How Basal Metabolism Affects Your Weight and How to Calculate It

The other day I was reading an article about basal metabolism and I realized I heard about it before, but I didn't really know what it was all about.

Here's what the average person thinks:

  • Slow metabolism: Even though I am on diet, I can't lose weight
  • Fast metabolism: I can eat whatever I want and how much I want, and I don't gain weight

With a slow basal metabolism we gain weight more easily

These are common thoughts.

But honestly, how many really know what basal metabolism is, and what role it has on our body regarding weight loss?

This is what that article that I read was talking about, and gave me some inspiration to explain a few things here as well.

Let's Learn More About Metabolism...

Basal metabolism, slow and "lazy", is blamed by many as the cause of excess body weight, but the reality is that it's almost never like that.

Instead, it is basically the quantity of energy that our body spends to make the engine run at minimum. In other words, the energy spend to provide vital functions such as:

  • Breathing
  • Blood flow
  • Heartbeat

and so on. Each one of us has their own basal metabolism and it is determined by parameters that we couldn't modify even if we wanted to.

  • First of all, after the age of 30 it decreases by 2% every 10 years because the bone mass stops growing after age 25.
  • In men it is higher than in women because it is proportional to the muscle mass, which is obviously bigger in males.
  • Between two persons of the same weight, the taller person burns more energy because they need more blood to flow through the body, more nutrients and so on.
  • Finally, metabolism depends also on genetics.

How Basal Metabolism Affects Our Weight

There are people that tend to save energy and people that tend to spend more, and that's not determined by one single gene, but rather by a bunch of factors.

Generally speaking, we tend to save energy because we inherited genes from our ancestors who lived when it was hard to find food.

A slow metabolism (that doesn't spend much energy) allowed them to survive during famine.

Today, instead, we have plenty of food (except in some third world countries, unfortunately) and our lifestyle is kinda sedentary. This is one of the main causes of today's people weight problems.

As a general rule, to avoid gaining weight we need to make sure that we burn the amount of energy that we take in with food (calories).

The amount of energy that we burn is given by basal metabolism and daily physical activity. Plus, we burn energy to digest food, breath, maintain the body temperature etc.

How to Measure the Basal Metabolism

Basal metabolism affects 60-70% of the energy we burn. In fact our body need lots of calories to just function even if we don't more and don't speak.

To measure it we can consider parameters like weight, height, age and gender.


Precise measurements can be obtained with an exam done on full rest where they measure the quantity of oxygen consumed and carbon dioxide produced (which are related to the amount of energy consumed). This method is called calorimetry.


Alternatively, as basal metabolism is proportional to our muscular mass, it can be measured by the resistance of the muscle tissues on small amounts of electric current.

This can tell us about the percentage of lean and fat mass in our body.

Skinfold ThicknessSkinfold Thickness

This exam estimates the subcutaneous fat percentage in specific body areas by pinching the skin with a special instrument.

Basal Metabolism and Thyroid

In the past the estimated basal metabolic rate was also used to evaluate the thyroid function, which affects the speed of energy consumption in our body.

However, it was a very specific method, because basal metabolism values are altered only for high degrees of thyroid dysfunction.

Still, thyroid plays a great role in how fast we consume energy.

ThyroidTogether with cortisol and adrenaline produced by the adrenal glands, thyroid hormones are those that most affect metabolism. They act on the respiratory cells, favor the production of nutrients, protein synthesis, splitting of fats and increase the use of glucose etc.

Therefore it is not surprising that the oxygen consumption of a hypothyroid patient, indicative of basal metabolism, decreases to 150 ml per minute compared to the standard 250 ml.

Those that, instead, suffer from hyperthyroidism, the oxygen consumption can increase up to 400 ml per minute.

Hypothyroidism and hyperthyroidism are, indeed, cases where the BMR (basal metabolic rate) is obviously different than normal.

The consequences affect the overall health status:

  • Hypothyroidism: the subject loses appetite (weight gain is usually very limited, unlike what many think), is often sleepy, cold and has a slower heart rate.
  • Hyperthyroidism: the subject can lose weight even if they eat more than usual, becomes restless, hyperactive, sweats more and has a higher heart rate.

 Low Calorie Diets Can Slow Down Basal Metabolism Even More

When there is no thyroid dysfunction, instead, our basal metabolism can be more or less slow, and it can affect weight changes only in minimal part.

Basal metabolism cycle

According to the experts, to have an idea of what our basal metabolism value is, it is important not to screw up our diet plan.

For example,

Long term hypocaloric diets that introduce less energy than what we need at rest are dangerous. Such diets shouldn't be followed for more than very few weeks and only under doctor's supervision.

When we provide fewer calories than required by the basal metabolism our body goes into alarm.

In fact, where does it take the energy from, and how does it react?

Unfortunately, it takes energy from muscle tissues and it inhibits the ability to reproduce.

Our body developed great survival skills over the millenniums. It takes energy away from the areas that are not critical.

For example, when you are in a very cold environment your hands and feet get very cold because the body takes the blood away from them to bring it to the vital organs.

When we're sick we lose the ability to reproduce because our body figures that we wouldn't be able to take care of kids anymore.

The Side Effects of Hypocaloric Diets

Our body is very smart, like a perfect machine.

Very low calorie diets can have bad consequences on our body, such as:

  • Heart and bones become more brittleWeakness
  • We get sick more easily
  • Inability to concentrate
  • Sleepiness
  • Can develop anorexia

Those are extreme side effects, but even without going that far, you are not going to get any good results with low-calorie diets.

They slow down metabolism. As I said before, prolonged lack of energy (calories) makes the body run at minimum consumption.

At that point most people get very frustrated because the are on such a hard diet and it doesn't even work anymore. They start eating more and gain weight again.

Fast solutions never work.

In conclusion, it is always better to lose weight gradually, and be on diets that allow the basal metabolism to be always active.

Hopefully this answers most of your questions on basal metabolism. Try to speed it up and you'll see real body changes!


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