Metabolism and Personal Choices: Exercise, Training and Diet

From 100 m to 10 km

A sprinter and a cross-country skier both need ATP, but their muscles get it in very different ways. A 100 m dash is over in about 10 seconds; a 10 km ski race can last half an hour. Understanding cellular respiration lets you understand why athletes train the way they do, and helps you make informed choices about exercise, food and supplements, which is exactly what expectation C1.2 asks: to assess how knowledge of metabolic processes is relevant to personal choices such as exercise and diet.

Three energy systems

Muscles regenerate ATP using three systems that overlap, but each dominates at a different time scale:

1. ATP–creatine phosphate (CP) system. Muscles store creatine phosphate. The enzyme creatine kinase transfers its phosphate to ADP: creatine phosphate + ADP → creatine + ATP. It's instant but runs out in roughly 10 seconds of all-out effort: a jump, a slapshot, a short sprint.
2. Anaerobic glycolysis (lactate fermentation). Glycolysis is fast and needs no O₂, so it provides much of the ATP for intense efforts of roughly 10 seconds to a couple of minutes (a 400 m race, a hard hockey shift). It yields only 2 ATP per glucose and produces lactate and H⁺, which contribute to fatigue.
3. Aerobic respiration. Slower to ramp up, because O₂ delivery has to increase, but it can run for hours using glucose, glycogen and fat. It dominates any effort longer than a few minutes, and also powers recovery between efforts.

In real activities, all three work together; the mix depends on intensity and duration.

How training changes your cells

Your cells respond to the demands you put on them.

Endurance training (running, cycling, skiing, swimming for 30 minutes or more):

• increases the number and size of mitochondria in muscle fibres, so more ATP can be made aerobically;
• increases the capillaries around muscle fibres and the amount of myoglobin, improving O₂ delivery;
• increases the enzymes for fat oxidation, so trained muscles spare glycogen;
• strengthens the heart, so more O₂-rich blood is pumped per beat, raising VO₂ max and the lactate threshold.

Sprint and strength training increases muscle size, stored creatine phosphate and glycolytic enzymes, improving short, powerful efforts.

Muscle fibres also differ. Slow-twitch fibres are packed with mitochondria and myoglobin and resist fatigue. Fast-twitch fibres are powerful but rely more on glycolysis and tire quickly. The proportion of each is partly inherited and partly trainable.

Diet: fuel and helpers

• Carbohydrates are stored as glycogen in the liver and muscles. Long, hard efforts (over about 90 minutes) can drain glycogen, and athletes describe "hitting the wall". This is why endurance athletes eat carbohydrate before and during long events.
• Fats are a huge energy store and the main fuel during long, moderate activity and at rest. Fat oxidation needs more O₂ per ATP and can't keep up with very high intensities.
• Proteins are mainly for building and repairing tissue; they are a minor fuel unless the body is short of other fuels.
• B vitamins are needed to make the carriers and coenzymes of respiration: niacin (B3) for NAD⁺, riboflavin (B2) for FAD, pantothenic acid (B5) for coenzyme A, thiamine (B1) for the enzyme of pyruvate oxidation.
• Iron is part of hemoglobin (O₂ transport) and of the cytochromes in the ETC. Iron deficiency is a common cause of tiredness, because less O₂ is delivered and the ETC works less well.

Canada's Food Guide recommends a variety of vegetables and fruits, whole-grain foods and protein foods, with water as the drink of choice. A balanced diet usually supplies enough of these nutrients; anyone considering supplements should talk to a doctor, pharmacist or registered dietitian.

Evaluating claims (A1.9)

Knowledge of metabolism helps you judge the products and advice you see online. Ask:

1. Does the mechanism make biological sense? (For example, a claim that a drink "gives you energy" without containing any fuel or caffeine doesn't.)
2. What is the evidence? Look for controlled studies in humans, not only testimonials.
3. Who benefits from the claim? Is someone selling something?
4. Is it safe? Some products interfere with respiration dangerously; for example, DNP, an uncoupler (Lesson 6), has been sold illegally as a "fat burner" and has caused deaths.

Not every supplement is useless: for example, research suggests that creatine supplements can improve performance in repeated short, high-intensity efforts in some people, which makes sense given the ATP–CP system. The skill is to weigh the evidence, not to accept or reject claims automatically.