Does Calorie Restriction Slow Metabolism?


When you start calorie restriction, you need to take into consideration a few changes that will happen in the body. Those are called metabolic adaptations, which is a fancy name of your body adapting to the new energy intake. In this article, I will explain everything you need to know about calorie restriction and metabolism, and how you can raise your metabolic rate after dieting.

Does calorie restriction slow metabolism?

In general, your metabolism does slow down after calorie restriction through the decrease of resting metabolic rate and decrease in the thermogenic effect of food. The largest component of your metabolic rate is BMR, which is highly associated with body size and body composition.

Larger people and people with high lean body mass burn more calories. So after losing weight when both total body weight and lean body weight goes down, your resting metabolic rate decrease. But this doesn’t mean you cannot do anything about it.

Why Calorie Restriction Lowers Basal Metabolic Rate?

In general, going on a diet and losing a significant amount of weight is the most robust non-pharmacological intervention to improve health and well-being. But together with a list of health-promoting benefits, calorie restriction also comes with a lower metabolic rate.

Basal metabolic rate is the amount of energy our body use at rest. And is dependent on the amount of lean body mass (muscle) we have. During dieting without efficient protein intake and regular strength training, people typically lose a lot of muscle mass, which leads to a lower metabolic rate.

Lower total body weight

Have you’ve ever wondered how we burn calories? The most metabolically expensive action in the body is breathing. Oxygen consumption takes up a humongous amount of calories.

Apart from that, we use calories at rest for cognitive functions, protein synthesis, and blood circulation. All those processes are involuntary and happen whether we want it or not.

FUN FACT: Master chess players can burn up to 6000 kcal while playing a chess tournament!

With lower body weight, it takes fewer calories per day to be used.

Lower lean body weight

Lean body weight is basically your muscle mass. Muscle is metabolically active and hard to keep. It’s like a farm of animals that all need their food on a daily basis.

So imagine if all of a sudden you sell 1/3 of your animals to your fellow farmer, your demand for food goes down. People with high muscle mass burn way more calories than people with more fat mass.

FUN FACT: Muscle mass and brain takes up the most amount of oxygen in the body

Lower food intake

Did you know that we burn calories to digest food? And here’s the kicker, when we diet, we consume less food, and therefore, we burn fewer calories for processing and digestion.

Some foods require fewer calories than others. More on that in a moment. Also, fewer calories mean we gonna lower core body temperature.

FUN FACT: Did you know that shivering from cold increases your metabolic rate by 5-fold? (source)

The key takeaways from that are:

  • The metabolic slowdown is gonna happen no matter what
  • Lower body weight means you will need less energy
  • Lower food intake means you will burn fewer calories

Severe Calorie Restriction Side Effects

Now once you know why metabolic rate drops let’s move on to the serious precautions before starting any calorie restriction program.

In the past, it has been documented that severe calorie restriction without sufficient protein and micronutrients leads to side effects. It can lead to starvation, vitamin deficiencies, malnutrition, impaired immune system functions, dehydration, and osteoporotic bone fractures to name a few.

As you can imagine, people don’t do research on starvation or malnutrition. It is too dangerous. In fact, it is strictly forbidden.

However, there is one famous study done in the 90s by Ancel Keys called the “Minnesota Starvation Experiment” (source) where they documented the dangers of not eating enough calories. Group of 32 lean and young men stayed consumed 40% of their normal calorie intake for 6 months.

Key takeaways from the study

  • On average, men lost ~25% of body weight
  • 67% of the weight was from fat
  • 17% of the weight was from muscle
  • fat-free mass

But the weight loss wasn’t the only thing that changed. Apart from anthropometric changes, there were some severe psychological and physiological side effects (source).

  • chronic weakness
  • reduced aerobic capacity
  • severe painful lower limb edema
  • emotional distress
  • confusion
  • apathy
  • depression
  • hysteria
  • hypochondriasis
  • suicidal thoughts
  • loss of sex drive

Minnesota Starvation Experiment

The most fascinating is the fact that in 2003, 18 out of the 36 men were interviewed and all expressed strong convictions about nonviolence and said they would do it again (source).

Remember that those people did this experiment from a political standpoint towards wanting to make a meaningful contribution after World War 2. They haven’t done this experiment to reduce excess weight to feel and look better.

Also, keep in mind that when calorie restriction is done under a supervised environment with enough proteins and nutrients the health benefits way exceed health risks. To learn more about the health benefits of calorie restriction check out my article here.

How To Prevent Metabolism From Slowing Down When Dieting?

So you already know that severe calorie restriction isn’t recommended as a way to reduce excess weight, let’s have a look at the strategies to prevent metabolism to slow down when dieting.

The most effective way to prevent metabolism from slowing down when dieting is by a combination of high-protein and high fiber intake together with strength training. Protein and strength workouts trigger muscle protein synthesis and high-fiber foods lower hunger and suppress appetite.

We know that lean body mass is the kind of calories. In fact, BMR is the main driver for burning calories at rest and is contributing that to muscle mass.

So by keeping muscle active, adding progressive overload, and having a high volume of proteins on a daily basis not only we can prevent the metabolic slowdown, but also improve the metabolic rate while doing calorie restriction.

Resistance training

The more you move, the more calories you burn. But if you don’t move around, you won’t give your body reason to burn more energy. Lifting weights, doing kettlebell swings, or simple bodyweight exercises are all effective easy to work your muscles.

Also, low-intensity cardio like daily walking, gardening, or taking stairs instead of elevators adds up, and by not exercising you just leave a lot of calories on the table.

FUN FACT: NEAT (non-exercise activity thermogenesis) which is your daily activity outside of the gym burns more calories than one hour on the elliptical (source).

Protein

Remember when I told you that some foods burn more calories than others? There you have it. Proteins are by far the most metabolically costly to digest. The process of digesting protein produces several by-products like urea, ammonium, and other waste.

Then, our body need to eliminate those from the body. All the process cost energy.

Proteins are also used in muscle protein synthesis (muscle growth). Calorie restriction enhances muscle protein breakdown. This means without efficient stimulus (strength training) and building blocks (proteins), we gonna be losing muscle more rapidly than building it.

FUN FACT: Eating around 1.6 – 2.2 grams of protein per kilogram of body weight (or 1 gram per 1 pound) will prevent slowing down the metabolism.

Fiber

Have you ever wonder what is the biggest secret of people who are dieting for a long time? Eating high fiber foods. This will control your hunger and appetite hormones. When you eat whole foods that have a high satiety score, you feel less hungry.

Nutrients from high-fiber and minimally processed foods need to be digested differently than let’s say, ultra-processed foods. If you have a baked potato, your body will ramp up your metabolic rate to the moon to digest it.

But when you eat potato chips, the food has been processed and all you eat is an extracted starch which is so much easier to digest. So all the work is been done. No calories to burn.

Conclusion

Lean body mass is the most valuable asset you have when it comes to your metabolic rate. So if we would reverse engineer the process, by keeping a high intake of protein and regular strength training with progressive overload, the body will minimize the drop in metabolism.

That is what you want to avoid any metabolic changes. Bear in mind that some changes will happen, but even with minor drops in your metabolism (you can always add more walking during the day to compensate) the health benefits of losing weight and keeping it off are much more superior.

Michal Sieroslawski

Michal is an exercise physiologist (MSc), nutrition coach, Ashtanga teacher, and fitness blogger. He shares his successes and failures to help busy men and women squash down 20, 50, or even 100 pounds of fat without leaving their home.

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