Metabolic Adaptation: We Get Fat

Javier Colomer 9 min. of reading Fitness Leave a comment

Is it the carbs… the fats… or junk food… If there’s one thing we’re known for, it’s blaming a single culprit for diet failures and why we gain weight instead of losing it… So, here’s my personal take on the matter

Are You Dieting and Not Losing Weight?

Interestingly, this is a problem that affects most of us, since the majority of the world’s population goes through a hypocaloric diet at some point in their lives. In fact, it’s estimated that 1.5 billion people are overweight, with 400 million of them being obese¹.

It’s no surprise then that more than 50% of people regain more fat than they lost while “dieting”². Although there’s no exact timeframe for how long it takes to regain lost weight, some studies point to 6 months – 1 year¹².

Don’t Diet, Keep Healthy Habits

I think this largely reflects the low focus on changing habits in people with overweight. The word “diet” makes people see it like the old military service:

…you join, follow it for a while, then quit and go back to your old ways…

With this mindset, we’re trapping people in a loop where for every kg lost, they gain 2 back, making the problem worse.

If you want your plan to succeed, bet on keeping healthy habits

How Does the Body React to a Hypocaloric Diet?

Our body weight is regulated by multiple mechanisms involving hormones secreted by:

The gastrointestinal tract (like ghrelin),
Pancreas (insulin),
Adipose tissue (leptin), the last being very important as it acts on the hypothalamus, regulating metabolism and hunger sensation³.

When we follow a hypocaloric diet to lose body fat, our body triggers a series of compensatory mechanisms both hormonally and at the hypothalamus level, leading to a reduction in levels of:

CCK⁴,
Leptin⁵,
Thyroid hormones¹³,
Testosterone¹⁴, and
Caloric expenditure⁶.

And an increase in levels of:

Ghrelin⁷, and
Appetite⁸.

It’s important to keep in mind all the variables involved in a weight loss process

What’s the Role of Leptin?

In short, leptin controls how much energy our body allows us to spend, which indirectly leads to greater fat loss.

By lowering leptin levels, the calories we burn in daily life (NEAT) and to maintain vital functions (BMR or basal metabolism) drop significantly.

Considering that the sum of our basal metabolism and daily routine expenditure (NEAT) accounts for 85% of the calories we burn daily, as shown in the graph, you can imagine how easy it is for our body to slow down our metabolism.

What’s the Role of Ghrelin?

On the other hand, ghrelin, a hormone secreted in the gastrointestinal tract, increases hunger and even cravings for high-calorie foods.

Some studies have shown that subcutaneous administration of this hormone increases calorie intake by 30%.

Generally, ghrelin rises during fasting or naturally before mealtime due to meal frequency⁹, but it also rises during a hypocaloric diet¹⁰, making hunger feelings much stronger when cutting calories.

The light blue line shows levels at 10 weeks of hypocaloric dieting, the black line at day zero.

This slower metabolism combined with increased calorie intake leads to the following:

When reducing calories to lose fat, our body adapts by burning fewer calories. However, this adaptation takes time, so we’ll lose fat without issues initially (which is why sedentary people lose fat in the first weeks).

Maintenance Phase

Next comes a “maintenance” phase, where calories consumed and burned balance out, and we maintain our weight. Then, we slightly increase calories without reaching the initial intake, but calories burned remain steady.

What happens? Our body fat percentage goes up. In some cases, this effect worsens because seeing weight regain causes anxiety, releasing more cortisol and increasing the chance of “binges”.

We’ve already covered how cortisol and weight gain affects us

This results in gaining more fat than initially lost. In less time than it takes a pre-beach to finish their post-workout shake, your body has won…

How Does Physical Exercise Affect Energy Expenditure?

Many might think that exercising can prevent metabolic adaptation since it burns more calories. That’s true, exercise increases expenditure obviously, but not all exercise types have the same effect.

Low-Intensity Exercise

Most people do low to moderate intensity exercise like walking, jogging, elliptical, zumba, etc. While it initially helps burn fat, the body adapts perfectly to this type of exercise, reducing oxygen consumption, calories burned, body temperature…

So, light to moderate exercise that once burned 400 kcal now burns only 200 kcal…

Your body becomes more efficient, needing less energy to do the same work

High-Intensity Exercise

That’s why I’ve never been a fan of the famous “train at 50-60% of your max heart rate because it burns the most fat”. That’s true, but what matters is burning calories, not just fat.

It’s proven that high-intensity exercise burns more calories than low-moderate intensity. The fact that the latter burns more fat during the workout doesn’t tell the whole story, since what counts is how much fat is burned the rest of the day (the famous EPOC).

Higher metabolic burn with HIIT

Benefits of High-Intensity Exercise

Although it mainly uses carbs as energy, it burns more calories throughout the day by speeding up metabolism and helps maintain more muscle mass.
As a result, high-intensity training leads to greater fat loss and less muscle loss. Also, adapting to high-intensity training is much harder than low or moderate, as oxygen consumption increases¹⁶.
A lesser-known effect of high-intensity training is that it reduces ad libitum hunger¹⁷, so it debunks the myth that higher intensity makes people hungrier or eat more as a “reward”.

This can help people finishing the maintenance phase to gradually increase calories without their expenditure falling behind, unlike the initial graph showed.

Conclusion

We can see that diet isn’t 70% of fat loss success, since our body shows us daily on a diet that a hypocaloric diet alone isn’t enough to lose fat.

Maybe that’s why people with constant calorie intake and training increase their body fat percentage, simply because their metabolism slows down¹⁸

Therefore, high-intensity training must be an essential factor for fat loss. We should avoid associating high intensity only with sprints; even a light bench press can be high intensity for an elderly or obese person. So, no excuses like “not everyone can do high intensity because it damages joints running” or “they might have a heart attack” and other phrases I’ve heard about HIIT, since even cancer patients have better survival with moderate-high intensity than low.

Does this mean we should stop doing low-intensity exercise?

No, in fact, it’s been shown that combining both is the best strategy to improve health and performance parameters. So, I encourage you to see fat loss as more than just cutting calories and running on a treadmill—your mind and body will thank you.

Sources

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