Exercises: The three groups of energy-providing nutrients

Classify the following foods from Mexican cuisine according to the nutrient group they mainly provide (carbohydrates, proteins, or lipids): corn tortilla, cooked beans, avocado, chicken breast, nuts, and rice. Briefly justify each choice.

Both the avocado and the cucumber are plant-based foods, but their energy content is very different. Explain, in terms of which nutrient each one contains more of (lipids or water and fiber), why the avocado provides much more energy per 100 g.

Carbohydrates provide about $4 \text{ kcal}$ per gram, proteins about $4 \text{ kcal}$ per gram, and lipids about $9 \text{ kcal}$ per gram. Order these 10 g portions of pure nutrient from lowest to highest energy content: 10 g of fat from butter, 10 g of protein from tuna, and 10 g of sugar (carbohydrate). Show the calculation for each one.

Before a race, many athletes eat pasta or rice for dinner instead of a high-fat meal. Using what you know about nutrient groups and their function in the body, explain two chemical and biological reasons why this choice makes sense.