Without PKU formula, your body can't get enough of essential nutrients called amino acids that are crucial for growth and general health.

CNN: Phenylketonuria (PKU)

To come up with his index, Drewnowski ranked foods based on their concentrations of nine essential nutrients (protein, fiber, calcium, iron, magnesium, potassium, vitamins A, C, E).

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The cave environment is a tough place in which to make a living, limited in food and essential nutrients, forcing microorganisms to fight over limited resources.

BBC: Cave bacteria could help develop future antibiotics

"I think that animal proteins provide essential nutrients that you are able to get in a unique package of foods for, let's say, about 200 calories -- a three ounce serving of meat, " Rodriguez said.

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