What Dieters Need to Know About Metabolism

What Dieters Need to Know About Metabolism

By wlr Contributor Dr Muhamad Usman MD

How metabolism works is important for people trying to lose weight, Dr Muhamad Usman explains, pointing out the metabolic issues that dieters need to be aware of.

Metabolism is the set of chemical reactions that essentially keep the human body alive. These chemical exchanges help you grow, stay healthy, reproduce and do everything that you normally do.

Of the several things metabolism controls; weight management is the most important.

Understanding your body’s metabolism better and knowing how to speed it up can help you lose weight faster.

You can expect to learn the following things as you read on:

  • You will get to know about terms like Catabolism, Anabolism, ATP, Calories, Basal Metabolic Rate (BMR) and Resting Metabolic Rate (RMR).
  • A brief understanding of human metabolism and how it works.
  • Factors that affect the rate of your metabolic rate.
  • Some tricks on how to boost your metabolism.
  • Things that slow down your metabolism.
  • Effects different ways of losing weight have on your metabolism.

Some Definitions

ATP: ATP stands for Adenosine Triphospate. It is the currency of energy in your body. Everything you eat eventually gets converted into ATP.

Calories: Calories is another way of measuring the amount of energy in food items. For instance, 1 oz. cereal contains 150 calories.

One ATP molecule is equal to 7.3 Calories (2).

Catabolism: Catabolism is that component of your metabolism in which food molecules are broken down into ATP.

Here is an example of a catabolic reaction.

Glucose + Oxygen  >>>  Carbon Dioxide + Water + ATP

Anabolism: Metabolism is not all about the breakdown of food stuff. Metabolism also includes the conversion of simpler energy sources into more complex forms that your body could use for later.

Here is an example of an anabolic process.

Glucose + Water  >>>  Glycogen

(Storage form of glucose found in muscles)

So like yin and yang, metabolism also represents an intricate balance between catabolism and anabolism.

=Basal Metabolic Rate (BMR): Basal Metabolic Rate or BMR measures the amount of energy you burn at rest. It is the number of calories you burn while maintaining only the most vital of bodily functions like breathing (3) (4).

The higher your BMR, the greater number of calories you burn just to stay alive.

Resting Metabolism Rate (RMR): BMR and RMR are usually used interchangeably but there is a difference between the two. RMR represents the number of calories you need to maintain the most basic of your body functions plus some minimal physical activity as well (3) (4).

The difference between BMR and RMR is more of scientific value. So these terms will be used interchangeably in the text to come.

Metabolism - How It Works?  

The food you eat fuels all your activities, no matter how small. But, how come the energy trapped in foodstuff gets converted into a form that your body could use?

The one word answer to this question is “metabolism”. Metabolism consists of a set of chemical reactions that continuously convert the food you eat into usable energy. Let’s break it down further.

All food items are made of three main energy sources: carbohydrates, proteins and fats. These fuel molecules have energy trapped in them in the form of chemical bonds.

Your body, through a series of chemical reactions, converts these molecules into an energy form called “ATP” (1).

Think of ATP as the currency of energy in your body. Whenever you need energy for any activity, ATP comes into play and provides it.

Here is a simplified representation of how this works in your body.

Metabolic rate is the speed at which your body burns the calories you eat. The higher your metabolic rate; the speedier is your calorie burning process.

How Speedy is Your Metabolism? Calculating BMR

To cut a long story short, all your weight loss efforts revolve around 3 factors:

  • Your BMR
  • Calories you get from food
  • Calories you burn through every day movement and exercise

If you were to represent this information in the form of an equation, this is what it would look like:

Weight Loss = Calories from food – (BMR + Calories exhausted through exercise)

If you look at this equation closely, you’ll see that BMR is a free ride. That is because it is the number of calories you burn without having to do anything.

Here is how to calculate your BMR:

The Mifflin St Jeor Equation


10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) + 5


10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) – 161.

If you do not want to do these calculations, here is a BMR calculator.

So for a 24 year old male who is 5 foot 10 inches tall and weighs 70 kg, the value of BMR would be 1750 calories. It means he is burning 1750 calories on daily basis just by sitting and breathing in air.

Knowing your BMR is important because: It gives you an idea of how many calories you should consume on daily basis.

Here is a table to help you with your daily caloric needs.

Level of Physical Activity Daily Calories Needed
Little/No exercise BMR * 1.2
Light exercise (exercising less than 3 days a week) BMR * 1.375
Moderate exercise (exercising 3-5 days a week) BMR * 1.55
Very active lifestyle (exercising more than 6-7 days a week) BMR * 1.725
Extra active lifestyle (involved in an extra physical job) BMR * 1.9

What Factors Affect Your Metabolic Rate?

Factors that affect the rate of your metabolism can be roughly divided into two categories: modifiable factors and non-modifiable factors.

Non-Modifiable Factors

These factors affect your metabolism based on your personal physiology. You can’t do anything to control or alter these factors. These factors include:

  • Sex: According to research, basal metabolic rate is 5 to 6% higher in men compared to women. This discrepancy is explained on the basis of greater fat content/lower muscle mass among females (6) (7).
  • Age: Age is another factor affecting your metabolic rate. This may sound grim but your metabolism slows down as you age. The cellular machinery responsible for carrying out metabolic processes slows down as you age. Moreover, with age, the amount of fat you carry increases in comparison to the amount of muscle tissue in body (6) (8).
  • Height: Though not super important, height does affect BMR in a direct fashion i.e. the taller you are, the higher your BMR (6) (9).
  • Genetics: There has been a lot of research lately on the effect of genetics on different human traits and conditions. Researchers have found that there exists a strong link between genetics and metabolic rate (10). In one research, scientists found that there can be a 40 to 50% chance that the members of a family, especially twins, would have a similar basal metabolic rate (11).  

Modifiable Factors

These are some of the factors that you can alter or change in one way or another. These factors include:

  • Body Composition: Fat Free Mass (FFM) or lean body mass is one of the most important factors affecting your metabolic rate. Higher fat free mass means a higher metabolic rate. According to research, the difference in basal metabolic rate in up to 90% cases is solely due to the difference in lean body mass (12-14). As per one equation; metabolic rate = 370 x FFM. It is quite evident from this equation that metabolic rate is directly linked to lean body mass.
  • Diet: In the 1990s, researchers coined a “food-habit” hypothesis to explain the relationship between food and BMR. According to this hypothesis, three factors determine whether a food would increase or decrease your BMR. These factors include: food availability, predictability and food quality. If the food availability and predictability do not change, the only factor that seems to affect the BMR is the food quality. The food quality, in turn, depends on its features like digestibility, constituents and presence or absence of certain secondary chemicals (15). In general, protein based foods are high BMR foods. Whereas, fats and carbs are low BMR foods. More detail about food and its relation with BMR is in the sections to come.
  • Hormones: Hormones represent another important factor affecting your metabolic rate. Different hormones have different effects on metabolic rate. Here is a table to sum things up for you.
    Hormone Effect on Metabolism
    Thyroid hormone (16) Increases metabolism
    Cortisol (17) Increases metabolism
    Growth Hormone (18) Increases metabolism
    Testosterone (19) Increases metabolism
    Leptin (25) Increases metabolism
    Ghrelin (25) Decreases metabolism
    Estrogen (20) Decreases metabolism
  • Physical Activity: Physical activity increases metabolism. Researchers have known this fact for quite some time since metabolic rate is much higher among athletes compared to non-athletes. Three factors related to physical activity seem to affect the metabolic rate: the type of physical activity, its duration and intensity.

    Researchers at West Virginia University studied the effects of different kinds of exercise on the metabolic status of participants. The study included 20 participants that were divided into two equal groups. Both the groups were given an 800 calorie liquid based diet on daily basis. One group did aerobic training while the other did resistance exercises. After 12 weeks, researchers found that although both groups lost weight, it was more so in the resistance training group. Moreover, the increase in basal metabolic rate was seen only in the resistance training group (21).

    Moreover, the duration of exercise also matters. Those who exercise regularly have higher resting metabolic rates. On the other hand, those who do not exercise regularly, experience a 7 to 10% decline in their metabolic rate (22).

    Finally, one thing that matters is the intensity of workout. Researchers have recently discovered that doing bouts of high intensity exercise followed by periods of absolute rest is far superior in promoting weight loss and boosting metabolic rate when compared to conventional low intensity cardio workouts (23) (24).
  • Sleep: Obesity and diabetes are common findings among individuals with chronic sleep deprivation. There can be three main reasons for this trend: an increase in appetite, alteration in glucose metabolism and decrease in the rate of metabolism. The fall in metabolism with sleep deprivation can be attributed to an increase in ghrelin, decline in leptin and decrease in physical activity (25).
  • Hydration Status: Proper hydration improves metabolism due to two main reasons. First, water is the fundamental component of all the reactions taking in our bodies. In one research, dehydrated cyclists experienced a decline in metabolism and decrease in physical performance when compared to their hydrated counterparts (26). Second, water has a thermogenic effect of its own too. Drinking 500 ml water can increase metabolism by 30% (27).

How Dieting Effects Metabolism?

According to research, resting metabolic rate accounts for at least 65-70% energy expenditure in sedentary individuals (28). In other words, your resting metabolic rate is an extremely important determinant for your weight loss. Anything that slows it down would make weight loss more difficult. That being said, it brings us to question the efficacy of one of the oldest methods of losing weight- dieting.

Does dieting really help? How does it alter the metabolic rate?

Well, you may have observed that dieting alone does little to help you with weight loss. Also, you tend to gain weight faster than you have lost it as soon as you are done with dieting. Why is that?

Human beings have always had to contend with periods of famine. Only in the last one century has there been such an abundance of food for most people.

Before that, we have always been struck by regular periods of drought and food shortage. So, the human body is naturally designed to conserve calories for a ‘rainy day’.

When you severely restrict the amount of calories in your diet, your body can become more efficient at storing calories. As you diet, your metabolic rate can decrease.

Researchers at the University of Vermont studied the effects of dieting on metabolic rates and patterns of fat oxidation in obese individuals. The research included 20 obese females who were inducted in an 11 week long dietary regime.

Researchers found that although the participants lost up to 9 kg weight on an average, most of the weight loss was due to a reduction in lean body mass and water content instead of a decrease in fat mass. Moreover, the participants experienced a 15% reduction in resting metabolic rate and 12% reduction in fat oxidation (29).

Similar results were obtained from another study where participants following a diet plan based on a significant caloric restriction experienced a significant reduction in their basal metabolic rates and rates of fat oxidation (30).

What is the meaning of this? Does caloric restriction have no role in weight loss? Should you throw it out of the window?

What you need to understand first is the fact that crash dieting and calorie control are different things. Crash dieting plans usually give you around 500-800 calories per day and most of the focus of such plans is calories coming from fiber and proteins. Most calorie controlled diets, on the other hand, encourage a balanced way of losing weight in which you aim to burn more calories than you consume – creating a daily deficit that will lead to steady weight loss of around 1-2lbs a week.

Going back to the basics, weight loss is all about calories in and calories out. You cannot expect to lose weight as long as the calories you burn outweigh the calories you consume. Simple as this! So you do need some form of caloric restriction if you’re going to lose weight.

Research shows that caloric restriction, no matter slow or crash, eventually leads to a decrease in resting rate of metabolism (31) (32). This is not surprising since your weight itself is part of the BMR equation above. If you weigh less, you need fewer calories.

What can you do to evade this? The answer to this lies in combining caloric restriction with exercise.

In one research, scientists evaluated as many as 22 human trials that studied the effects of caloric restriction, with or without exercise, on weight loss and metabolism.

Researchers concluded that both the groups in almost all the studies lost weight and experienced a drop in resting metabolic rate too. But, the decrease in metabolism was smaller for dieters who exercised compared to dieters who did not (33).

Bottom line: The bottom line is that a decline in resting metabolic rate is an inevitable consequence of caloric restriction or any other pattern of dieting. The only way you can attenuate this is by coupling your dietary regimen with a proper exercise plan.

The Seesaw of Metabolism; How to Keep Things in Balance?

Now you know the factors that control your metabolism, let’s move on to discuss what you can do to boost it.  

Diet: Diet can have a big effect when it comes to boosting metabolism naturally.

Here are some foods that can boost your metabolism.

Spices like Chilies:

Spices really spice things up for you and your metabolism. These include turmeric, cinnamon, chilies, black pepper, white pepper, garlic, ginger etc.

In one piece of research, scientists studied the effects of chili consumption on the rate of metabolism in 12 subjects. Researchers found that  resting metabolic rate increased up to 20% within few minutes after consuming 5 gram chilies and the effect lasted for up to 30 minutes (34).


Protein has gained a lot of attention lately when it comes to weight loss. Rightly too because it curbs down appetite and increases the rate of metabolism.

Proteins are highly thermogenic foods. It means your body needs to burn a lot of calories to digest them, which is not the case with fat and carbs. In other words, eating more protein in your diet can boost your metabolism.

Researchers studied the thermogenic effects of high protein and high fat foods in obese females. They found that protein based diet caused 3 times greater thermogenesis when compared to fat rich diet (35).

Green Tea:

Green tea contains a number of useful chemicals like polyphenols and catechins that are known to promote weight loss, enhance fat oxidation and improve satiety (36) (37).

In one research, participants were randomly divided into two groups: one group received green tea on daily basis and the other received placebo. There was no difference in the dietary interventions among both groups.

At the end of the study, the green tea group lost more weight compared to the placebo group. Moreover, the green tea group experienced a significant boost in metabolism, where they burned an extra 200 calories per day on daily basis. Also, the green tea group experienced a significant improvement in fat oxidation (36).

Vegan vs. High Protein Diet:

There is no one single way to say which diet is superior but when it comes to boosting metabolism, a high protein diet holds the upper hand. Although a fibre rich diet leads to weight loss. This effect is largely due to the appetite suppressing effects of fibre rather than its metabolism boosting effects (38) (39).

Still, having a source of fibre in your diet is a great addition to your weight loss regimen.  


Caffeinated beverages have some benefits when it comes to weight loss. Caffeine promotes basal metabolic rate and fat oxidation. Drinking as much as 4 mg caffeine/ kg body weight can be a powerful metabolism booster (40).


As mentioned before, water has thermogenic effect of its own and serves a vital role in all cellular processes. Drinking 500 ml water can increase your metabolism by 30%. So you should aim on drinking at least 2 to 3 litres of plain water daily, depending on the local climate and level of your physical activity.

Physical Activity

A proper workout routine is one of the most effective ways to boost your metabolic rate. Although all forms and types of exercise improve metabolism, some methods work better than others.

That being said, the most effective metabolism boosting workout routine is alternating bouts of high intensity workout followed by periods of rest. This is known as High Intensity Interval Training (HIIT). In one piece research, scientists compared the metabolism boosting effects of traditional training and HIIT workouts. Results showed that HIIT was far superior to traditional training methods in increasing the resting rate of metabolism (41).

You should aim at working out 3-5 times and a total of around 150 minutes a week.


This is one unique way to improve your metabolic rate. You may have observed that you shiver when you go out in the cold or bathe in cold water. Shivering is your body’s natural way to generate heat by triggering your metabolism.

That is the working principle of hydrotherapy. In hydrotherapy, you expose yourself to cold water. According to researchers at the University of Ulster, immersion in cold water (< 15 C) for up to 5 minutes a day can significantly boost the rate of metabolism, increase fat oxidation and enhance recovery time in case of injuries (42).

Adequate Sleep:

Sleeplessness disturbs your energy balance and distorts your metabolism in several ways, as mentioned before.

Sleep deprivation has transient benefit in terms of improving metabolism, but it deteriorates your metabolic profile in the longer run. Getting the right hours of sleep at the right time can undo this imbalance and bring your metabolic profile back to normal.

So you should at aim at getting 7 to 8 hours of uninterrupted sleep at night.

Talk to A Doctor For a Hormonal Disorder:

If you are doing everything right and still have failed to lose weight then the problem maybe deeper than simple lack of exercise or overeating. There may be something wrong with your hormones. As mentioned before, the lack or excess of a couple of hormones can slow down your metabolism and can make weight loss very hard for you. For that, you need to talk to your doctor. Go to your doctor and get yourself assessed for a possible hormonal disorder that may be disturbing your metabolic health.

Metabolism Boosting Supplements:

You can cheat your way in boosting your metabolism by eating certain supplements.

But, always consult your healthcare provider before starting with a supplement. Never use these supplements on your own.

Here are some supplements that have shown to be effective in improving metabolism.

Supplement Main Effects Safety Profile
Omega 3 fatty acids (43) Increases energy expenditure by 14%, resting metabolic rate by 10%, fat oxidation by 19%. Mostly safe
Bitter Orange (44) Possible metabolism boosting effects Some safety concerns. Observe caution
Caffeine (44) Stimulates metabolism and fat oxidation Safe as long as daily dose is less than 400 mg
Fucoxanthin (44) Increases energy expenditure and fat oxidation No safety concerns reported
Green Tea (44) Increases energy expenditure and fat oxidation Largely safe
Pyruvate (44) Increases energy expenditure Some safety concerns. Observe caution

Take Our Free Trial

... to find out how many calories you are burning each day, and how many you are eating. It takes just 2 minutes to get instant access to all our great tools:

  • Daily Calorie Allowance - tailored to you;
  • Interactive Goal Setting tool - set a goal that works for you;
  • Online Food and Exercise Diary - the easy way to count calories;
  • Calorie and Nutrition Database - quickly find how many calories are in the foods and drinks you consume;
  • and Exercise Database - see how many extra calories you burn doing the activities you enjoy.

Genuinely free for 24 hours — no credit card details required.

Take Our FREE Trial »


  1. Nutrient Utilization in Humans: Metabolism Pathways
  2. ATP and calories.
  3. BMR vs. RMR
  4. Difference Between Basal Metabolism & Resting Metabolism
  5. BMR calculator
  6. The Factors Affecting Normal Basal Metabolism
  7. Is resting metabolic rate different between men and women?
  8. Relationship between basal metabolic rate, gender, age, and body composition
  9. Basal metabolic rate and body composition
  10. Lower metabolic rates of post-obese versus lean women
  11. Genetic effect in resting and exercise metabolic rates
  12. Body composition as a determinant of energy expenditure
  13. Reexamination of the relationship of resting metabolic rate to fat-free mass and to the metabolically active components of fat-free mass in humans
  14. Basal metabolic rate, fat-free mass, and body cell mass during energy restriction.
  15. The Relationship between Diet Quality and Basal Metabolic Rat
  16. Thyroid hormone regulation of metabolism.
  17. Relationship between basal metabolic rate and cortisol secretion 
  18. Basal metabolic rate in adults with growth hormone deficiency
  19. Effect of testosterone on metabolic rate and body composition
  20. Sex hormones, body fat distribution, resting metabolic rate 
  21. Effects of resistance vs. aerobic training combined with an 800 calorie liquid diet on lean body mass and resting metabolic rate.
  22. Impact of energy intake and exercise on resting metabolic rate.
  23. Effects of exercise intensity and duration on the excess post-exercise oxygen consumption
  24. The effects of increasing exercise intensity on muscle fuel utilisation in humans
  25. The Metabolic Consequences of Sleep Deprivation
  26. The effect of dehydration on muscle metabolism and time trial performance during prolonged cycling in males
  27. Water-induced thermogenesis.
  28. Effect of caloric restriction and excessive caloric intake on energy expenditure. 
  29. Decrease in fat oxidation following a meal in weight-reduced individuals
  30. Effect of caloric restriction and excessive caloric intake on energy expenditure
  31. Short-Term Changes in Body Composition and Metabolism With Severe Dieting and Resistance Exercise
  32. Long-term calorie restriction decreases metabolism
  33. Effects of dieting and exercise on resting metabolic rate and implications for weight management
  34. Effect of chili pepper (Capsicum frutescens) ingestion on plasma glucose response and metabolic rate in Thai women.
  35. Diet-induced thermogenesis
  36. Effectiveness of green tea on weight reduction in obese Thais: A randomized, controlled trial.
  37. The effects of green tea consumption and resistance training on body composition and resting metabolic rate in overweight or obese women.
  38. Basal metabolic rate in carnivores is associated with diet after controlling for phylogeny
  39. Resting metabolic rate and thermogenic effect of food in vegetarian diets compared with Mediterranean diets.
  40. Caffeine and coffee
  41. HIIT vs Traditional training
  42. Hydrotherapy based improvement in BMR
  43. Omega-3 Fatty Acid Supplementation
  44. Weight Loss supplements
Lose a Stone for Christmas Challenge

If you enjoyed this article, try our newsletter. It's free.

Receive the latest on what works for weight loss straight to your inbox. We won't share your email address. Privacy policy