Nine of the 20 standard amino acids are "essential" amino acids for humans. They cannot be built (synthesised) from other compounds by the human body, and so must be taken in as food. Others may be essential for some ages or medical conditions. Essential amino acids may also differ between species. Herbivores have to get their essential amino acids from their diet, which for some is almost entirely grass. Ruminants such as cows get some amino acids via microbes in the first two stomach chambers.