Body Composition, Metabolism and Weight Loss


Today’s post is a bit of a long one, so apologies for that but it has some useful information to help you get up to speed with your body composition, boosting your metabolism and losing weight.


What is body composition?

In a previous blog I spoke about the three macronutrients (carbs, proteins and fats) that we need as part of a healthy diet.

What the hell are Macros??

In terms of body composition, fats are the biggest problem.   Body composition generally refers to the percentage of non-essential fat stored in our body (i.e., fat that serves no purpose and is not necessary for the body to function) or the percentage of your whole body mass or weight that is made up of fat.  Therefore, fats impact our body composition the most!


What is Metabolism?

Your body composition is determined by your metabolism.  Metabolism is the chemical activity that occurs in our body which regulates how quickly your body burns the calories and fat you eat to power your bodily processes.

So while there is only one way calories can enter your body (i.e., from eating!), there are many ways for calories to leave it. Here are the three major factors that affect your metabolism:

  • Physical Activity and Exercise
  • Nutrition
  • Basal Metabolic Rate

The three factors that affect Metabolism

  1. Physical Activity & Exercise (Calories to move) – this accounts for only about 20% of the calories you burn!  So, a daily 45 minute to 1 hour workout metabolises or burns only about 17% of all the energy we burn in a day, burning from 250-500 calories depending on the intensity of the exercise.  Most of us think that if we work out, we have burnt a large amount of calories and therefore can eat more or have worked off that non-nutritious meal or alcohol that we had on the weekend, which is why gyms are usually flat out on a Monday!  But, as an example, a hamburger alone can contain over 300 calories, which is just one meal, or an iced coffee with full cream milk and icecream can be around 250 calories, so you may not even work either of these off during one 45 minute session!  Other activities while working on the job (e.g., typing, carrying heavy loads, standing, fidgeting) and having fun (shopping, playing, singing) are also factored in here.  So you need to focus on what you are eating for the remaining hours of the day.

2. Nutrition or Food Thermogenesis (Calories to digest) –  Nutrition or the energy you consume to digest and absorb food accounts for only about 10% of all energy metabolized within a day.  Of all the macronutrients, protein requires the most work to digest followed by carbs and fat, which is why eating a high protein diet is beneficial to boost your metabolism. This is also why nutrition and exercise often work hand in hand because if you mess up your nutritional choices, you may not achieve the results you want.  So, eating the right foods and the right amount of calories based on your daily allowance can create the perfect metabolism for fat loss and better health.

But what about the remaining 70% of energy burned?

 3. Basal Metabolic Rate (Calories to Survive) – around 60-70% of all the energy we metabolize each day comes down to our basal metabolic rate (BMR) or the number of calories your body needs at rest to support its vital functions that keep you alive (breathing, digesting, filtering waste).  There are a number of factors that affect your BMR including:

  • Body size – A bigger individual requires more calories to sustain their body at rest
  • Age: Your BMR is higher when you are younger – as you age, you slowly gain fat mass and lose muscle mass.
  • Genetics: Some people are born with higher (or lower) BMR than others.
  • Hormones: Particular hormones cause your thyroid gland to release more or less of these hormones, leading to a change in BMR.
  • Health: Your BMR is higher when you are fighting off an infection or healing from a major wound because your body is working harder at these times.

But the most important factor affecting our BMR is:

  • Body composition, or the amount of muscle we have:  Muscle is more metabolically active than fat, meaning more calories must be burnt to maintain muscle compared to fat. So for example, because most women have more fat mass than muscle mass compared to men, they have lower BMR’s compared to men.  Also, as mentioned earlier, as we get older we lose muscle mass and gain fat mass. So, the key to increasing your metabolism and burning calories is to do exercise that builds or preserves muscle, which is basically resistance or weight training.


Calories in Vs Calories Out??

Many people simply make the mistake of focusing on burning off the calories that they eat through exercise, thinking that calories in minus calories out is the key to weight loss. But, as you can see, it’s not that simple!

For example, you might be in a hurry to get to work and have a choc-chip muffin and coffee for breakfast and then tell yourself that because you ate so many calories it may be best to have a light lunch and then go to the gym after work.    The muffin you ate is likely to be made up of saturated fats, which take longer for the body to break down, therefore even if you do have a light salad for lunch, the fats you have consumed are still hanging around in your body which may be harder to shift with exercise and simply convert to excess body fat.

So, it’s not just about the number of calories you have per day and whether you are burning off those calories through exercise, but it’s about the types of foods that make up the calories you are eating each day and the type of exercise that you are doing.


What do I need to be doing then?

Well, in terms of exercise, you need to be doing some form of resistance or weight training that is enabling your muscles to contract.  You should aim to achieve around 2 sessions per week in order to build muscle mass, strength and endurance which is vital for our metabolism, and as we talked about in another blog, aim for HIIT sessions to get that afterburn effect and burn more calories post exercise.

In terms of nutrition, we need to focus on the proportion of carbs, proteins and fats that we are eating as part of your daily energy or calorie allowance to boost metabolism. To do this, you have to first work out how many calories you should be having per day to maintain a healthy weight for you.  Then, you can work out the percentage of carbs, proteins and fats you should be eating.

According to the Australian Dietary Guidelines, the estimated acceptable ranges that we should consume daily to reduce our risk of chronic disease are:

  • 20–35% of total energy intake from fat (with a maximum of 10% being from saturated fats) (1g of fat = 9 calories)
  • 45–65% from carbohydrate (1 gram of carbs = 4 calories)
  • 15–25% from protein (1 gram of protein = 4 calories)

This is just a starting point. You need to find the proper amounts for your own body, depending on your health and fitness goals, which might constantly change.  So for example, if you are training consistently 3-4 times per week, I would aim for a higher protein diet to help with muscle repair and growth (around 40% protein, 30% carb, 30% fat).  A fitness professional can help you work this out and make adjustments as necessary, or there are many apps that can help, such as My Fitness Pal, plus there are online nutritional profile calculators, such as:  or or

Track your exercise and eating habits

So jump on one of these websites and work out how many calories you should be consuming per day as well as the right proportion of carbs, proteins and fats.  Once you know this, the hard part is tracking what you eat so that you can stay accountable.  Keeping a diary can help with this, or as mentioned earlier, you can use an app like “My Fitness Pal” which is fantastic for helping you work out in real time exactly the proportion of macronutrients you are consuming, as well as your calories, to help you stay on track.