How do we spend our calories?
Let’s now look at the energy output side
of the energy balance equation, that is, how we spend our calories. Our body uses most
of the energy for two purposes: basal metabolism and physical activity. On top of that, we
also need to spend a little amount of energy to digest and utilize the food we eat. Our basal metabolism, or basal metabolic rate
is the energy we need to spend to perform our basic vital functions, such as breathing,
pumping our heart and allowing all of our organs to function. In other words, this is
the energy we need to just survive without doing anything. Ideally, this would be calculated
for a resting body in a fasting state, in a warm and quiet environment.
But in real life, most of the time we are not completely rested, not completely fasted
and not in a quiet and warm environment, and so we need to spend a little energy on top
of that. For practical purposes, this energy is still included in the calculation of our
basal metabolism. What is not included, however, is the extra
energy we need to perform all of our physical activities, such as walking or washing the
dishes, as well as the energy we need to digest and utilize food. For a sedentary person, basal metabolism accounts
for 60 to 70% of the whole energy expenditure, and it is influenced by many different factors.
One is body composition: our lean body mass, such as our muscles, needs more energy even
at rest compared to our adipose tissue, and so the more lean mass we have, the higher
our basal metabolism. This is also the reason why – when everything else is equal – men
have higher basal metabolism than women, since women are genetically designed to have more
fat mass. And this is also the main strategy we have
to change our basal metabolism: by doing physical activity to increase our lean mass, we can
increase our basal metabolism and therefore burn more calories even when we are at rest,
on top of those spent to perform the activity itself.
Another factor increasing basal metabolism is a larger body surface area per body volume:
taller people have larger body surface areas, and for this reason they use more calories
at rest than shorter people, and are less prone to obesity.
The temperature of our environment also affects basal metabolism. In particular, temperature
extremes will always increase basal metabolism because we need to spend energy to keep our
body temperature stable. Raising our body temperature if it’s cold, and keeping it
cool if it’s warm, both require to spend some energy, just like our air conditioners
need electricity both to warm up and to cool down our buildings. Particularly cold environmental
conditions will also cause some involuntary physical activity, such as increased muscle
tone, fidgeting or shivering, further increasing energy expenditure. Thyroid hormones directly
affect our basal metabolism because they set the pace of our body’s metabolic rate. Hyperthyroidism
results in higher basal metabolism and difficulty to gain weight, vice versa hypothyroidism
results in a lower basal metabolism and therefore a tendency to gain weight.
Stress also increases our basal metabolism, mainly through the activity of our nervous
system and the higher circulating levels of stress hormones, such as adrenaline.
Our basal metabolism can also be increased by thermogenic substances, that is substances
which increase our energy expenditure, such as caffeine, nicotine, ephedra, capsaicin,
and drugs such as amphetamine and ephedrine. For this reason, coffee, green tea, red peppers
or ephedra tea have been used to help in many weight loss diets. Cigarette smoking is another
option, but it is definitely not a smart way to induce thermogenesis.
Finally, our basal metabolism is affected by homeostatic adjustments to food intake.
As we said before, our body tries to match our energy intake to our energy expenditure,
so if we eat a little bit more than what we spend, it will try to dissipate some of that
extra energy for example by dissipating a little heat, or involuntarily shaking a foot,
or fidgeting with a pen. Some people are more efficient than others at maintaining body
weight in spite of overeating thanks to thermogenesis. On the other hand, if we eat a little less
than what we spend, our metabolic rate will also slow down to save some energy. If our
food intake suddenly drops to a severely restricted caloric intake, our basal metabolism also
drops immediately by about 10 to 20%, because our body recognizes the situation as an emergency
and switches to ‘conservation mode’, it’s sort of our body’s way of going to standby.
For example, our thyroid hormones production will drop, while our cells become more efficient
at storing fat, for example by increasing the activity of the enzyme lipoprotein lipase. Finally, our genes have once again an important
word. Every other factor being equal, different persons will still have different basal metabolic
rates, and this depends on their different genetic makeup. So, let’s recap the main factors that influence
our basal metabolism. These are body composition, body surface area, temperature, thyroid hormones,
stress, use of thermogenic substances, homeostatic adjustments to food intake, and our genetic
makeup. One question that is often asked by people
interested in losing weight is, can I increase my basal metabolism so that I can burn more
calories even when I’m at rest? From what we said, there are indeed some ways to increase
one’s basal metabolism. The best way is to change body composition, by doing physical
activity to increase the amount of muscle, because lean mass requires more energy to
be maintained. Another strategy is to lower the temperature of our home by one or two
degrees, as long as it’s still comfortable. Finally, we can make a moderate use of safe
thermogenic substances, such as caffeine. Instead, abusing thermogenic substances, smoking,
or using thermogenic or thyroid stimulating drugs just for the purpose of increasing basal
metabolism, is dangerous and absolutely to be avoided. Let’s now examine the other two components
of energy expenditure, which are the energy spent for physical activity, and the thermogenic
effect of food. The energy spent for physical activity is
greatly variable from person to person depending on how active they are. A very active athlete
may spend more energy for physical activity than he spends for his basal metabolism, and
will have a very high total energy requirement. Most individuals, however, even if physically
active, will normally spend less energy for physical activity than they need for their
basal metabolism. A sedentary person will spend less than 20% of his energy for physical
activity. The energy spent for physical activity is
not only the energy required to perform the activity itself, which is basically muscle
contraction, but it is also the energy that is needed afterwards to repair tissues and
restore normal conditions, for example, to clear lactic acid. For this reason, energy
expenditure stays elevated for a while even after the physical activity itself has ended.
This is another concept that people interested in losing weight should keep in mind about
physical activity: number one, we spend calories to perform the activity itself, but also number
two, we keep spending calories even after we have exercised to repair and restore our
tissues, and on top of all that, number three, we increase our basal metabolic rate by increasing
our muscle mass, which means we spend more calories even when we sleep. Let’s now move on to the last component
of energy expenditure, which is called thermic effect of food. This is the energy we need
to spend to be able to digest, absorb and process nutrients from food. The three macronutrients
contribute differently to the thermic effect: lipids require the least energy to be used,
while protein require the most. About 20 to 30% of the energy contained in proteins needs
to be used just to make these proteins available. The thermic effect of carbohydrates is 5 to
10% of their energy content, while the one of fats is between 0 and 3%. When we say that
proteins provide 4 calories per gram, we have already discounted the thermic effect of food.
If we burn one gram of protein, it will actually release 5.65 calories, but we estimate that
that 1.65 on top, is what need to be spent on average just to process them. The same
goes for the 9 calories per gram of lipids, and the 4 calories per gram of carb. These
values are actually approximations used to discount thermogenesis, and are called Atwater
conversion factors, from the name of the scientist who first proposed them about a century ago.
In reality, the total thermic effect of a meal is variable and also depends on the total
amount of food that we eat: if we eat a lot at the same time, the thermic effect is a
little higher. Alcohol also has a thermic effect, which is about 20% of its energy content. So let’s recap. Our energy expenditure,
the energy output side of the energy balance equation, is the sum of three components:
our basal metabolism, which is the energy we need to spend to survive and perform our
basic vital functions, the energy spent to perform our physical activities, from walking
to washing the dishes to exercising at the gym, and the thermic effect of food, the energy
we spend to digest, absorb and metabolize nutrients from food.
Particular physiologic states such as growth, pregnancy or lactation, of course further
increase energy expenditure, as well as the stress of illness, the hypermetabolic state
due to fever, infection, inflammation, wounds, trauma, disease or surgery, when extra energy
is spent to raise body temperature, to repair damaged tissues, and to synthesize white blood
cells and inflammatory mediators to respond to an infection. Some conditions, such as
extensive burns or cystic fibrosis, are extremely hypercatabolic and cause extremely high energy