IB Sports, exercise and health science A.2.2.1 Macronutrients as Energy Sources and the Factors Influencing Their Use Notes

Macronutrients as energy sources

1. The three primary macronutrients carbohydrates, proteins, and lipids contribute differently to energy metabolism based on factors such as exercise intensity, duration, and an individual's physiological state.
2. They support growth, rest, and physical activity, with each macronutrient playing a unique role.

Carbohydrates are the main fuel for high intensity exercise

1. Carbohydrates serve as the primary fuel source for most bodily functions and physical activities, particularly during high-intensity exercise.

Carbohydrates are classified by the number of sugar units

Monosaccharides

1. The simplest form of carbohydrate, consisting of a single sugar molecule.
2. Easily absorbed by the body.
3. Examples include glucose, fructose, and galactose.

Disaccharides

1. Composed of two monosaccharides linked together.
2. Examples include sucrose (glucose + fructose) and lactose (glucose + galactose).

Oligosaccharides

1. Carbohydrates that contain 3 to 9 sugar units.
2. An example is maltodextrin, often used in processed foods and sports drinks for quick energy release.

Polysaccharides

1. Long chains of monosaccharides, such as starch and glycogen, that provide long-lasting energy.
2. Broken down into glucose during digestion.

Dietary Fiber

1. Some polysaccharides, like cellulose, cannot be digested by the human body and are considered dietary fiber.
2. Although not a direct energy source, fiber is crucial for digestive health.

Energy yield

1. Carbohydrate energy density: ~1,760 kJ per 100 g.
2. Storage capacity: Limited to about 400 g muscle glycogen + 100 g liver glycogen in a healthy adult.
3. Other functions: Contribute to cell membrane structure and to DNA/RNA synthesis, but the dominant role is fuel.

Carbohydrates are digested to glucose and stored as glycogen

1. Digested carbohydrates are broken down into glucose, which enters the bloodstream.
2. Excess glucose is stored in the liver and muscles as glycogen through a process called glycogenesis.
3. During exercise, glycogen is broken down into glucose via glycogenolysis and used for ATP production.

Metabolic Pathways of Carbohydrates

1. In anaerobic metabolism, carbohydrates provide rapid energy without requiring oxygen (e.g., during sprinting).
2. In aerobic metabolism, glucose is oxidised efficiently to produce ATP for prolonged exercise.

Lipids provide long term energy and support aerobic exercise

1. Lipids, commonly known as fats, are the body's most energy-dense macronutrient, providing a long-term energy reserve.
2. Fats are used predominantly during low-to-moderate intensity exercise, especially when exercise duration is long.

Lipids include triglycerides, phospholipids, and sterols

1. Triglycerides: The most common form of fat in the body, composed of one glycerol molecule and three fatty acids. Major energy storage molecule.
2. Phospholipids: Major components of cell membranes, made up of two fatty acids and a phosphate group. Crucial for maintaining cell structure and function.
3. Sterols: Including cholesterol. Important for producing hormones and maintaining cell membrane integrity.

Fatty acids are classified by the degree of saturation

Saturated Fatty Acids (SFA)

1. These have the maximum number of hydrogen atoms attached to each carbon atom, making them solid at room temperature.
2. Commonly found in animal products.

Unsaturated Fatty Acids

1. These fats have one or more double bonds between carbon atoms, making them liquid at room temperature.
2. Unsaturated fats can be further classified into:
   1. Monounsaturated Fatty Acids (MUFA): Contain one double bond, found in foods like olive oil.
   2. Polyunsaturated Fatty Acids (PUFA): Contain multiple double bonds, with examples like omega-3 and omega-6 fatty acids.

Metabolic Role

1. Stored as triglycerides in adipose tissue and muscle.
2. Broken down through lipolysis into glycerol and free fatty acids (FFAs).
3. FFAs enter the bloodstream and undergo beta-oxidation to generate ATP in mitochondria.
4. Lipid energy density: ~4,000 kJ per 100 g, roughly three times that of carbohydrate. Stored as triglycerides in adipose tissue, which also provides physical protection for organs and thermal insulation.

Energy Contribution

1. Dominant fuel source during rest and low-intensity activities (e.g., walking, yoga).
2. Contributes more energy as exercise duration increases and glycogen stores decrease.
3. Requires oxygen to be metabolised, making it inefficient for high-intensity activity.

Proteins as an energy source

1. Proteins primarily serve a structural and functional role in the body but can be used for energy when carbohydrate and fat stores are low.
2. Amino acids are the building blocks of proteins.

Amino acids

1. Total amino acids in human protein: 20.
2. Essential amino acids: Nine that must come from food: isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine, histidine.
3. Non-essential amino acids: The other 11, which the body can synthesise (e.g., alanine, asparagine, glutamine).

Protein quality and food sources

1. Complete proteins: Animal protein (meat, fish, milk, dairy, eggs) typically contains all nine essential amino acids and is highly digestible.
2. Incomplete proteins: Plant proteins are usually short of one or more essentials. Pulses are low in isoleucine and lysine; grains are low in methionine and tryptophan.
3. Complementary combining: Combining pulses and grains (rice and beans, hummus and bread) covers the gap.

Metabolic Role

1. Proteins are made of amino acids, which are used for muscle repair, enzyme production, and immune function.
2. Under normal conditions, proteins contribute less than 5% of total energy production.
3. Protein energy density: ~1,720 kJ per 100 g, similar to carbohydrate. The body avoids burning protein because every protein molecule has a specific biological job.
4. During prolonged exercise or starvation, proteins undergo gluconeogenesis, where amino acids are converted into glucose in the liver.

Energy Contribution

1. Low contribution during short-term exercise.
2. Becomes more significant in prolonged endurance exercise (e.g., ultra-marathons).
3. BCAAs (branched-chain amino acids: leucine, isoleucine, valine) can be directly oxidised by muscles for ATP production.

Macronutrient use changes with the athlete and the activity

1. The contribution of macronutrients to energy production is not static.
2. It varies based on individual factors, including body composition, age, sex, and activity level.

Body Composition

1. Muscle Mass: Individuals with higher muscle mass have higher energy demands and rely more on carbohydrates and proteins to fuel muscles.
2. Fat Mass: Individuals with higher body fat percentage tend to rely on lipids for energy during endurance exercise because the body can utilise fat stores more efficiently.

Age

1. Children: Need more protein per kg of body mass for growth, more carbohydrate to fuel play, and adequate fat for brain development.
2. Older adults: Need adequate protein to maintain muscle mass and prevent sarcopenia.

Sex Differences

1. Biological males: Typically have higher basal metabolic rate and more lean muscle mass, so they tend to need slightly more carbohydrate for fuel and slightly more protein for muscle.
2. Biological females: Show hormonal fluctuations across the menstrual cycle that influence carbohydrate metabolism and fat storage.

Activity Level and Exercise Type

1. Higher levels of physical activity require more carbohydrate to fuel energy needs.
2. Strength training raises protein requirements to build new muscle.
3. Endurance training raises both carbohydrate and protein needs to replace glycogen and repair connective tissue.