Eat food

We ingest nutrients by eating organic substances originating from other life forms, like plants and animals. We need food as a source of energy and for cell growth.

Photosynthesis is the ultimate source of food for nearly all life on earth: Plants combine water (H2O), carbon dioxide (CO2) and sun light into sugar, the basic building block of all carbohydrates.

Our body can convert back and forth between sugars, fatty acids, amino acids, etc, often using the liver. But we cannot make all amino acids, the building blocks of protein.

The limiting amino acid is the essential amino acid which is furthest from meeting nutritional requirements. This concept is important when determining the selection, number, and amount of foods to consume because even when total protein and all other essential amino acids are satisfied if the limiting amino acid is not satisfied then the meal is considered to be nutritionally limited by that amino acid.