5 Factors that Contribute to a Fast or Slow Metabolism


5 Factors that Contribute to a Fast or Slow Metabolism

You’re struggling to lose weight and convinced that you have a slow metabolism. After all, you’re exercising, eating the right foods, and watching portion sizes but still, the body fat won’t budge. Surely it must be your metabolism! How do you know whether you have a fast or slow one anyway?

Metabolism Defined

First, what do we mean when we talk about metabolism? Your metabolic rate actually has three components: basal metabolic rate, thermic effect of exercise, and the thermic effect of food. Let’s look briefly at each component.

Your basal metabolic rate is the quantity of energy your body uses when you’re resting and haven’t eaten a meal in the last 12 hours. In physiological terms, it’s a measure of the energy your body needs to sustain life. Remember, even when you are doing nothing, you’re breathing, your heart is beating, and your organs and cells are active. These activities all require energy.

To measure basal metabolic rate, you need to spend the night in a lab with a controlled temperature and measurements would be taken as soon as you wake up while the room is still dark. Because basal metabolism is hard to measure, resting metabolic rate is used instead. To measure resting metabolism, you don’t spend the night in a lab and you don’t have to fast for as long prior to measuring. Resting metabolic rate makes up between 60% and 80% of your daily energy expenditure.

When you eat something, your resting metabolism temporarily increases to help digest and absorb what you eat. This is called the thermic effect of food. This component of your metabolism is fairly small, making up around 10%. It’s slightly higher when you eat a protein-rich meal.

Finally, the thermic effect of exercise is the additional energy your body expends when you’re actively moving your body and includes unstructured exercise as well. Thermic effect of exercise varies from about 10%, in inactive people, to 30% in highly active individuals.

Since the resting metabolic component makes up the lion’s share of the calories you burn each day, a slow metabolism refers to a slow resting metabolic rate. If you actually have a slow metabolism, you might wonder why. The rate at which you expend energy at rest comes down to a few factors.

Slow Metabolism: Your Age

You’ve probably seen teenagers who could eat huge quantities of food, often unhealthy food, and not gain a pound. Although many teenagers are active and burn more calories than adults through exercise, they also have a higher resting metabolic rate than a 50 or 60-year-old. Resting metabolic rate declines somewhat with age, partially due to the loss of lean body mass that goes with aging.

How to Fight It:

Do what you can to preserve muscle tissue. Muscle is metabolically active tissue and the more you have, the more calories you burn even when you’re not working out – and not just any type of exercise – strength training. You can’t stop the hands of time, but you can slow them by preserving as much muscle tissue as possible to keep your metabolism humming along.

Slow Metabolism: Your Gender

Sounds unfair, but men have a slightly higher resting metabolic rate relative to women (around 10% higher). You can attribute some of this discrepancy to differences in lean body mass. Men have more muscle than women. However, even when you take differences in lean muscle into account, men still have a resting metabolic rate that’s 3% higher than women. Some of this difference is likely due to hormonal differences. It’s not just differences in muscle, men have larger organs and every organ in your body needs energy to function and larger organs need more.

How to Fight It:

Again, strength train to build as much metabolically active muscle as you can. By doing so, you increase the number of calories your body burns at rest. You can’t do much about the 3% difference between men and women that’s not explained by differences in muscle mass.

Slow Metabolism: Your Genetics

Genetics impacts almost everything, or so it seems. In fact, genetics can explain between 15% and 20% of your resting metabolic rate. You can’t change your parents or the genes you inherited, although lifestyle and diet can impact the expression of genes through a process called epigenetics. What are the best practices? Avoid processed foods, exercise, sleep enough, and learn how to manage stress. This will not only help you avoid weight gain, but these healthy habits may also impact gene expression through epigenetics.

How to Fight It:

Lead a healthy lifestyle. Also, get your thyroid function checked. Some types of hypothyroidism, or an underactive thyroid, tend to run in families. When your thyroid is underactive, one of the consequences is, you’ll likely gain weight. Not surprising since thyroid hormones regulate your metabolic rate.

Slow Metabolism: Your Eating Habits

Being in chronic “diet” mode, where you’re always restricting calories can slow your resting metabolic rate. Remember, your body wants to ensure you have enough stored calories to meet energy demands. It does this by slowing your resting metabolic rate so that you conserve fuel. If you lose muscle mass through dieting, your metabolic rate will decline even more. Some studies show that exercising while on a calorie restriction diet reduces the slowdown in metabolism but it doesn’t completely prevent it.

How to Fight It:

Don’t be a habitual dieter or try to lose weight quickly by drastically dropping your calorie intake. Take a slower path to healthy weight loss by making smarter food choices and reduce your calorie intake by no more than 15 to 20%, even if you’re trying to lose weight. You need adequate food intake to maximize your metabolism. Give yourself breaks from dieting, even if you’re trying to shed body fat. You don’t want your body to sense a low energy state and slow your metabolic rate down. Make sure you’re exercising and doing strength training to gain muscle.

Slow Metabolism: Your Gut Microbiome

Your gut microbiome is the bacteria that reside in your large intestinal tract. We now believe these bacteria influence a variety of physiological functions, including metabolic rate. A study carried out at the University of Iowa showed that mice treated with drugs to change their gut bacteria became obese due to a reduction in resting metabolism.

How to Fight It:

Keeping a healthy gut microbiome is important for weight control. Although it’s not clear what bacterial populations are ideal for a healthy metabolism, studies suggest that a diverse bacterial population is best. To cultivate this diversity, avoid taking antibiotics unless you absolutely have to. Also, eat more fermented foods, like yogurt, kimchi, fresh sauerkraut, and other fermented vegetables that can populate your gut with friendly, bacterial species.

The Bottom Line

Now, you know the factors that impact your metabolism and some ways to maximize your metabolic rate. That’s important since your metabolic rate directly impacts your ability to lose weight.



ACE Fitness. “Resting Metabolic Rate: Best Ways to Measure It—And Raise It, Too”
Body Recomposition. “Metabolic Rate Overview”
Medscape.com. “Can Diet and Exercise Really Change Metabolism?”
Science Daily. “Altered microbiome burns fewer calories”


Related Articles By Cathe:

Is Muscle Loss the Only Reason Your Metabolism Slows with Age?

Does Your Metabolism Have to Decrease With Age?

Why Metabolic Rate Calculators Aren’t Always Accurate

Can You Speed Up Your Metabolism Through Lifestyle?


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