Trait#87: Basal Metabolic Rate (UCP2)
Dr Haran Sivapalan
December 14, 2020
How much energy do I use each day?
Like other living organisms, we use energy to move, grow, reproduce, break down and use nutrients, and carry out other essential biochemical processes that allow us to sustain life.
The total amount of energy that we use each day for these processes is known as our total energy expenditure (TEE).
There are three main components of our total energy expenditure:
- Basal metabolic rate (or resting energy expenditure [REE]) – the energy used at rest to maintain basic life-sustaining functions (e.g. breathing, blood circulation, metabolic reactions, regulation of fluid balance). Basal metabolic rate accounts for around 60-80% of our total energy expenditure.
- Diet-induced energy expenditure [DEE] (also known as diet-induced thermogenesis [DIT] or the thermic effect of food) – the energy used to digest and absorb food. This accounts for about 10% of our total energy expenditure.
- Activity-induced energy expenditure [AEE] – the amount of energy spent on physical activity (PA), including purposeful activity such as exercise, and non-purposeful activity such as fidgeting. AEE makes up 15-30% of total energy expenditure.
The chart below shows the relative contributions of basal metabolic rate, diet-induced energy expenditure (or DIT) and activity-induced energy expenditure (or PA (physical activity) to our total energy expenditure. It also shows some of the factors that can affect each component of our total energy expenditure.
Our total energy expenditure is the sum of these three components:
The SI units for energy (energy expenditure) are joules (J), but most of us are perhaps more familiar with calories (kcals). One calorie (kcal) is equivalent to 4.184 kJ of energy. *
Another way of looking at your total energy expenditure, then, is to say that it’s simply the total number of calories (kcals) you burn each day.
(*To confuse matters, 1 kcal (kilocalorie) is known as ‘large calorie’ (sometimes denoted Calorie with a capital ‘C’). A small calorie (cal) is one thousandth of a large calorie and is equivalent to amount of energy required to raise the temperature of 1 gram (g) of water by 1º Celsius (º C). For simplicity, we will use the term ‘calorie’ to mean ‘kcal’).
- We expend energy on life-sustaining processes, digestion and absorption of food (diet-induced energy expenditure), and physical activity (activity-induced energy expenditure).
- Our total energy expenditure (TEE) is the total amount of energy we use each day on these activities.
- Basal metabolic rate (BMR) is the amount of energy we use at rest on life-sustaining processes (e.g. breathing, brain activity).
- Total energy expenditure = BMR + Diet-induced Energy Expenditure + Activity-induced energy expenditure.
- Our total energy expenditure refers to the amount of calories (kcals) we burn each day.
How do I lose weight?
In order to expend energy on movement, growth, digestion, and all the other processes that make up our total energy expenditure (described previously), we first need to take in energy. We of course do this by eating food, which contains chemical energy (measured in calories). The total amount of calories we consume in food each day is known as our energy intake.
The difference between our energy intake and total energy expenditure is known as our energy balance. You may have heard this more popularly referred to as the ‘Calories in vs. calories out’ equation.
According to the first law of thermodynamics, (also known as the principle of conservation of energy), energy cannot be created or destroyed, it can only be converted from one form into another. The human body is no exception to this law.
If we were to go for a run, for example, the chemical energy from our food gets converted into kinetic energy as we move, while some gets turned into (“lost” as) heat energy. The total energy of the system, however, remains the same.
It’s easy to see this thermodynamic law being obeyed when we have a perfect energy balance – that, is when Energy intake = Total Energy Expenditure.
In this scenario, the amount of energy (number of calories) we get from food is exactly equal to the amount of energy (number of calories) spent on physical activity (AEE), digestion and absorption of food (DEE), and basic life-sustaining processes (BMR). No energy has been created or destroyed.
But, what happens when energy intake is not equal to (i.e. greater than or less than) total energy expenditure?
Weight gain results from a positive energy balance
A positive energy balance describes when our energy intake is greater than our total energy expenditure. In other words, the number of calories we eat exceeds the number of calories expended.
So, what happens to the excess energy we’ve consumed?
Remember that the first law of thermodynamics dictates that we cannot simply destroy this excess energy. Instead, the excess energy gets stored as chemical energy in the body.
This stored chemical energy can take various forms, including fat (triglyceride) stores, glycogen stores, glucose, new proteins (e.g. muscle proteins), bone, other new tissues and cell components.
Consequently, when our energy intake exceeds our total energy expenditure, we end up gaining weight. Much of this weight gain is due to increases in body fat, with studies suggesting that body fat accounts for 60-80% of weight gain.
From an evolutionary perspective, this is because fat is a very useful, highly concentrated energy store, with a gram of anhydrous fat providing more than 6 times as much energy as a gram of glycogen.
Consequently, we mainly store excess energy as fat, with the typical 70kg man having 100,000 kcals of chemical energy stored as fat (triglycerides), 25,000 kcals as protein (mainly in muscle), 600 kcals as glycogen, and 40 kcal in glucose.
Weight loss requires a negative energy balance
When our total energy expenditure exceeds our energy intake, it is known as a negative energy balance or energy/caloric deficit.
In other words, in a negative energy balance, the number of calories we burn per day is greater than the number of calories we consume in food.
Again, the question arises: how can we spend energy that we haven’t taken in through our food?
The answer is that we use our body’s energy stores. For example, we may break down fat (triglyceride) stores into fatty acids, and then, through a process called fat oxidation, burn these to release energy.
As a result of breaking down these energy stores, a negative energy balance causes us to lose weight. Studies suggest that between 60 and 80% of this weight loss is attributable to reductions in body fat.
The bottom line of all this is that, thermodynamically speaking, if we want to lose weight, our total energy expenditure needs to be higher than our energy intake.
- Energy balance is the difference between Energy Intake ('calories in') and Total Energy Expenditure ('calories out').
- If your total energy expenditure is less than the amount of calories you eat (a positive energy balance), you will gain weight.
- If your total energy expenditure is higher than the amount of calories you eat (a negative energy balance), you will lose weight.
What is Basal Metabolic Rate (BMR)?
Your Basal Metabolic Rate (BMR) is the amount of energy used at rest to perform basic, life-sustaining functions. This includes energy needed to maintain cell homeostasis, heart function, brain activity, nerve function etc.
As mentioned earlier, your basal metabolic rate makes up the biggest portion (60-80%) of your total energy expenditure.
The terms ‘basal metabolic rate (BMR)’ and ‘resting energy expenditure (REE)’ or ‘resting metabolic rate (RMR)’ are often used interchangeably. Technically speaking, they are slightly different, with BMR measured under stricter conditions (after 24 hours of no exercise, following an overnight fast).
- Your basal metabolic rate (BMR) is the number of calories you burn each day in order to fuel life-sustaining processes (e.g. cell homeostasis, breathing, nerve function).
- BMR makes up about 60-80% of your total energy expenditure.
What factors affect my BMR?
Several different lifestyle and genetic factors can influence your basal metabolic rate. Some of these include:
- Body mass
Generally speaking, the main determinant of your basal metabolic rate is your body mass. The heavier you are, the higher your BMR as you expend more energy to meet the needs of more cells in your body. On a similar note, your BMR rises with your height too.
- Body composition
Muscle uses more calories at rest compared to fat tissue. Therefore, people with greater muscle mass (and a lower body fat percentage) tend to have higher BMRs.
Another way of phrasing this is to say that BMR positively correlates with your fat-free mass (i.e. muscle, bone, internal organs, connective tissue – basically anything that isn’t fat). This is illustrated in the graph below.
Women tend to have lower BMRs compared to men. This is largely due to differences in body composition, with men typically having greater muscle mass then women.
Your BMR tends to decrease as you get older due to losses in muscle mass, hormonal, and other age-related physiological changes.
Various hormones, including your thyroid hormones, can alter your basal metabolic rate. For example, in your Thyroid function trait, we found out that the hormones T3 (triiodothyronine) and T4 (thyroxine) increase your basal metabolic rate.
- Ambient temperature
Many of the chemical reactions that make up our basal metabolism generate heat energy. Making use of this, we have evolved to increase our basal metabolic rate to warm the body when the ambient temperature is cold. This process is known as non-shivering thermogenesis.
Drugs, both licit and illicit, can alter your basal metabolic rate. An obvious example is caffeine, which can increase BMR. On this note, one study found that a 100 mg dose of caffeine increased basal metabolic rate by 3-4% over 150 minutes.
Your genetic make-up has a significant influence on your basal metabolic rate. One study suggested that BMR has a heritability of 40%. This means that 40% of the variation in BMR can be explained by genetic differences. Variants of single genes, such as your UCP2 gene (the subject of this trait), contribute to these genetic differences.
- The main factor that affects your basal metabolic rate is your fat-free mass (including muscle, bone, connective tissue).
- Your genetic makeup and various other lifestyle factors also affect basal metabolic rate.
How do I calculate my BMR?
Given the impact of the previously-described factors on BMR, several different formulae have been created to estimate a person’s BMR. Two of the most widely-used formulae are the Harris-Benedict Equation and the Mifflin St Jeor Equation.
Both equations factor in the effects of sex, weight, height, and age on BMR.
BMR = 66.5 + (13.75 x Weight [kg]) + (5.003 x Height [cm]) - (6.775 x Age [years])
BMR = 655.1 + (9.563 x Weight [kg]) + (1.850 x Height [cm]) - (4.676 x Age [years])
Mifflin St Jeor Equation
BMR = (10 × Weight [kg]) + (6.25 × Height [cm]) - (5 × Age [years]) + 5
BMR = (10 × Weight [kg]) + (6.25 × Height [cm]) - (5 × Age [years]) - 161
How does BMR affect weight loss?
As established earlier, we need a negative energy balance in order to achieve weight loss. This means that our total energy expenditure needs to exceed our total energy intake.
Given that our basal metabolic rate (BMR) makes up the largest proportion of our total energy expenditure, it follows that, ceteris paribus (all other things being equal), a higher BMR will lead to a higher total energy expenditure.
In turn, a higher total energy expenditure will make it easier to attain a negative energy balance and lose weight. Putting these together, weight loss ought to be easier for someone with a higher BMR, who burns more calories at rest, compared to someone with a lower BMR.
Conversely, a lower BMR may make it more difficult to lose weight. Furthermore, a person with low BMR ought to achieve a positive energy balance more easily and therefore be at greater risk of weight gain.
In line with this, a study of Pima Indians showed that lower BMR (adjusted for fat-free mass and fat mass) was associated with greater weight-gain over a 2 year period. Similarly, a meta-analysis found that people who were formerly obese had an adjusted BMR that was 3-5% lower compared to normal weight control subjects.
It’s important to bear in mind that, in practice, the relationship between BMR, energy balance and weight loss is more complicated than as suggested above.
Firstly, it’s important to understand that, compared to BMR, changes in energy intake (i.e. how many calories we consume) and activity-induced energy expenditure (i.e. how many calories burned by physical activity) have a much larger effect on our overall energy balance.
If you’re looking to lose weight, strategies such as diet (which targets energy intake) and exercise (which targets activity-induced energy expenditure) are much more viable than trying to enhance your basal metabolic rate.
Secondly, remember that your BMR changes as you gain or lose weight. If you successfully lose weight, your BMR will drop as your fat and fat-free mass drop. This is shown in the diagram below.
Thirdly, the impact of differences in BMR on adiposity (the relative amount of fat or ‘adipose’ tissue) is complex. Some studies have found that having a BMR that is significantly higher or lower than predicted by weight, height, sex, and age does not influence body fat percentage.
- A higher BMR may contribute to increased total energy expenditure, making it easier to achieve a negative energy balance and lose weight.
- A lower BMR may make it harder to achieve a negative energy balance and lose weight.
- BMR is likely to have a lesser effect on changes in energy balance compared to your energy intake and physical activity.
What is UCP2?
UCP2 stands for Uncoupling Protein 2.
We’ve encountered this protein before in your Fuel usage (UCP2) trait. Readers are encouraged to revisit the accompanying blog article for a more in-depth overview of how the protein works in mitochondria.
When it comes to energy balance, UCP2 regulates the way mitochondria use glucose and fats to yield chemical energy. Generally speaking, greater burning or ‘oxidation’ of glucose and fats as fuel will increase our total energy expenditure.
Furthermore, UCP2 indirectly activates a receptor called the melanocortin-4 receptor, which stimulates brain circuits (located in a part of the brain called the hypothalamus) that inhibit food (energy) intake and increase our basal metabolic rate.
UCP2 also plays a role in insulin secretion. Insulin is a hormone that affects glucose and fat metabolism and influences how we store energy.
The UCP2 protein is encoded by your UCP2 gene. Your Basal Metabolic Rate (UCP2) trait looks at different variants of your UCP2 gene that have been shown to affect energy balance.
- UCP2 stands for Uncoupling Protein 2.
- UCP2 plays a role in glucose and fat metabolism, insulin secretion, and control of energy balance.
- Your UCP2 protein is coded for by your UCP2 gene.
How do UCP2 gene variants affect BMR?
Some variants of the UCP2 gene are associated with changes in the levels of UCP2 protein produced in different tissues.
We say that these gene variants are associated with changes in gene expression – the process by which the instructions in our DNA are used to make proteins.
It is thought that higher expression of the UCP2 gene, which leads to higher UCP2 protein levels in muscle and adipose tissue, is associated with a higher basal metabolic rate.
Conversely, lower UCP2 expression and protein levels are thought to contribute to a lower basal metabolic rate.
On this note, a SNP (Single Nucleotide Polymorphism) of the UCP2 gene, designated rs659366, creates two different gene variants or ‘alleles’: the ‘G’ allele and the ‘A’ allele.
Various other UCP2 gene variants, such as those created by the rs660339 SNP and the 45-bp insertion/deletion polymorphism, are also shown to alter UCP2 expression and influence BMR.
For example, a study of Pima Indians found that those with UCP2 gene variants linked to increased UCP2 expression had higher BMRs, burning, on average, about 150 kcals more per day.
- UCP2 gene variants may affect gene expression and therefore alter UCP2 protein levels.
- Higher UCP2 expression/protein levels are linked to a higher basal metabolic rate.
- Lower UCP2 expression/protein levels are linked to a lower basal metabolic rate.
How do UCP2 gene variants affect weight loss?
A small study showed that UCP2 gene expression is positively correlated with weight loss when following a six-week low-calorie diet. As shown in the graphs below, people with higher levels of UCP2 expression also experienced greater absolute and percentage weight-loss.
It is possible that gene variants associated with higher UCP2 gene expression facilitate greater weight loss, although more research is needed to establish this.
Other studies have also found that gene variants linked to higher UCP2 expression, such as the ‘A’ allele created by the rs659366 SNP, are associated with greater weight-loss in response to gastric bypass and gastric banding surgery.
- Some studies have found UCP2 gene variants linked to higher UCP2 expression/ protein levels to be associated with greater weight loss.
Your Basal Metabolic Rate (UCP2) trait
Your Basal Metabolic Rate (UCP2) trait analyses several different variants of your UCP2 gene, including those created by rs659366, rs660339, and the 3’UTR 45-bp insertion/deletion polymorphism.
Based on these gene variants, you will be classified into one of three groups:
- Increased basal metabolic rate
- Moderate basal metabolic rate
- Reduced basal metabolic rate
To find out your result, please login to truefeed.