How to scientists know what food is the most healthy for a certain species? (for example: Humans)

Question

owlfred · Answer

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anonymous · Answer

A food is healthy for a species like for humans, our foods must possess carbohydrates for energy, proteins for growth and lipids for storage of energy. Scientists conduct test for certain foods like if the food has carbohydrate, one test is by putting iodine solution to the food and when it turns black it is positive. For protein, there is the Beirut test.

anonymous · Answer

Each species(animals in it) has certain daily requirements and has certain definite modes to meet them. Even quantity of food taken by human beings change according to the work they are doing.....sedentary people needs less  calories , compared to quarry workers.
        The calorie value is determined by separating the material to be tested(each group have a specific property which is used in separation) and burning them in a controlled media to measure how much heat energy is produced which is later quantified.
       Each molecule of protein has a net protein value , which can produce useful contribution to human growth.(eg: egg) 
       The food we eat depends on what we can digest too....Herbivore animals have certain microorganisms in their gut which breaks down cellulose, which animals like human beings do not have. So when we eat grass or hay we won't be able to extract energy from it.

matthewrlee · Answer

If you eat it and it kills you or makes you sick it goes on the "do not eat" list. That is rule number one. I don't know rule number two.