Vitamin D Comes From Sunshine, Vitamin B12 From Bacteria
How can a diet consisting mainly of leaves give gorillas enough protein to grow big and strong? It’s because leaves contain protein. Leaves are low in calories, but a lot of their calories come from protein. In fact, most ordinary plant foods contain more than enough protein to meet human nutritional needs. Nutrition scientists have known for nearly a hundred years that as long as you are eating any reasonable plant-based diet, if you take care of the calories, the protein takes care of itself.
It’s hard even to design a plant-based diet that would be deficient in protein while providing enough calories. You’d have to include nothing but some low-protein fruit, such as apples. Or you could cheat and use sugars and fats that have been extracted from plants, leaving the protein and other nutrients behind.
Not only do plants provide enough protein for human nutrition, the proteins they contain are nutritionally “complete,” as far as human protein needs go. That means that they contain enough of all of the different amino acid “building blocks” that human bodies need.
The only “incomplete” protein you are likely to find on your dinner plate is gelatin, which comes from animal bones. Gelatin is incomplete because tryptophan is destroyed in the manufacturing process. You’d get very sick if you tried to use gelatin as your sole source of protein.
Plant foods also contain the minerals that are essential for human nutrition. Plants absorb these minerals, such as calcium and iron, from the soil. After all, where did the cows get the calcium that goes into their milk? Where did they get the iron that goes into their flesh?
Plants also provide nearly all of the vitamins that are essential in human nutrition. The exceptions are vitamin B12, which is made by bacteria, and vitamin D, which your body can make for itself if you go outside in the sunshine. People who eat a purely plant-based diet are generally advised to take a vitamin B12 supplement. Whether you need a vitamin D supplement depends on how dark your skin is and where you live. Your doctor can do a simple blood test to see whether you have enough of both of these vitamins.