What is the role of fat macronutrient in relation to generating energy for exercise and physical activity?

Many physiological and nutritional demands occur within the body during exercise. As muscles contract, the demand for oxygen, hydrogen and other key nutrients increases. The human body requires a continuous supply of energy to perform its many functions. As energy demands increase with exercise, additional energy must be supplied or the exercise will end.

Factors of performance

Whether a recreational athlete or an elite athlete, many factors influence performance including, but not limited to, diet, hydration, fitness level, intensity and duration. There are many factors that predict what source of fuel will be used. Proteins, fats and carbohydrates are all possible sources of fuel for exercise and muscle contraction.

During moderate-intensity exercise, roughly half of the energy is derived from glycogen, while the other half comes from glucose in the blood and fatty acids. Carbohydrates (glucose/glycogen) serve as the primary source of fuel as duration and intensity increase. If exercise continues for a significant period of time, fatty acids will serve as the fuel source when glycogen stores are nearly depleted. It must be noted that fat metabolism cannot occur without the presence of glucose, and thus muscle glycogen and blood glucose are the limiting factors in performance. Protein or, more specifically, amino acids, will only be used as an energy source if other calories are insufficient.

Food choices

A person’s diet will influence which source of fuel is used and therefore, performance level. If a person consumes a high-carbohydrate diet, more glycogen will be used for fuel. If the diet is high in fat, fat will be used as the fuel source. A high-fat diet is not recommended as even the leanest person has plenty of stored fat for long endurance exercise. A high-fat, low-carbohydrate diet can lead to poor performance due to low glycogen stores. As a guideline for endurance athletes, roughly 60–70 percent of calories should come from carbohydrates, 10–15 percent protein and 20–30 percent from fat. You should consume a well-balanced diet containing carbohydrates, protein and fat during training periods.

Carbohydrate intake before, during and after exercise is crucial. A high-carbohydrate pre-exercise meal not only prevents hunger pangs during exercise, it also provides optimal blood glucose levels for endurance exercising and increases glycogen stores. Avoid high-fat foods in a pre-exercise meal as it delays stomach emptying and takes longer to digest. This meal should be three to four hours before an event.

Marathon runners talk about “hitting a brick wall.” This refers to the time when fuel sources have been drained and not replaced. When glycogen and blood glucose levels are low, the body is out of fuel and cannot keep going no matter how fast an athlete wants to go.

For exercise lasting longer than an hour, you should ingest carbohydrates to fuel the brain and muscles. You can maintain a sufficient supply of energy by consuming 26–30 grams of carbohydrates every 30 minutes during exercise. Most sports drinks provide 15–20 grams of carbohydrate, so consuming 8–12 ounces every 15–30 minutes is recommended. As for protein, only a few amino acids can actually be used directly as energy. Thus, protein consumption during exercise is not advantageous.

Fluid intake

Muscle glycogen stores must be replaced after endurance exercise. Resynthesis of muscle glycogen is promoted when carbohydrates are consumed immediately after exercise. Unfortunately, due to an elevated body temperature, appetite is usually depressed and many athletes have difficulty consuming foods immediately after exercise. Drinking carbohydrates via a sports drink or shake provides carbohydrates and promotes rehydration.

Adequate fluid intake is also crucial for any athlete. You should weigh yourself before and after an endurance event, especially during hot weather. For each pound lost during exercise, drink three cups of fluid. Fluids should not be restricted before, during or after an event. Athletes should not rely on thirst as a sign of fluid loss. Consume roughly 14–22 ounces of fluid before an event, 6–12 ounces every 15–30 minutes during an event, and after the event, 16–24 ounces for every pound of body weight lost.

Nutrients are substances needed for growth, energy provision and other body functions. Macronutrients are those nutrients required in large amounts that provide the energy needed to maintain body functions and carry out the activities of daily life. There are 3 macronutrients – carbohydrates, proteins and fats.

Macronutrients give us energy

Although each of these macronutrients supplies the energy needed to run body functions, the amount of energy that each provides varies.

Carbohydrates and proteins each provide 17kJ/g whereas fats provide 37kJ/g. 1 kilojoule (kJ) = 1000 joules.

4.2 joules is the energy needed to raise the temperature of 1g of water by 1°C.

Nutritional research evidence shows that the relative proportion of energy-giving foods in the diet can increase or decrease the likelihood of problems such as heart disease. A balance of energy-giving nutrients is suggested.

For example, if an active teenager’s energy requirements are around 12,000kJ per day, an intake for energy purposes of about 388g of carbohydrate along with some protein (110g) and fat (97g) would meet this need. These values equate approximately to 55% of energy needed from carbohydrate, 30% from fats and 15% from protein.

Why do we need carbohydrates?

Carbohydrates, in the form of starches and sugars, are the macronutrients required in the largest amounts. When eaten and broken down, carbohydrates provide the major source of energy to fuel our daily activities. It is recommended that carbohydrates should supply 45–65% of our total daily energy needs.

Some of the carbohydrate we consume is converted into a type of starch known as glycogen, which is stored in the liver and muscles for later use as an energy source.

Not all of the carbohydrates found in foods are digestible. For example, cellulose is a non-digestible carbohydrate present in fruits and vegetables. Although unable to be used as an energy source, this type of carbohydrate plays a very important role in maintaining the health of the large intestine and assisting with the removal of body waste. It is often referred to as ‘dietary fibre

Why do we need proteins?

The proteins we consume as part of our diet are broken down in the gut to amino acids. The body can then use these amino acids in 3 main ways:

• As ‘building blocks’ in the production of ‘new’ proteins needed for growth and repair of tissues, making essential hormones and enzymes and supporting immune function.
• As an energy source.
• As starting materials in the production of other compounds needed by the body.

All the proteins in the body are made up of arrangements of up to 20 different amino acids. Eight of these amino acids are described as ‘essential’, which means that the food we eat must contain proteins capable of supplying them. The other amino acids can be synthesised by the liver if not provided by the diet.

Protein in the diet that comes from animal sources contains all of the essential amino acids needed, whereas plant sources of protein do not. However, by eating a variety of plant sources, the essential amino acids can be supplied.

Why do we need fats?

Although fats have received a bad reputation in relation to heart disease and weight gain, some fat in the diet is essential for health and wellbeing.

It is recommended that 20–35% of our daily energy requirement should be supplied through the consumption of fats and oils. In addition to supplying energy, fats are needed to:

• supply fatty acids that the body needs but cannot make (such as omega-3)
• assist with absorption of the fat-soluble vitamins A, D, E and K and carotenoids
• provide foods with flavour and texture.

Dietary fats are of 3 main types:

• Saturated fat – found in foods like meat, butter and cream (animal sources).
• Unsaturated fat – found in foods like olive oil, avocados, nuts and canola oil (plant sources)
• Trans fats – found in commercially produced baked goods, snack foods, fast foods and some margarines.

Replacing saturated fats and trans fats in the diet with unsaturated fats has been shown to decrease the risk of developing heart disease.

Over the last 50 years, the recommendations of nutrition researchers about balancing carbohydrate, protein and fat intake have changed. This highlights the ‘feedback process’ nature of science. If new evidence gained from research indicates a modification needs to be made to a recommendation, that is what eventually happens.

Listen to this RNZ podcast for more information on macronutrients and diet.

The human body uses carbohydrate, fat, and protein in food and from body stores for energy to fuel physical activity. These essential nutrients are needed regardless of the intensity of the activity you are doing. If you are lying down and reading a book or running a marathon, these macronutrients are always needed in the body. However, in order for these nutrients to be used as fuel for the body, their energy must be transferred into the high energy molecule known as adenosine triphosphate (ATP). ATP is the body’s immediate fuel source and can be generated either with in the presence of oxygen or without the presence of oxygen. The type of metabolism that is predominately used during physical activity is determined by the availability of oxygen and how much carbohydrate, fat, and protein are used.

Anaerobic and Aerobic Metabolism

Anaerobic metabolism occurs in the cytosol of the muscle cells. As seen in Figure 10.1., a small amount of ATP is produced in the cytosol without the presence of oxygen. Anaerobic metabolism uses glucose as its only source of fuel and produces pyruvate and lactic acid. Pyruvate can then be used as fuel for aerobic metabolism. Aerobic metabolism takes place in mitochondria of the cell and is able to use carbohydrates, protein, or fat as fuel sources. Aerobic metabolism is a much slower process than anaerobic metabolism, but it can produce much more ATP and is the process by which the majority of the ATP in the body is generated.

Figure 10.1. Anaerobic vs aerobic metabolism. Note that carbohydrate is the only fuel utilized in anaerobic metabolism, but all three macronutrients can be used for fuel during aerobic metabolism.

Physical Activity Duration and Fuel Use

The respiratory system plays a vital role in the uptake and delivery of oxygen to muscle cells throughout the body. Oxygen is inhaled by the lungs and transferred from the lungs to the blood, where the cardiovascular system circulates the oxygen-rich blood to the muscles. The oxygen is then taken up by the muscles and can be used to generate ATP. When the body is at rest, the heart and lungs are able to supply the muscles with adequate amounts of oxygen to meet the energy needs for aerobic metabolism. However, during physical activity, your muscles need more energy and oxygen. In order to provide more oxygen to the muscle cells, your heart rate and breathing rate will increase. The amount of oxygen that is delivered to the tissues via the cardiovascular and respiratory systems during exercise depend on the duration, intensity and physical conditioning of the individual.

• During the first few steps of exercise, your muscles are the first to respond to the change in activity level. Your lungs and heart do not react as quickly, and during those beginning steps, they can’t yet increase the delivery of oxygen. In order for our bodies to get the energy that is needed in these beginning steps, the muscles rely on a small amount of ATP that is stored in resting muscles. The stored ATP is able to provide energy for only a few seconds before it is depleted. Once the stored ATP is just about used up, the body resorts to another high-energy molecule known as to convert ADP (adenosine diphosphate) to ATP. After about 10 seconds, the stored creatine phosphate in the muscle cells is also depleted as well.
• About 15 seconds into exercise, the stored ATP and creatine phosphate are used up in the muscles. The heart and lungs have still not adapted to the increased oxygen need, so the muscles must begin to produce ATP by anaerobic metabolism (without oxygen). Anaerobic metabolism can produce ATP at a rapid pace but only uses glucose as its fuel source. The glucose is obtained from muscle glycogen. At around 30 seconds, anaerobic pathways are operating at their full capacity, but because the availability of glucose is limited, it cannot continue for a long period of time.
• As your exercise reaches two to three minutes, your heart rate and breathing rate have increased to supply more oxygen to your muscles. Aerobic metabolism is the most efficient way of producing ATP; it produces significantly more ATP for each molecule of glucose than anaerobic metabolism. Although the primary source of ATP in aerobic metabolism is carbohydrates, fatty acids and protein can also be used as fuel to generate ATP.

Figure 10.2. Energy systems used to fuel exercise change with duration of exercise. The ATP-creatine phosphate system is used up within seconds. The short-term and long-term systems kick in and provide energy for exercise as the duration of the workout goes on.

The fuel sources for anaerobic and aerobic metabolism will change depending on the amount of nutrients available and the type of metabolism.

• Glucose may come from blood glucose (which is from dietary carbohydrates, liver glycogen, and glucose synthesis) or muscle glycogen. Glucose is the primary energy source for both anaerobic and aerobic metabolism.
• Fatty acids are stored as triglycerides in muscles, but about 90 percent of stored energy is found in adipose tissue. As low- to moderate-intensity exercise continues using aerobic metabolism, fatty acids become the predominant fuel source for exercising muscles.
• Although protein is not considered a major energy source, small amounts of amino acids are used while resting or doing an activity. The amount of amino acids used for energy metabolism increases if the total energy intake from your diet does not meet your nutrient needs or if you are involved in long endurance exercise. When amino acids are broken down and the nitrogen-containing amine group is removed, the remaining carbon molecule can be broken down into ATP via aerobic metabolism, or it can be used to make glucose. When exercise continues for many hours, amino acid use will increase as an energy source and for glucose synthesis.

Figure 10.3. Fuel sources for anaerobic and aerobic metabolism. Both dietary sources and body storage of carbohydrates, fat, and protein can all be used to fuel activity. Amount varies depending on duration and intensity of the activity.

Physical Activity Intensity and Fuel Use

Exercise intensity determines the contribution of different fuel sources used for ATP production. Both anaerobic and aerobic metabolism combine during exercise to ensure that the muscles are equipped with enough ATP to carry out the demands placed on them. The contribution from each type of metabolism depends on the intensity of an activity. During low-intensity activities, aerobic metabolism is used to supply enough ATP to muscles. However, during high-intensity activities, more ATP is needed, so the muscles must rely on both anaerobic and aerobic metabolism to meet the body’s demands.

Table 10.2. Summary of fuels used for activities of different intensities and durations.

During low-intensity activities, the body will use aerobic metabolism over anaerobic metabolism, because it is more efficient and produces larger amounts of ATP. Fatty acids are the primary energy source during low-intensity activity. With fat reserves in the body being almost unlimited, low-intensity activities are able to continue for a long time. Along with fatty acids, a small amount of glucose is used as well. Glucose differs from fatty acids, because glycogen storages can be depleted. As glycogen stores are depleted, the glucose supply becomes depleted, and fatigue will eventually set in.

Figure 10.4. The effect of exercise intensity on fuel sources. Anaerobic exercise utilizes only glucose for fuel. As activities become more aerobic, the body can utilize fatty acids and, to a small extent, amino acids, for energy production.

One important clarification about exercise intensity and fuel sources is the concept of the fat-burning zone. Many people think that in order to lose body fat, they should exercise at a lower intensity so that fat is the primary fuel source. The fat-burning zone is typically referred to as a low-intensity aerobic activity that keeps your heart rate between 60 and 69 percent of maximum heart rate. The cardio zone, on the other hand, is a high-intensity aerobic activity that keeps the heart rate between about 70 and 85 percent of maximum heart rate. So which zone do you burn the most fat in? Technically, your body burns a higher percentage of calories from fat during a low-intensity aerobic activity. When you begin a low-intensity activity, about 50% of the calories burned come from fat, whereas in the cardio zone only 40% come from fat. However, this isn’t the whole story. High-intensity activity burns more total calories per minute. At this higher rate of energy expenditure, you can burn just as much or more total fat and more total calories as during a lower intensity activity. If weight loss is one of your goals, high-intensity activities will burn more total calories, helping to shift to negative energy balance, and will promote a greater level of fitness. However, the best exercise program is one that is enjoyable, sustainable, and safe for you; if you’re just starting out, it’s wise to begin with low- to moderate-intensity activities and work your way up from there.

Figure 10.5. The fat-burning zone. While a greater percentage of calories burned in lower intensity exercise come from fat, the overall total calorie burn is greater in higher intensity exercise.

Attributions:

Image Credits:

• Figure 10.1. “Anaerobic vs Aerobic Metabolism” by Allison Calabrese is licensed under CC BY 4.0
• Figure 10.2. “Energy systems used to fuel exercise change with duration of exercise” by Alice Callahan is licensed under CC BY 4.0, with images: Instant energy sprint photo and short-term energy women on track photos by Nicolas Hoizey; long-term energy race finish photo by Peter Boccia, all on Unsplash (license information)
• Figure 10.3. “Fuel Sources for Anaerobic and Aerobic Metabolism” by Allison Calabrese is licensed under CC BY 4.0
• Table 10.2. “Summary of Fuels” by Tamberly Powell is licensed under CC BY-NC-SA 2.0
• Figure 10.4. “The Effect of Exercise Intensity on Fuel Sources” by Allison Calabrese is licensed under CC BY 4.0
• Figure 10.5. “The Fat-burning Zone” by Allison Calabrese is licensed under CC BY 4.0