The Essential Role of Body Fat in Our Health- Part 2

Amber Whittemore RD, BSN, MHSc

Fat stores provide our body with energy during rest and physical activity

Our bodies rely on more than just glucose from carbohydrates, stored in the muscles and liver as glycogen, for energy provision and maintenance. They also depend on fatty acids from our adipose tissue (fat stores) and, to a smaller degree, amino acids from protein for energy production!

The human body is extremely efficient at utilizing a combination of glucose, fatty acids, and amino acids to provide our body with the energy it needs both for rest as well as to maintain our strength and stamina while exercising.

As you may be familiar with, there are two primary modes of energy production that our bodies engage in during different forms of movement: aerobic and anaerobic. Aerobic energy production, or metabolism, takes place in our cells with the assistance of oxygen. This is typically the pathway our bodies receive energy from during rest states as well as steady states of movement. Conversely, our bodies rely on anaerobic metabolism during short bursts of energy, since our cells have not been provided with enough oxygen yet to keep up with the quick increase in demand for energy.

While at rest our lungs and heart are adapted and easily able to supply oxygen to our body’s cells to support aerobic metabolism. While glucose is the primary source of energy for aerobic metabolism, fatty acids from our adipose tissue also provide fuel to optimize this process.

When we shift into movement during physical activity, there is a temporary shift to anaerobic metabolism until our heart and breathing rate can adapt to the increased need for oxygen provision to the cells. Therefore, during quick bursts of high-intensity exercise, such as HIIT or weightlifting, our bodies are producing energy for the cells in the absence of adequate oxygen – this is anaerobic metabolism. The only fuel source utilized during this pathway is glucose.

As our physical activity reaches the two-to-three-minute3 mark our bodies enter into a steady-state of movement. This occurs with an exercise such as jogging or rowing. During steady state movement, our heart and breathing rates adaptively increase to supply more oxygen to the muscles. Therefore, our bodies shift back to the aerobic pathway, meaning we rely once again on both glucose and fatty acids primarily for energy provision. As low to moderate-intensity activity continues, we burn through our glycogen (glucose from carbohydrates) stores and soon fatty acids become our primary fuel source for exercising our muscles!

If our bodies do not have adequate fat stores to fuel our muscles during exercise, they will inevitably turn to breaking down muscle for energy. This is counterintuitive to most people’s goals who are engaging in exercise. The following illustration3 depicts our body’s predominant energy sources during rest, moderate-intensity, and high-intensity activity as we shift between aerobic and anaerobic metabolism. Notice how our body relies on fatty acids for adipose tissue to produce energy during rest and steady-state movement!

Chart, treemap chart

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Fat stores maintain our basal metabolic rate (BMR)

Every single function that our body performs requires energy, from the more subtle functions such as thinking, breathing, and digesting, to the more obvious ones like chewing our food, walking, and lifting weights. The rate at which our body requires energy to perform the more subtle daily tasks that keep us alive refers to our basal metabolic rate (BMR). This is the minimum amount of food energy we need to ingest to keep these essential functions running smoothly. When a person is at rest, the following chart2 shows where the daily food energy is diverted to:

Organ% BMR
Liver & spleen27%

Therefore, in a well-nourished and functioning body, nearly 50% of our daily energy is spent keeping our liver, spleen and brain functioning normally – even when we are at complete rest! This is why we need to continue to nourish our bodies even when we are taking a rest from physical activity.

To keep our BMR functioning optimally we need to ensure we consume adequate calories and macronutrients as well as enough muscle and fat mass to promote overall health. If we are restricting calories or fall below our required fat mass then our bodies shift into survival mode, which is accompanied by a decrease in BMR. This decrease occurs in an effort for our bodies to store as much fuel as we can rather than burning it, therefore our body begins prioritizing only essential daily functions of living – while slowing down the less life-essential processes such as optimal mental processing and digestion. 

Recall the previous function that fat stores play in providing us with energy. If our fat stores fall below where we need them, our bodies turn to breaking down our muscles to provide essential fuel. As this occurs, there is a further accompanying decrease in BMR since muscle is the most metabolically active tissue in the human body.