Diet and energy sources

Diet is only one of several lifestyle choices affecting a measure such as VO2 max. Asked how diet affects it, candidates sometimes answer with an unrelated choice — smoking or poor sleep — instead of eating. Stay on the named choice: give a diet-specific mechanism, e.g. carbohydrate-rich eating fuelling better training. Re-read the stem for WHICH choice is asked.

Asked for the effect of an unbalanced diet, candidates often give a generic answer ("you will feel tired") with no activity-linked consequence. A mark-scheme answer needs a chain: a distance runner who eats too little carbohydrate stores less glycogen, so it runs out earlier and they cannot maintain pace. The activity-linked consequence scores, not "tiredness".

Glucose is the simple sugar from digesting carbohydrate, used directly in respiration to release energy. Glycogen is its storage form — glucose not needed becomes glycogen, stored in the muscles and liver and converted back later. Candidates swap the words: "glycogen is used in respiration" (no — that is glucose) or "glucose is stored" (no — it is stored as glycogen).

A nutrient question usually needs the function AND a plausible food source; naming only a food, or only a function, drops a mark. "Avocado" alone does not state that fat is a reserve energy source, and "growth and repair" alone does not name a protein source such as eggs. Give both halves, and match the source to the nutrient.

Energy balance = intake - expenditure, so a positive result means MORE was taken in than used (a surplus, stored as fat) and a negative means less (a deficit, weight loss). Candidates subtract in the wrong order and flip the sign, drop the minus on a deficit, or call a negative balance "positive" because the number looks large. Keep intake first and trust the sign.

Carbohydrate is the body's main (primary) energy source for muscle contraction; fat is the secondary/reserve source, which also insulates and protects organs. Candidates reverse these — fat as the main source, or carbohydrate as the reserve — which loses the mark. Carbohydrate is the primary supply used first; fat is the reserve backup.

Students assume obesity comes from eating too much fat, but it results from a positive energy balance — more energy in than expended — whatever nutrient the surplus is. Excess carbohydrate or protein is stored as fat too, if intake exceeds expenditure. State it as "intake greater than expenditure", not "too much fatty food".