Let’s Demystify the Biological Concept of Homeostasis

Hamna Qasim
10 min readDec 30, 2020


Image Source: iStock

Introduction to Energy Homeostasis and Energy Balance

Remember, it’s a critical topic that forms the basis for our understanding of weight gain and obesity. So let’s start by looking a bit more into the regulation of food intake.

The energy balance is the balance between energy intake and energy expenditure.

It’s quite fascinating how most people can almost perfectly match energy intake to their energy expenditure. At least in the long-term, you will read texts and study cartoons that will teach you the basics of hunger and satiety. Next, read the article and spend quite a bit of time on the energy value of nutrients: fats, proteins, and carbs.

They provide energy and calories, but how does that work? And how did we figure out that one gram of carbohydrate contains four calories and nine calories for one gram of fat?

Some people say: “a calorie is a calorie,” but what do we mean by that? And is it true? Energy expenditure balances energy intake. And did you know that maintenance covers about 60–70% of our daily energy expenditure? And that physical activity only takes up about 30–40%, at least for most of us?

So how can we increase energy expenditure? There are two ways:

  1. By building muscles
  2. By being more physically active

And obviously, these two are somewhat interrelated.

Energy Balance

Energy balance is the balance between energy coming in (energy intake) and energy going out (mainly energy expenditure). When energy coming in exceeds the energy going out (people consume more calories than they burn), people gain weight. When energy expenditure exceeds energy intake (people consume fewer calories than they burn), and lose weight.

The three macronutrients carbohydrate, fat, and protein, contain energy and thus contribute to energy intake. The energy consumed and not immediately used as fuel can be stored in the body, mainly body fat. The stored energy can be utilized when there is a need for it. If energy intake equals energy expenditure, the energy storage size does not change, and a person is in energy balance.

Energy balance is the balance between total energy intake and expenditure. Strictly, this is not correct. The reason is that energy expenditure is not the only way by which the body loses energy; some energy is also lost in the stools and urine.

However, it is essential to realize that the loss of energy in stools and urine is already considered when calculating food content and energy intake.

When energy coming in exceeds energy going out (left picture), a person is in a state of positive energy balance — the excess energy stores in the body, mainly as body fat. When energy going out exceeds energy coming in (right picture), energy is lost from the body, resulting in a decrease in fat stores.

Regulation of Food Intake

Food Intake

First, we need to have a common understanding of the terminology relevant to food intake. Hunger is the uncomfortable sensation caused by a lack of food that makes people look for it. Appetite is the integrated response to the sight, smell, thought, or taste of food that triggers eating. Sometimes appetite is driven by hunger, but it is often due to cravings, habits, food availability, boredom, or other social and emotional factors.

Hunger is a very dominant sensation that impairs a person’s ability to focus on other tasks. People can have an appetite without being hungry. Otherwise, nobody would order anything from the dessert menu! You can also have no desire for food even though you are hungry, which may happen in a stressful situation or during an illness.

Satiation and Satiety

The two other terms that require some explanation are satiation and satiety.

Satiation is the feeling of satisfaction and fullness during a meal, which makes you stop eating. It determines how much food is consumed during a meal and is your immediate reaction to the ingestion of food: the drive that causes you to stop eating.

On the other hand, satiety is the feeling of satisfaction that occurs after a meal and prevents you from eating. Satiety determines how much time passes between meals.

Please note that the distinction between satiation and satiety is not in standard (colloquial) language, and the two terms are often used interchangeably.

Take a careful look at the two meals below. Which meal is more satiating? Let me know in the comments section below!

The correct answer is the meal on the right as it has much more volume, water, and fiber. It will probably take nearly an hour to finish this meal, whereas one could finish the left meal in 15 minutes or less. Because a meal such as the left one is not very satiating, it promotes overeating. Therefore, the brain doesn’t receive the sensory input that tells it to stop eating.

Foods that promote satiation are generally very bulky, have a high water and fiber content, and carry vital sensory attributes. Eating vegetables causes satiation. It is essential to realize that food’s satiating properties are not determined by its nutrient (and water) content. The texture is also significant.

For example, there is evidence that eating an apple is more satiating than drinking apple juice, even if that apple juice was prepared in a blender using the entire apple. It has been observed that consuming energy-containing beverages, which ingests quickly, does not make people eat less of other foods after that, perhaps because calories that swiftly ingest are not sensed correctly.

Prolonging the exposure time to foods (chewing!) causes people to quit eating sooner and consume less energy in total. These findings suggest that consumption of sugar-containing beverages may lead to overeating, which may contribute to the growing occurrence of obesity in many countries.

The beverage industry (and some nutrition scientists) oppose the notion that energy from fluids is less satiating than from solids and that consumption of sugar-containing beverages may promote overeating. However, it seems that some companies are starting to give in and accept this notion.

Energy Value of Nutrients

Consumption of food provides energy in the form of 3 macronutrients: carbohydrate, fat, and protein. The proportion of total energy intake contributed by these macronutrients varies from day to day and between individuals.

In most western societies, protein provides about 10–15% of energy intake, carbohydrates about 50%, and fat the remaining 35% — but these are averages.

Does this mean that we eat more than twice as much fat as protein? The answer is no.

Because fat is more energy-dense, which means that it contains more energy per unit of weight, fat contains 9 calories per gram, whereas protein and carbohydrate only have 4 calories per gram.

Kcal is an abbreviation for kilocalories and is a unit of energy. We often ignore the kilo, and we talk about calories. How did we get at these numbers? How do we get at the 9 calories for fat, the 4 calories for protein and carbohydrate? These factors are called the Atwater factors. Thus, you must know the body’s flow of dietary energy to understand the Atwater factors.

The total amount of energy in food is known as gross energy. The food’s gross energy is calculated by burning the food in a particular device called a bomb calorimeter and measuring how much heat liberates. However, not all the energy consumed in the form of the three macronutrients enters the body.

A small portion leaves the body undigested in the stools. It’s assumed that the absorbed proportion, which we call digestibility, is a constant value for each of these macronutrients. This digestibility is assumed to be 98% for carbohydrates, fat 95%, and protein 92%. It means that 92% of the energy present in dietary protein is taken up into the body, whereas the remaining 8% leaves the body undigested. The amount of energy that is absorbed is called digestible energy.

But there is additional loss of energy once the macronutrients absorb into the body.

This is because the body can not completely utilize the energy present in dietary protein.

About 20% of the energy present in absorbed amino acids is lost in the urine, as part of urea. Absorbed fat and carbohydrate completely combust to carbon dioxide and water; therefore, there is no additional energy loss once these two macronutrients are absorbed.

After the energy loss in the urine, the amount of left energy is called the metabolizable energy. Therefore, it is the energy available for the body to use after losing dietary energy in stools and the urine.

The critical thing to realize is that the energy content of foods (described on the food label on the package) is calculated based on metabolizable energy. It does not represent the total amount of energy present in food, which is the gross energy.

As we pointed out, the metabolizable energy for carbohydrate is 4 calories/gram, 9 calories/gram of fat, and 4 calories/gram of protein — these numbers are known as the Atwater factors. They base upon W.O. Atwater and his colleagues at the United States Department of Agriculture at the end of the 19th century.

Using these Atwater factors, you can easily calculate the energy content of food based on macronutrient content. For example: after determining what the whole milk contains, you can easily calculate 4, 5 grams of carbohydrate, 4 grams of fat, and 3, 5 grams of protein/100 grams of the product. Thus, you can easily calculate the energy content of 100 grams of whole milk.

This is what we refer to as the Atwater factor. The critical point I want to emphasize is that we use metabolizable energy to refer to the energy content of a food as written on the food package.

Flow Chart of Energy

The picture provides a graphical illustration of the different types of energy. The total energy coming in (gross energy) is equal to the energy going out in the energy balance.

The latter consists of two components:

  1. The energy lost in stools and urine
  2. The energy lost as energy burned (expenditure)

As indicated already, the energy content of foods and our energy intake calculation is based on metabolizable energy. It means that energy loss in stools and urine has already been taken into account in energy intake analysis. Hence, in people that are in energy balance, energy intake (as metabolizable energy) equals energy expenditure (energy going out). The metabolizable energy is available for combustion to yield net energy.

Net energy is available for performing various tasks in the body. These energy-consuming tasks divide into three main categories:

  1. Maintenance
  2. Physical activity
  3. Growth

Daily energy expenditure is about 2000 Kcal for an average female and about 2500 Kcal for an average male. These numbers can be substantially higher or lower depending on body size, illness, and physical activity.

Weight Gain and Loss

Image Source: scientificamerican.com

So do you know what happens during weight gain and loss from an energetic point of view?

Let’s analyze the situation when you would eat a little bit more food daily. For instance, every day, you would increase your energy intake by one can of coke. What you may expect is that your weight will continuously grow because you will enter into a state of a positive energy balance. You are consuming more energy than you are expending, and as a result, you will gain weight.

But at a certain point, that weight gain will level off, so your actual weight gain is different than what you’d expected. The reason is that your energy expenditure will increase. As you become more solemn, your energy expenditure will increase as you need more energy for your maintenance, your basal metabolic rate will increase as you will become heavier. But also, physical activity will take up more power as you will gain weight.

Lifting an arm and walking will cost more energy. As a result, you will reach a new equilibrium between energy intake and expenditure, but now at a higher level of input and expenditure and higher body weight. So what about the opposite situation?

This is a fundamental issue for many people. They reduce energy intake by a certain amount, for instance, 10% or 20%. They experience that after several weeks of very rapid weight loss, a decrease in weight starts to level off. And at a certain point, they don’t lose any more weight at all. They’ve reached a new equilibrium, where their energy expenditure and intake are balanced or equal. It’s now the opposite situation as we saw for weight gain.

As your energy intake goes down, you start losing weight. Your energy expenditure will also go down because you will consume less energy sitting or lying down as a smaller, less bulky person. Still, even during physical activity, you have less body weight to move around.

As a result, your total energy expenditure will go down. You will reach a new equilibrium between energy expenditure and intake at lower body weight. That’s also very frustrating for many people because they have to decrease energy intake when they want to lose weight.

That’s a crucial point that I want to get across.

As seen on:

  1. https://www.linkedin.com/pulse/lets-demystify-biological-concept-homeostasis-hamna-qasim/?published=t
  2. https://hamnaqasim.com/2020/12/30/lets-demystify-the-biological-concept-of-homeostasis/
  3. https://issuu.com/hamnaqasim90/docs/let_s_demystify_homeostasis_.docx



Hamna Qasim

A Millionaire in the Making || Driven Entrepreneur || COO at Inteliract