Metabolism: Myths and Realities

Manoj Pandey*

The word metabolism is quite often tossed in our conversations on health. Interestingly, people carry different notions about metabolism even when talking about it with the same group of people for a long time. So, if you feel exhausted, you tend to blame it on a weak metabolism; you seem to be having an unusual metabolism when you gain weight; your health is always poor as you have inherited a defective metabolism; you must do strenuous exercises to improve your metabolism; and so on.

Most of us have ingrained this expression by hearing from others, from the web or from the doctor. We tend to leave out the context in which the word was (correctly) used, and we blame it for many of the unwanted changes or disorders relating to our weight, energy level and overall health.

Let’s try to clear the fog around metabolism.

What is metabolism?

Metabolism is nothing but a collective term for the chemical reactions taking place all the time in our body. 

Chemical reactions are what sustain all the biological processes. Without them, we would not be able to extract nutrients out of the food we eat, or breathe, or excrete the waste out of the body. Even a seemingly non-chemical action such as seeing is the result of a series of complex chemical reactions. 

So, our body is all the time engaged in what can be called metabolism.

Metabolism has to be two-way: some reactions should build body-parts and store energy, while others should utilize the available energy and clean the body of the old and sick body-parts. In a healthy living being, both types of metabolism happen in a balanced manner. 

If you have never thought about it, it might fascinate you to know that trillions of chemical reactions keep taking place in our body even when we are in deep sleep. At the molecular level, even lifting an eyelid would need millions of chemical reactions taking place in a split-second. There is an enormous amount of intelligence contained in the genetic material, which determines what, how and when a particular type of chemical reaction should occur in which part of the body. This intelligence gives direction to another set of wonderful chemicals such as hormones and enzymes, and neurochemicals. These, together with resultant chemicals in each stage of reaction, determine the functioning of each cell, tissue and organ – and the entire body.

Generally speaking, when big molecules are broken into smaller ones, their chemical bonds are broken with the help of enzymes, and energy is released. For example, a molecule of glucose breaks into carbon di-oxide and water through a series of chemical reactions, and releases a tiny amount of energy. When the body has to make bigger molecules out of the small ones (e.g. for growth of organs, repair of broken tissues, converting extra energy into fat), chemical bonds must be formed, needing a lot of energy. This happens all the time in living beings in an enormously intelligent and immensely precise manner.  

Metabolism in humans: does that sounds something different?

Metabolism in humans happens the same way as it happens in an elephant, a dog, a fish.

You read that right. Some of the chemical reactions through which food is broken down and then energy is produced from the food are similar in almost all living beings. 

But when we talk of metabolism in common parlance, we do not look at the enormity of chemical reactions, and we do not include all types of processes; we mostly refer to the utilisation of food by the body. In fact, we narrow it down further to the intake and use of energy

With that background, let me flash a jargon: metabolic rate. Metabolic rate is a measurement (in Calorie or kilocalorie, which is a unit of energy) that a person’s body spends in a day. When you are very active, your body works a lot, which in turn needs a lot of energy, and, therefore, your metabolic rate goes up.   

That brings us to another oft-touted term, basal metabolic rate or BMR. This is the measurement of energy spent without doing any physical work. Thus, this is the energy spent on inner body functions on which we have little control. Another related term is resting metabolic rate (RMR). It is the metabolic rate measured while you rest. Some low-key physical activities are added to the sleeping stage, and, thus, it is nearly as much as the BMR, with a few calories added. 

Activities that keep happening all the time in the human body – heart beat and flow of blood in arteries and veins, breathing, and overall maintenance – consume about half of the total energy spent by the person. When he/ she is not active, his/ her body uses up as much as 80 percent of the total energy in these functions. 

One of the very important roles of metabolism in humans and other warm-bodied animals is maintaining a constant body temperature. It needs a lot of energy to keep the body warm when outside temperature is low, and it also needs energy to keep the body cool when the outside temperature is high. 

BMR/ RMR depend on more than a dozen things. Body mass is one big factor. In general, more muscular and heavily built people have a higher BMR. 

Since muscles take up a lot of energy for their maintenance (obviously, their energy usage when not at work is part of BMR), people with a higher ‘lean body mass’ or musculature spend more energy even without doing much work. On the other hand, weight due to fat deposition slows BMR, because fatty tissue does not need much energy for its maintenance.

How much food you consume is one big factor affecting the BMR. If you take less than the required amount of food, the body tends to adjust by lowering BMR. That explains why regular fasting or dieting becomes less and less effective in reducing weight of obese people.  

Young people need much more energy for growth and also for maintaining their body temperature due to an active routine. Aged people, for the opposite reasons (no growth, less activity) and receding muscle mass, have a low BMR. That explains, to a good extent, why many people start gaining weight once they cross 50 or so. 

Hormones play a significant role in metabolism, and the effects of imbalance in thyroid hormones are the most pronounced. When the thyroid gland becomes very active (called hyper-thyroidism), the body becomes very active and thus spends a lot of energy all the time, resulting in marked weight loss. On the other hand, when thyroid becomes lethargic (hypo-thyroidism), the body becomes less active, resulting in an unusual weight gain. 

Heredity plays a significant role too, but in most cases the differences among individuals are fine and they occur gradually. Only in some cases, the effect of defective genes is prominent. For example, different types of food-intolerances happen, because the body is not able to digest those types of chemicals in the food. 

Exercise not only directly expends energy, it helps in weight-loss by building muscles and thus using up energy even when one is not doing exercise. However, that gain is not permanent. Studies have found that even strenuous exercise does not lead to a sustained increase in the metabolic rate. These facts should explain why exercise is generally the most important part of any weight-loss regimen, and also why the majority of fat people regain weight when they abandon their exercise routine. 

Do you know that quite a significant amount of energy is taken up in digestion itself? While fats and carbohydrates need lower amounts of energy for their digestion, protein intake can raise BMR by up to 30%. The digestive system starts sucking a lot of energy as soon as you start eating and it peaks in about 2-3 hours. No wonder, you feel sleepy after a heavy meal.

Since in the fitness parlance, weight is highly correlated with metabolism, let me throw another term: the body mass index or BMI. It is a measurement obtained by dividing a person’s weight by height. When BMI is lower than normal levels, one is said to be underweight and when it is higher, the person is dubbed overweight. As you would have already sensed, BMI is a rather crude measurement because the weight does not differentiate between a muscular body and a rotund one. 

An argument is often heard among roly-polies: my weight is due to my slow basal metabolism, and I cannot do much about it. Generally speaking, that is one main – but not big – reason for some people gaining weight faster than others. But one cannot solely blame inherited metabolism for unusual weight, for more reasons than one.

Ironically, overweight people tend to have a high overall metabolic rate, because their basal metabolic rate itself is quite high (because their bulky bodies need that much more energy just for inner functions). So, blaming a slow metabolism might, in many cases, is just an excuse at the cost of one’s health. Since we do not have much control over the BMR, it helps to check other factors. The activities we undertake, the food we eat and the overall healthy/ unhealthy lifestyle that we live determine how much fat we’d carry in our belly and buttocks. 

The nitty-gritty of conversion of food energy into activity and body fat

When we consume food, its major constituents (proteins, carbohydrates and fats) are broken down to very small molecules and these are absorbed by our small intestine. The nutrient-laden blood from the intestines is pumped into the liver. 

The small molecules of protein (called amino acids) are synthesised back into proteins that our body needs for giving it structure, making muscles and other tissues, making enzymes (which facilitate working of other molecules), etc. Carbohydrates are absorbed as simple sugars (mostly glucose, and some fructose and galactose -and these two are also converted to glucose in the liver). Glucose is used directly in providing energy to different parts of the body to function, and the excess of glucose is stored in the liver in the form of glycogen. When there is excess glucose in the blood and the liver is unable to convert it into glycogen (due to defects which are commonly called diabetes, or due to glucose being so much that even a healthy liver cannot convert it all), it gets converted into fat, which is deposited in special cells found mostly under the skin. The fats that we consume in our food are broken down as fatty acids, which are used in various bodily functions; excess of these molecules is also converted into fat, which ends up in fat deposits. When the body does not get sufficient amount of energy from food and stored glycogen, the stored fat is broken down to release energy. When no regular source of energy (sugar and fat) is available, proteins are broken down, as happens in starving people.

A hormone secreted by pancreas, called insulin, controls conversion of sugar into glycogen and back. When the blood starts receiving sugar molecules after eating food, insulin increases the flow of sugar to muscles so that it is consumed and temporarily stored there. It asks the liver to convert excess sugar into glycogen. This glycogen is broken into glucose as and when muscles and other body parts need energy for their functioning. Insulin also controls sugar metabolism by regulating the break-down of stored fat into sugar.

If insulin is not present in adequate amount, sugar level in the blood shoots up after meals and dips precariously when food has not been taken for many hours. That is the reason why diabetic patients have problems immediately after eating and when they have not eaten for hours; and that also explains why blood sugar levels are measured in the morning before breakfast and about two hours after eating a meal. 

Measuring metabolism

If you are not into calorie-counting, you might be unsure about these units of measuring energy stored in food, as they are used in the media and on food containers: calorie, Calorie/ kcal, and kilojoule/ kj.

Calorie (written with C in caps) or kilocalorie or kcal equals 1000 calories. As calorie is a very small unit, Calorie and kcal are commonly used when talking about ‘calorific value’ of foods, and energy spent in exercise. A term in use in food packets, especially in some European countries, is kilojoule. A Calorie equals approximately 4.2 kilojoules. 

Let’s also look at the scales of the commonly used measurements discussed in this article.

If I were to give a highly generalized figure of BMR of an Indian adult, it is around 1500-1700 kcal. As mentioned earlier, it depends on age, sex and many other factors.

There are many formulae for calculating MBR/ RMR based on weight, height and age, but none of them can measure the actual energy usage for a particular individual. One of the most widely used calculations is Harris-Benedict Equation. Even this formula is found to be only up to 70% accurate.

If you want to calculate your BMR, you can use this formula:

  • Men: BMR = 88.362 + (13.397 x weight in kg) + (4.799 x height in cm) – (5.677 x age in years)
  • Women: BMR = 447.593 + (9.247 x weight in kg) + (3.098 x height in cm) – (4.330 x age in years)

Coming to the body mass index or BMI, a person with BMI between 18.5 and 24.9 is taken as of normal weight. Those with BMR less than 18.5 are said to be underweight, while those  with BMI between 25 and 29.9 are taken as overweight. A person with BMI 30 or more is branded obese. 

The formula for calculating one’s BMI is simple. Divide your weight in kilograms by the square of your height in meters. For example, if your weight is 70 kg and height 1.68 m, your BMI will be 70 divided by 1.68 x 1.68 =  24.8

How about some takeaways?

This article is not from a medico or a weight trainer. At the same time, a proper understanding of metabolism gives us some handy (and sound) pointers for overall health – and, so, allow me to share a few of them here.

For losing weight, we should not go unmindfully for fasting. Fasting is good for health, but regular fasting can lead to many problems such as nutrient deficiencies. From metabolism point of view, fasting can be effective in reducing weight only to an extent, because a reduced food intake gives a signal to the body to slow down metabolism. This, in turn, lessens the effectiveness of fasting in reducing body weight. In addition, severe fasting results in the loss of muscle mass, which further dampens the metabolic rate.

If we are physically very active due to a healthy lifestyle or needs, we do not need further exercise, speaking solely from the weight angle. This category would cover farm-workers and labourers and those who do all their household chores themselves. In all other cases, we should exercise regularly, and adjust it according to our age and lifestyle. From the metabolic point of view, exercising helps in building muscle mass (in younger age) and retaining it (in older age). That gives us good weight without unwanted fat deposition. As explained above, muscles take up a lot of energy, and when your body has good muscle mass, it helps in burning calories and thus controlling the weight.

Health and fitness experts have different views about how much one should exercise, and it depends on factors specific to each individual. At the same time, they generally advise at least half an hour of moderate-intensity physical activity every day. If you are overweight, you may like to do more strenuous exercise. However, the type and amount of exercise need to be decided based on factors such as age, overall fitness, injuries, special situations (e.g. pregnancy) and so on.

If you have been gaining weight after your 50th birthday or later, there is a chance that your muscle mass is reducing and the body is not utilising the energy that you obtain from your food, thus creating a positive energy balance, which in turn is converting into body fat. If so, increase your physical activity. 

A number of fitness apps and wearable devices have come up in recent years. These tend to focus on various numeric values related to health. Out of them, calculations related to metabolism are among the most used. Fitness freaks are seen regularly checking their calorie intake and other measurements, and taking actions based on them, thus undermining the traditional wisdom and body’s natural demands. A more common-sensical approach towards food intake and exercises, perhaps, works the best except in case of special situations and disorders. I have seen experts’ warnings online that an obsession with weight is leading to grave nutrition deficiencies and psychological disorders in the society, especially among young women.

Since body weight is not just the balance between calories taken up (as food) and calories burnt (by physical activity), focusing mainly on calculations based on these numbers is closing one’s eyes to other ways of keeping fit. For example, except for some forms of asanas, most yoga and pranayama postures/ activities neither tone up muscles nor expend too much energy; yet, by toning up the endocrine and nervous system and other ways, they seem to be optimising one’s metabolism. 

Before we say goodbye

Using this calculator, you can calculate the amount of energy you are likely to spend in a day, based on your physique and level of activity:

Manoj Pandey is a former civil servant. He does not like to call himself a rationalist, but insists on scrutiny of apparent myths as well as what are supposed to be immutable scientific facts. He maintains a personal blog, Th_ink

Disclaimer: The views expressed in this article are the personal opinion of the author and do not reflect the views of which does not assume any responsibility for the same.


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