interesting that being slightly overweight (BMI 25-27 ) is not necessarily linked to any additional health risks. The health risks tend to occur when the BMI creeps over 30 and even then, 20% of obese individuals do not suffer from any health issues.

So what’s wrong with BMI? It does not take into account your fat percentage and fat distribution which are now recognised as being more relevant markers for your health. You can be ‘skinny fat’ (small boned and carrying alot of excessive fat even if your BMI stays within the healthy range) which is unhealthy while you can also be a ‘chunky muscular healthy monkey‘ with a BMI in the overweight range yet with a high percentage of muscle and a low percentage of body fat.

As a very rough guide to fat percentages – for women anything up to about 33% is acceptable while for men up to 24% is acceptable. The biggest health concern is visceral fat, intra-abdominal fat which is stored around the waist and strongly linked to diabetes and heart disease.

Invest in scales which measure your fat percentage and don’t get down hearted if your weight is not coming off – focus instead on your fat percentage and visceral fat levels.

Muscle does not weigh more than fat and you can’t convert muscle to fat

Every time I hear the statement ‘Muscle weighs more than fat’ it makes me smile. A kg of fat weighs exactly the same as a kg of muscle and it is impossible to ‘convert’ one tissue to another. Muscle is more densely packed so a kg of muscle occupies less space. So for example, if you lose fat and gain muscle, it is possible to look and feel slimmer even if your weight remains the same.

You can be ‘a bit fat and fit’

Hurrah! This is one of the reasons why exercise is so important. Your metabolic markers (cholesterol, trigylcerides, blood sugar etc) are the most important indicators of your health. Exercise helps to improve all of these markers, so you can indeed be overweight, but exercising regularly and with metabolic markers within the healthy ranges.

If you fall into this category, try taking the pressure off yourself to lose weight and simply focus on eating healthily.

Muscle only burns slightly more calories at rest than fat and you may not actually be ‘burning‘ huge amounts of calories when you exercise!

There seems to be an urban myth that muscle tissue burns much more calories than fat. The reality is that 0.5kg of muscle burns 5-6 calories a day at rest while 0.5kg of fat burns…..2 calories per day a rest. Not a huge difference! So what about after we exercise, do we burn more calories then? This is known as the ‘after burn’ and, yes we do burn more calories due to the additional oxygen consumption to return our muscles back to their pre exercise state, but again we are not talking about huge amounts (50-120 calories per hour of aerobic activity)

Be wary of gym machines as the evidence shows that they can dramatically over estimate calories burnt by anything from 19% (treadmill) to 42 % (Elliptical machines)

Of course, every calorie that we ‘burn’ helps with weight control and exercise does help to build and conserve muscle mass but in general people dramatically over estimate the calories expended during exercise. There are so many reasons for our health why we should exercise but most exercise on its own is not really effective for weight loss.

Intermittent fasting and high intermittent intensity training (HIIT) seem to be very effective approaches for losing fat while maintaining or increasing muscle mass.

Intermittent fasting (having periods of time where you either don’t eat or eat very little) seems to have the edge over other diets in that it optimises insulin sensitivity which helps the body become more efficient at burning fuel (fat). Research also indicates that this approach helps to preserve muscle mass and optimise the production of hormones and enzymes which increase our metabolic rate.

As for HIIT? It increase your ‘oxidative capacity’ (after burn) possibly even more than steady cardiovascular activity and like all forms of exercise improves our insulin sensitivity.

So how can all this be summarised? Focus on your fat percentage rather than your weight. Eat for your health rather than ‘to diet’. Enjoy exercise for all the health benefits it brings, but don’t focus on exercise exclusively as a means of weight loss and burning additional calories. If you are fit and healthy, it definitely worth looking at intermittent fasting and HIIT as there is mounting evidence to show that these techniques can help to optimise both your health and weight.

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