 Protein-rich foods, including animal and plant-based proteins, can also vary enormously in their quality and their implications for our long-term health. Proteins are often said to act as building blocks for the lean tissues in our bodies, but they serve many other important functions in body regulation, support of immune function, and a variety of other roles in our physiology. There are twenty types of amino acids needed to fulfill all of these functions, but only nine of them are essential, meaning that our bodies can't make enough of them, so we rely on getting these from our food. In general, animal sources of protein, like fish and eggs, provide all of the essential amino acids in high enough concentrations that these foods are called complete protein sources. In contrast, plant-based protein sources like beans, lentils, nuts, and tofu tend to be incomplete sources of protein. It might seem that since plant-based proteins are incomplete in their nutrient content, that they're nutritionally inferior compared with animal-based proteins, but in fact, the health benefits of substituting plant-based proteins for animal-based ones, ideally a few days a week, this far outweighs the risk of falling short on essential amino acids. Plant-based proteins can be combined with other foods to provide a complete amino acid profile. In fact, many traditional food combinations like corn and black beans, or rice and lentils, are based on the principle of combining complementary proteins. Meals that contain vegetarian sources of protein also contain more fiber and less fat, especially saturated fat than meals that feature animal-based protein. And even though saturated fat may not be as harmful as we once thought, it can still contribute to elevated levels of LDL cholesterol if we eat too much of it, so moderating our intake of red meat, for example, is still a sensible idea. In general, the typical Western diet contains more animal-based protein than we need to sustain good health. People who eat lots of plant foods eat a plant-based diet tend to have much better health and better longevity than people who eat a heavy meat diet. So if you're thinking of trying a low-carb, high-protein diet, it's important to pay attention to the kinds of proteins that are being consumed and in what quantities. A diet high in animal protein, especially if it's poor quality animal protein, like processed meat or high-fat cuts of meat, this kind of diet can be harmful to our health, even if it leads to weight loss in the short term. Processed meats often contain nitrates used as a preservative which can damage blood vessels and contribute to hardening of the arteries. These meats also tend to be very high in sodium, which can be a contributor to high blood pressure. The most sensible diets are usually those that encourage us to eat moderate amounts of high-quality protein foods. These should come from a variety of different sources, including some fish, if possible, as well as plenty of vegetarian sources of protein that are combined for completeness.