Eggs are a good source of low-cost high-quality protein, providing 5.5 grams of protein (11.1% of the daily value for protein) in one egg for a caloric cost of only 68 calories. The structure of humans and animals is built on protein. We rely on animal and vegetable protein for our supply of amino acids, and then our bodies rearrange the nitrogen to create the pattern of amino acids we require. Another health benefit of eggs is their contribution to the diet as a source of choline. Although our bodies can produce some choline, we cannot make enough to make up for an inadequate supply in our diets, and choline deficiency can also cause deficiency of another B vitamin critically important for health, folic acid. Choline is a key component of many fat-containing structures in cell membranes, whose flexibility and integrity depend on adequate supplies of choline. Two fat-like molecules in the brain, phosphatidylcholine and sphingomyelin, account for an unusually high percentage of the brain's total mass, so choline is particularly important for brain function. In addition, choline is a highly important molecule in a cellular process called methylation. Many important chemical events in the body are made possible by methylation, in which methyl groups are transferred from one place to another. For example, genes in the body can be switched on or turned off in this way, and cells use methylation to send messages back and forth. Choline, which contains three methyl groups, is highly active in this process. Choline is also a key component of acetylcholine. A neurotrasmitter that carries messages from and to nerves, acetylcholine is the body's primary chemical means of sending messages between nerves and muscles.