The truth about your slow metabolism

How often have you heard someone say "I can't seem to lose weight I have a slow metabolism". Be honest with yourself here, have you been the one to say that? Unfortunately I'm here to tell you the truth, you probably don't have a slow metabolism and let me tell you why. 

Your metabolism is the rate of calories burned to maintain normal body function at rest. It can also be known as Basal Metabolic Rate or BMR. It contributes the largest part to overall daily energy expenditure. Normal body function encompasses all active tissue including organs, fat and muscle. As more tissue (or weight) is added it contributes to BMR and therefore increases total calories burned.

Real Examples of BMR:

Examples from Introduction to Nutrition and Metabolism, metabolic rate increases (speeds up) with increased weight. That means the more you weigh the faster your metabolism is!

  • 60kg man burns 1,552 kcal per day.
  • 70kg man burns 1,672 kcal per day.
  • 80kg man burns 1,910 kcal per day.

What Happens When We Exercise?

Exercise contributes to your total daily energy expenditure, or calories burned. Likewise, non-exercise activity thermogensis or NEAT adds a huge chunk of calories burned to daily energy expenditure. Think of exercise and NEAT as multipliers. You take your starting resting BMR levels and multiply them by the level of exercise you do in a day. Different research studies and health assessment centers use different activity ratings. For this example we are going to use 1.4 (low), 1.8 (moderate) and 2.2 (high) taken from the book.

Since exercise and NEAT are your multipliers if you take 2 individuals with different body weights the heavier of the 2 will burn more calories doing the same exercise. Which makes sense when you think about it. Walking to the shops is easier than walking back carrying shopping bags as extra weight. The same applies with body weight whether it is fat or muscle. Carrying more weight is more difficult, requires more energy and therefore calories burned.

Calories Burned at 60kg vs 80kg:

The 60kg man burns:

Rest: 1,552 kcal per day
Low: 2,269 kcal per day
Moderate: 2,866 kcal per day
High: 3,463 kcal per day

Meanwhile the 80kg man:

Rest: 1,910 kcal per day
Low: 2,508 kcal per day
Moderate: 3,344 kcal per day
High: 4,180 kcal per day

Why is weight loss difficult then?

Now that we know the truth about your slow metabolism, that it's actually a myth or maybe even an excuse, why is weight loss still so difficult?

Well this is when metabolism starts to go against us. With any diet or weight loss programme the aim is to create a calorie deficit compared to maintenance levels. A deficit of 500 calories is estimated to lead to 1lb of weight loss per week. This is theoretical weight loss. It assumes a constant 500 calorie deficit from beginning to target end weight.

However, observed weight loss is much slower and tends not to reach the target end weight as shown:

Slow metabolism

Weight loss causes metabolism to slow down.

Reduced Food Intake

Would you believe me if I said eating less food is why you are struggling to lose weight?

Well this is a strange one because it might just be the case. When you eat less food your level of diet induced thermogenesis is lower. This is the amount of calories burned while digesting and processing food. One of the main factors is how protein is handled when in a calorie restriction. Usually your body breaks down and rebuilds protein structures such as muscle fibres. This process requires energy and therefore burns calories. When in a caloric deficit you start to preserve protein stores and there is a lower rate of turnover.

Now before you quit your diet believing that you are doomed just know that weight loss is still possible despite these mechanisms slowing your metabolism.

Creating Calorie Deficits

So how do we overcome this slowing metabolism problem. Well before I mentioned a 500 calorie deficit is enough to lose 1lb of fat per week. Well, this is true but the problem is we only calculate our maintenance level to create that deficit once and that is at the start of our diets. For continued weight loss we need to recalculate our calorie maintenance level as we know our BMR will lower as we lose weight. By doing so you are ensuring you stay at a deficit of 500 calories each day. I'd recommend calculating this every 2 weeks or whenever you achieve significant weight loss.

Alternatively, if you are not one for calculations simply restrict by a further 100-200 calories every 2 weeks or whenever weight loss starts to slow down.

who has the slower metabolism?

A 60kg or 80kg man? Click the icon to reveal the answer...