Final answer:
Fatty acids are the most energy-dense macronutrient, providing nine calories per gram, compared to carbohydrates and proteins which both offer only four calories per gram. Fatty foods are calorie-dense and more efficient for long-term energy storage in animals.
Step-by-step explanation:
When comparing the amount of energy gained from the mass of macronutrients, fatty acids are the most dense source of energy. One gram of carbohydrates, including glucose and glycogen, supplies four calories of energy, whereas a gram of protein also provides four calories. However, one gram of fat, which is what fatty acids are made of, provides a substantial nine calories. This higher caloric content makes fatty acids the macronutrient with the greatest energy density.
Fatty foods are thus considered calorie-dense, having more calories per unit mass than carbohydrates or proteins. Additionally, while glycogen is an energy reserve stored in the liver and skeletal muscles, it is hydrophilic and binds water, which increases its mass and reduces its energy density compared to fats that are stored in a more reduced, anhydrous form.
For energy storage, animals, including humans, often convert excess energy into fat, which is more efficient for long-term energy storage due to its high energy density. This adaptation is particularly important for animals that experience periods where food is scarce, such as hibernating animals.