Nutrition and Physical Activity: Key Concepts and Applications

Nutrition and Physical Activity

Energy Needs

Energy requirements for physical activity depend on several factors, including the type and characteristics of the activity and the individual. For athletes, both the source and amount of dietary energy are crucial for optimal performance and health.

• Intensity, duration, and frequency of activity: Higher intensity, longer duration, and more frequent activity increase energy needs.

• Characteristics of the exerciser: Age, sex, body size, and metabolic rate influence energy requirements.

• Location: Environmental factors such as temperature and altitude can affect energy expenditure.

• Source vs. amount of energy: The macronutrient composition (carbohydrates, fats, proteins) is as important as total caloric intake for athletes.

Relationship of Exercise to Intake

Physical activity increases the need for certain vitamins and minerals, and the proportion of macronutrients in the diet should be adjusted to support athletic performance.

• B vitamins: Essential for energy metabolism.

• Antioxidant vitamins (C and E): Protect against oxidative damage caused by increased oxygen use during exercise.

• Iron: Prevents anemia, which can impair oxygen transport and performance.

• Calcium: Important for bone health and muscle function.

Recommended macronutrient distribution for athletes and healthy individuals:

• Carbohydrates: 45–65% of total energy

• Fat: 20–35% of total energy

• Protein: 10–35% of total energy (slightly higher for athletes, but usually met with food)

Female Athlete Triad

The Female Athlete Triad is a syndrome observed in physically active females, involving three interrelated health problems:

• Restrictive eating patterns: Insufficient energy intake due to dieting or disordered eating.

• Eating disorders: Such as anorexia nervosa or bulimia nervosa.

• Amenorrhea: Absence of menstruation due to low energy availability.

• Bone formation issues: Increased risk of osteoporosis and bone fractures.

Sports Anemia

Sports anemia is a temporary condition seen in athletes, especially those engaged in endurance sports. It is characterized by reduced hemoglobin levels due to an increase in plasma volume, which dilutes red blood cells. This adaptation is generally beneficial and does not indicate true iron-deficiency anemia.

• Normal: Normal concentration of red blood cells in plasma.

• Sports anemia: Lower concentration due to increased plasma volume.

Additional info: True anemia should be distinguished from sports anemia by assessing iron status and clinical symptoms.

Fluid Needs for Physical Activity

Proper hydration is essential for maintaining performance and preventing heat-related illnesses during physical activity.

• Functions of fluid: Eliminates heat, transports oxygen and nutrients, removes waste products (e.g., lactic acid).

• Hydration and heat dissipation: The body's ability to lose heat depends on hydration status.

• Daily water loss: At rest in a temperate climate, about 4.5 cups of water are lost per day through evaporation from the skin and lungs.

• Dehydration: Occurs when fluid losses are not replaced, leading to impaired performance and increased health risks.

Performance and Dehydration

As dehydration increases, physical performance declines. Even a 2% loss of body weight from fluid can significantly impair endurance and strength.

Humidex and the Risk of Heat-Related Illness

The humidex is an index that combines temperature and humidity to estimate the risk of heat-related illness during physical activity. As humidex values rise, the risk increases, and precautions should be taken.

Humidex (°C)	Response
30–32	Monitor for symptoms; encourage hydration
33–35	Take additional breaks; reduce intensity
36–39	Consider rescheduling or moving activity indoors
40+	Cancel or postpone outdoor activity
Additional info: Table adapted from Figure 13.23; actual recommendations may vary by organization.

Recommended Fluid Intake

Fluid needs vary before, during, and after exercise. Proper hydration strategies help maintain performance and prevent dehydration.

Timing	Recommendations
Before Exercise	Begin exercise well hydrated by consuming generous amounts of fluid in the 24 hours before exercise. Consume 5 to 10 mL/kg body weight of fluids 2 to 4 hours before exercise.
During Exercise	Replace water lost from sweating (typically 0.4 to 0.8 L per hour). For exercise lasting <60 minutes, plain water is sufficient. For exercise >60 minutes, fluids containing 6–8% carbohydrate and electrolytes may improve endurance.
After Exercise	Begin fluid replacement immediately after exercise. Consume 500–750 mL (16–24 ounces) of fluid for each 0.5 kg (1 lb) of weight lost.

Hyponatremia

Hyponatremia is a condition caused by low blood sodium levels, often resulting from excessive water intake without adequate sodium replacement during prolonged exercise. Symptoms include nausea, headache, confusion, and in severe cases, seizures or coma.

• Normal: Blood sodium concentration is maintained.

• During exercise: Sodium is lost in sweat.

• Overhydration: Replacing only water dilutes blood sodium, leading to hyponatremia.

What to Eat: Nutrition Strategies for Athletes

Proper nutrition before, during, and after exercise maximizes performance, recovery, and overall health.

• Maximize glycogen storage: Glycogen is the primary fuel for endurance exercise.

• Carbohydrate loading: Increases muscle glycogen stores; beneficial for endurance events >90 minutes.

• Pre-exercise: Consume fluids and 60–70% of calories from carbohydrates 2–3 hours before the event.

• During exercise: For events >1 hour, consume 30–60 g carbohydrate per hour and adequate fluids.

• After exercise: Replenish fluids, electrolytes, glycogen, and protein for muscle repair.

Label Literacy: Sports Bar Comparison

Understanding nutrition labels helps athletes choose appropriate snacks for energy and recovery. The table below compares the nutrient content of different sports bars.

Nutrient (per bar)	Energy Bar (Before Exercise)	Protein Bar (After Exercise)	Milk Chocolate Bar
Calories	200	300	220
Total Fat (g)	2.2	8	11
Saturated Fat (g)	1	3	6
Total Carbohydrate (g)	43	31	26
Fibre (g)	4.5	2	1
Protein (g)	5	20	3
Vitamin A (% DV)	27	0	0
Vitamin C (% DV)	25	0	0
Calcium (% DV)	15	15	8
Iron (% DV)	15	15	6
Additional info: Other vitamins and minerals may be present in these bars.