Macronutrients

Overview

Nutrition is the foundation of athletic performance. The food athletes consume provides the energy for movement, the building blocks for tissue repair and growth, and the regulatory substances that control metabolic processes. Understanding macronutrients (carbohydrates, fats, proteins) and micronutrients (vitamins, minerals) is essential for optimizing training adaptations, competition performance, and recovery. This topic covers the structure, function, energy yield, dietary sources, and sport-specific applications of each nutrient category.

Energy and Calories

What is Energy?

Energy is the capacity to do work. In nutrition and exercise, energy refers to the chemical energy stored in food that is converted to mechanical energy for movement and heat.

Units of Energy

Note: "Calories" on food labels typically refers to kilocalories (kcal).

Energy Density of Macronutrients

Why These Values Matter

• Fats provide more than twice the energy per gram compared to carbohydrates or proteins

• This makes fats efficient for energy storage (less weight for same energy)

• However, carbohydrates are more efficient for high-intensity exercise (more ATP per oxygen used)

Carbohydrates (CHO)

Definition

Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen atoms, typically in a ratio of 1:2:1 (CH₂O)ₙ. They are the body's preferred and most readily available source of energy, especially during moderate to high-intensity exercise.

Energy Yield

$\text{Carbohydrates} = 4 \text{ kcal/g} = 17 \text{ kJ/g}$

Classification of Carbohydrates

1. Simple Carbohydrates (Sugars)

Monosaccharides (Single Sugar Units)

Disaccharides (Two Sugar Units)

2. Complex Carbohydrates (Polysaccharides)

Carbohydrate Digestion and Absorption

1. Mouth: Salivary amylase begins starch breakdown

2. Stomach: Acidic environment halts amylase activity

3. Small intestine: Pancreatic amylase continues digestion; brush border enzymes (maltase, sucrase, lactase) complete breakdown to monosaccharides

4. Absorption: Monosaccharides absorbed into bloodstream via intestinal wall

5. Transport: Glucose transported to liver and tissues

Carbohydrate Storage

Glycogen

The storage form of carbohydrates in animals (including humans).

Total carbohydrate stores: ~400-600 g (1600-2400 kcal)

Key limitation: Glycogen stores are limited and can be depleted within 60-90 minutes of intense exercise.

Functions of Carbohydrates

Carbohydrate Metabolism During Exercise

Key insight: As exercise intensity increases, reliance on carbohydrates increases because:

• Carbohydrates produce more ATP per unit oxygen

• Fat oxidation cannot keep pace with high energy demands

• Anaerobic glycolysis can supplement aerobic metabolism

Carbohydrate Requirements for Athletes

Dietary Sources of Carbohydrates

High-Quality Sources (Nutrient-Dense)

Quick-Release Sources (Performance)

Fiber

Definition

Dietary fiber consists of non-digestible carbohydrates and lignin that are intrinsic and intact in plants.

Types of Fiber

Fiber Recommendations

• General: 25-38 g/day

• Athletes: Similar, but may need to reduce before competition to prevent GI distress

Fiber Considerations for Athletes

• Benefits: Satiety, blood sugar control, gut health

• Caution: High fiber before competition may cause GI distress

• Strategy: Reduce fiber 24-48 hours before important events

Fats (Lipids)

Definition

Fats are organic compounds composed of carbon, hydrogen, and oxygen, but with a lower proportion of oxygen than carbohydrates. They are the most energy-dense macronutrient and serve crucial structural and metabolic functions.

Energy Yield

$\text{Fats} = 9 \text{ kcal/g} = 37 \text{ kJ/g}$

Fats provide more than twice the energy per gram compared to carbohydrates or proteins.

Classification of Fats

1. Triglycerides (95% of Dietary Fat)

Structure: Glycerol backbone + 3 fatty acid chains

        Fatty Acid 1
       /
Glycerol — Fatty Acid 2
       \
        Fatty Acid 3

2. Fatty Acid Types

Based on Saturation (Double Bonds)

3. Essential Fatty Acids

The body cannot synthesize these, so they must be obtained from diet:

Omega-3 to Omega-6 Ratio:

• Recommended: 1:1 to 1:4

• Typical Western diet: 1:15 to 1:20 (too much omega-6)

• Athletes should increase omega-3 intake for anti-inflammatory effects

Fat Digestion and Absorption

1. Mouth: Lingual lipase begins minimal fat digestion

2. Stomach: Gastric lipase continues digestion; fats slow gastric emptying

3. Small intestine: Bile emulsifies fats; pancreatic lipase breaks down triglycerides

4. Absorption: Fatty acids and monoglycerides absorbed; reassembled into triglycerides

5. Transport: Packaged into chylomicrons; enter lymphatic system, then bloodstream

Fat Storage

Key advantage: Fat stores are essentially unlimited compared to glycogen stores.

Functions of Fats

Fat Metabolism During Exercise

Lipolysis

The breakdown of triglycerides into glycerol and free fatty acids (FFA): $\text{Triglyceride} \xrightarrow{\text{Lipase}} \text{Glycerol} + 3 \text{ Free Fatty Acids}$

Beta-Oxidation

The breakdown of fatty acids in mitochondria to produce acetyl-CoA for the Krebs cycle:

• Occurs in mitochondria

• Requires oxygen (aerobic only)

• Slower than glycolysis but produces more ATP per molecule

Factors Affecting Fat Oxidation

Fat Requirements for Athletes

Absolute intake: Approximately 1-1.5 g/kg/day for most athletes

Dietary Sources of Fats

Healthy Fat Sources

Sources to Limit

Fat Adaptation and Ketogenic Diets

Fat Adaptation

Training the body to rely more on fat oxidation through:

• Low-carbohydrate diets

• Fasted training

• High-fat diets

Potential benefits:

• Glycogen sparing

• Sustained energy for ultra-endurance events

Limitations:

• Impairs high-intensity performance

• Reduces exercise economy (more oxygen needed per ATP)

• May impair carbohydrate metabolism capacity

Ketogenic Diets for Athletes

Very low carbohydrate (< 50 g/day), high fat diet that induces ketosis.

Current evidence: Not recommended for most athletes, especially those requiring high-intensity efforts.

Proteins

Definition

Proteins are large, complex molecules composed of amino acids linked by peptide bonds. They are the primary structural components of body tissues and play essential roles in virtually all biological processes.

Energy Yield

$\text{Proteins} = 4 \text{ kcal/g} = 17 \text{ kJ/g}$

Same energy density as carbohydrates, but proteins are not a preferred energy source.

Structure of Proteins

Amino Acids

The building blocks of proteins. Each amino acid contains:

• Amino group (-NH₂)

• Carboxyl group (-COOH)

• Side chain (R group) — determines properties

Types of Amino Acids

Branched-Chain Amino Acids (BCAAs)

Three essential amino acids with branched side chains:

• Leucine: Most important for muscle protein synthesis

• Isoleucine: Energy production, blood sugar regulation

• Valine: Muscle metabolism, tissue repair

BCAAs are metabolized directly in muscle (not liver) and are popular supplements for athletes.

Protein Quality

Complete vs Incomplete Proteins

Limiting Amino Acid

The essential amino acid present in the lowest amount relative to requirements. This limits the protein's usability for synthesis.

Complementary Proteins

Combining incomplete proteins to provide all essential amino acids:

• Rice + beans

• Peanut butter + whole wheat bread

• Hummus + pita

Note: Complementary proteins do not need to be consumed at the same meal; daily intake is sufficient.

Protein Quality Scores

Protein Digestion and Absorption

1. Stomach: Hydrochloric acid denatures proteins; pepsin begins breakdown

2. Small intestine: Pancreatic proteases (trypsin, chymotrypsin) continue digestion; peptidases complete breakdown to amino acids

3. Absorption: Amino acids absorbed into bloodstream via intestinal wall

4. Transport: Amino acids transported to liver and tissues

5. Utilization: Used for protein synthesis or other metabolic processes

Functions of Proteins

Protein Metabolism

Protein Turnover

The continuous process of protein breakdown (proteolysis) and synthesis:

• ~300-400 g of protein turned over daily in adults

• Balance between synthesis and breakdown determines net protein status

Nitrogen Balance

$\text{Nitrogen Balance} = \text{Nitrogen Intake} - \text{Nitrogen Excretion}$

Factors Affecting Protein Synthesis

Protein Requirements for Athletes

General Population

• RDA: 0.8 g/kg/day (minimum to prevent deficiency)

Athletes

Upper limit: No additional benefit beyond ~2.2-2.5 g/kg/day for most athletes.

Example Calculation

An 80 kg strength athlete needs: $80 \text{ kg} \times 1.8 \text{ g/kg} = 144 \text{ g protein/day}$

Protein Distribution

Timing Matters

Research suggests distributing protein throughout the day is more effective than consuming large amounts at once.

Optimal approach:

• 20-40 g of protein per meal

• 4-5 eating occasions per day

• Include protein post-exercise (anabolic window)

Muscle Full Effect

Muscle protein synthesis becomes maximally stimulated at ~20-40 g of high-quality protein. Additional protein beyond this provides diminishing returns for that meal (though it still provides amino acids for other functions).

Dietary Sources of Protein

Animal Sources (Complete Proteins)

Plant Sources

Protein and Exercise

Resistance Exercise

• Stimulates muscle protein synthesis for 24-48 hours

• Requires protein intake to provide amino acids

• 20-40 g post-exercise optimizes synthesis

• Leucine content is particularly important (≥2-3 g)

Endurance Exercise

• Increases protein oxidation during exercise

• Requires protein for repair and adaptation

• 20-25 g post-exercise supports recovery

The "Anabolic Window"

The period after exercise when muscle is most sensitive to protein intake:

• Traditional view: 30-60 minutes post-exercise

• Current understanding: 2-3 hour window; less critical if pre-exercise protein was consumed

• Practical recommendation: Consume protein within 2 hours post-exercise

Micronutrients for Sport

Definition

Micronutrients are nutrients required in small amounts (milligrams or micrograms) that are essential for normal physiological function. They include vitamins and minerals.

Vitamins

Classification

Key Vitamins for Athletes

Vitamin D — Critical for Athletes

Functions:

• Calcium absorption and bone health

• Muscle function and strength

• Immune function

• Inflammation regulation

• May affect testosterone levels

Deficiency Consequences:

• Impaired bone health (stress fractures)

• Muscle weakness

• Increased injury risk

• Impaired immune function

• Fatigue

At-Risk Athletes:

• Indoor sport athletes

• Athletes in northern latitudes

• Dark-skinned athletes

• Athletes who cover skin (religious/cultural reasons)

• Winter sport athletes (despite sun, skin covered)

Recommendations:

• General: 600-800 IU/day

• Athletes (especially at-risk): 1000-4000 IU/day

• Monitor blood levels (target: 40-60 ng/mL or 100-150 nmol/L)

Minerals

Key Minerals for Athletes

Iron — Critical for Endurance Athletes

Functions:

• Hemoglobin (oxygen transport in blood)

• Myoglobin (oxygen storage in muscle)

• Cytochromes (electron transport chain)

• Energy metabolism enzymes

Iron Deficiency Stages:

1. Iron depletion: Low ferritin, normal hemoglobin

2. Iron-deficient erythropoiesis: Low iron, low transferrin saturation

3. Iron deficiency anemia: Low hemoglobin, impaired oxygen transport

At-Risk Athletes:

• Female athletes (menstrual losses)

• Endurance athletes (foot-strike hemolysis, GI bleeding, sweat losses)

• Vegetarian/vegan athletes

• Athletes with restricted energy intake

• Adolescent athletes (growth demands)

Heme vs Non-Heme Iron:

Enhancing Iron Absorption:

• Consume vitamin C with iron-rich foods

• Include some heme iron with non-heme sources

• Avoid tea/coffee with meals (tannins inhibit absorption)

• Avoid calcium supplements with iron-rich meals

Iron Recommendations:

• Males: 8 mg/day

• Females (premenopausal): 18 mg/day

• Female athletes may need higher intake (up to 18-25 mg/day)

Calcium — Essential for Bone Health

Functions:

• Bone and teeth structure (99% of body calcium)

• Muscle contraction

• Nerve impulse transmission

• Blood clotting

Deficiency Consequences:

• Decreased bone mineral density

• Stress fractures

• Osteoporosis (long-term)

• Impaired muscle function

At-Risk Athletes:

• Athletes with energy restriction

• Athletes with eating disorders

• Amenorrheic female athletes

• Lactose-intolerant athletes avoiding dairy

• Vegan athletes

Recommendations:

• Adults: 1000-1300 mg/day

• Athletes: Same, but ensure adequate intake

Calcium-Rich Foods:

Antioxidants in Sport

Oxidative Stress and Exercise

Exercise increases oxygen consumption and metabolic rate, generating reactive oxygen species (ROS) and free radicals. While some ROS is necessary for adaptation, excessive oxidative stress can:

• Damage cell membranes

• Impair muscle function

• Delay recovery

• Contribute to inflammation

Key Antioxidant Nutrients

Supplementation Considerations

• Food-first approach: Obtain antioxidants from whole foods

• Excessive supplementation may be harmful: High-dose antioxidants may blunt training adaptations by interfering with necessary ROS signaling

• Research: Mixed results on antioxidant supplementation for performance

• Recommendation: Eat a varied diet rich in fruits and vegetables; avoid megadoses

Electrolytes

Definition

Electrolytes are minerals that carry an electrical charge when dissolved in body fluids. They are essential for hydration, nerve function, and muscle contraction.

Key Electrolytes for Athletes

Sodium is the most important electrolyte to replace during and after prolonged exercise (> 60-90 minutes) due to significant sweat losses.

Summary: Macronutrient Comparison

Practical Recommendations for Athletes

Daily Nutrition Framework

Micronutrient Priorities

1. Eat a varied diet with fruits, vegetables, whole grains, lean proteins

2. Prioritize vitamin D (sunlight, fortified foods, supplementation if needed)

3. Monitor iron status (especially female and endurance athletes)

4. Ensure adequate calcium for bone health

5. Replace electrolytes during prolonged exercise

Food-First Approach

• Prioritize whole foods over supplements

• Supplements only when dietary intake is inadequate or needs are elevated

• Consult sports dietitian for individualized recommendations

Exam Tips

1. Know energy values: CHO = 4 kcal/g, Fat = 9 kcal/g, Protein = 4 kcal/g

2. Understand fuel selection: CHO dominant at high intensity; fat dominant at low intensity

3. Know glycogen limitations: ~2000 kcal storage; depleted in 60-90 min of intense exercise

4. Essential fatty acids: Omega-3 and omega-6 must come from diet

5. Essential amino acids: 9 that cannot be synthesized

6. Protein timing: 20-40 g per meal; distribute throughout day

7. Key micronutrients: Iron (oxygen transport), calcium (bone), vitamin D (bone + muscle)

8. At-risk populations: Female athletes, vegetarians, restricted energy intake

9. Calculate requirements: Be able to calculate g/kg needs for different athletes

10. Apply to sport scenarios: Recommend nutrition strategies for different sports/situations