Vegetarian food is rich in linoleic acid, a short-chain omega-6 fatty acid, and it contains only small amounts of the short-chain omega-3 alpha linolenic acid (ALA). Furthermore, vegetarian food is practically absent of the long-chain fatty acids EPA and DHA that are extra important for good health. These are often missing since they mainly come from fish. Women who are vegetarians have less DHA in their mother's milk than none-vegetarians. They can have as little as 0.05 percent compared to Inuit women who have 1.40 percent, this is 28 times as much. Chinese women on the island Zhangzi have up to 2.78 percent DHA in their milk (Fidlera et al.).
Herbivores are animals who eat only plants. Their bodies are made for a vegetarian diet, and they have enzymes in the liver that can transform the short-chain fatty acids to their long-chain derivatives. Predators cannot do this, instead they must eat the herbivores. The human body is able to transform only small amount of ALA to EPA and DHA. It is not known how efficient this transformation is, studies have shown different results. A limiting factor is that the same enzymes are used to transform both linoleic acid (omega-6) and ALA (omega-3), and it can not be done simultaneously.
A study from 2001 (Pawlosky et al.) indicates that only 0.2 percent of the ALA is transformed to EPA, but that the further transformation to DHA is more efficient. However, there are other studies where the transformation to EPA have been slightly more efficient. The process still seems to be inefficient and depending on many factors. This is why it is important to include the long-chain fatty acids EPA and DHA in the diet. Deficiency of DHA can impair eyes and brain in fetus, it is therefore extra important for pregnant women to include DHA in their diet. For those who don't eat animal products, there will probably be supplements available in the future with EPA and DHA extracted from marine plants.