The term “slow metabolism” is thrown around a lot in health and fitness and often carries a negative connotation, but what exactly is a slow metabolism? When someone says “I have a slow metabolism” what they are actually saying is that their body burns calories very slowly, and this is why it may be difficult for them to burn fat. Another way to look at it as that their body is very efficient with calories, which throughout evolution where food was scarce would have been an advantage. But in a society where food is plentiful and the challenge is to abstain from food rather than find it, an efficient, or slow, metabolism has actually become a disadvantage because it increases the propensity to store fat and all the negative side effects that come with that. So, how can you tell if you have a slow metabolism?
A tell-tale sign of a slow metabolism is gaining weight on a relatively low caloric intake i.e. not eating that much food and still gaining weight consistently. However, there are numerous variables involved in weight gain, so to take out some of the guesswork, I think using a tracking-centric framework is the best way to determine if you have a slow metabolism (or are experiencing negative metabolic adaptation). A slow metabolism can be loosely defined as a maintenance calorie set point that is lower than what would be expected given your height, weight, and biological sex. Consequently, this makes it much easier to exceed maintenance calories and increases the probability that those excess calories will be stored as fat. To understand where your maintenance calories should be, you can use a equations such as the Katch-McArdle or Muller to establish Basal Metabolic Rate (I usually use both then average the BMR), then calculate your maintenance calories by multiplying your BMR by your activity factor (which can be found using a quick Google search). After establishing your predicted maintenance calories, you should then compare them to your “real-world” maintenance calories (the number of calories you can eat/day and not gain or lose any weight). This can be achieved through tracking calories and recording the number of calories needed to maintain weight over a ~2 week period. Once you have both your predicted maintenance calories and your real-world maintenance calories, you should be able to determine how your metabolism stacks up to what would be expected given your biology. Take make things simpler, here’s an example:
Name: Jane Doe
Biological Sex: Female
Weight: (61.4 kg) 135 lbs
Lean Body Mass: (54 kg) 118.8
Activity Factor: 1.55 (Moderate Activity)
Katch-McArdle: BMR= 21.6 x LBM (kg) + 370
BMR= 21.6 x 54 kg + 370
BMR= 1,536 kcals
Maintenance Calories= BMR x Activity Factor
= 1,536 kcals x 1.55
= 2,381 kcals
Jane began tracking her calories and observed that to maintain a constant 135lb over two weeks she had to eat ~1,600 calories, which is about 800 calories less than her predicted maintenance calories and could be considered a slow (or really efficient) metabolism. To make weight loss/weight gain more manageable, Jane can slowly introduce more calories to bring her maintenance calorie set point to a more desirable number i.e. what is feasible for her to eat regularly and fits her fitness/body comp goals.
It’s worth stating that any given person’s metabolism isn’t fixed for their entire lifetime. You’re not born with a fast or slow metabolism then destined to be underweight or overweight, respectively, for the rest of your life, although genetic predisposition does play a large part in body composition. Your metabolism can be shaped to become slower (more efficient with calories) or faster (less efficient with calories) if the proper principles are applied. Specifically, my article “What Is Metabolic Adaptation?” (found via the Mind Pump Blog and PassioInventa) provides a detailed blueprint for how to slow down or speed up your metabolism by re-establishing maintenance calories.