That being said there are a few nutrients that are hard if not impossible to get sufficient amounts of from plant based foods, making it important to be aware of them and supplement your diet with them to maintain optimal health and physical performance.
Vitamin B12 is almost exclusively found in animal sourced foods such as eggs, fish, meat, and dairy products. This nutrient is also known as cobalamin, it is water soluble and involved in developing red blood cells as well as maintaining nerves and normal brain function. Vegans who don’t take supplements are at a higher risk of having a vitamin B12 deficiency than vegetarians; symptoms include but are not limited to fatigue, weakness, impaired brain function, neurological disorders, psychiatric disorders, and megaloblastic anemia.
Creatine is found in animal sourced foods, most of this nutrient is stored in muscles but significant amounts are also concentrated in the brain. Creatine functions as an energy reserve for muscle cells to give them greater strength and endurance. Supplementing may have significant benefits such as improving physical performance and improvements in brain functions like memory.
Carnosine is important to muscle function and is only found in animal sourced foods, it is an antioxidant that is concentrated in the muscles and brain. Carnosine is not an essential nutrient, but supplementing it may be a good way to boost muscles to improve endurance and increase muscle mass.
Vitamin D3 is the sunshine vitamin which the skin produces when exposed to the sun. There are 2 forms of dietary vitamin D: cholecalciferol(D3) found in animal based foods, and ergocalciferol(D2) found in plants; D3 increases blood levels of absorbable vitamin D more efficiently. The best source of D3 are egg yolks, fatty fish, and cod liver oil. Deficiency is linked to increased risk of cancer, heart disease, depression, impaired brain function, multiple sclerosis, osteoporosis, muscle wasting, and reduced strength.
Docosahexaenoic acid is mainly found in fatty fish, fish oil, and certain types of microalgae. It ia an essential omega-3 fatty acid that is important for normal brain development and function. DHA can be made from ALA found in walnuts, chia seeds, and flax seeds but the conversion is inefficient may not increase blood levels of DHA sufficiently.
Heme iron is only found in meat, especially red meat, and helps to improve absorption of non-heme iron from plant foods. Non-heme iron is poorly absorbed, and this can be limited by antinutrients present in plant foods such as phytic acid.
Taurine is only found in animal sourced foods, it is a sulfur compound found in various body tissues, including the kidneys, brain, and heart. This nutrient appears to play a role in muscle function, bile salt formation, and antioxidant defenses.
When they are well planned vegetarian and vegan diets are very healthy, unfortunately there are several nutrients these diets may not provide. If you are eliminating animal sourced foods from your diet make sure to investigate which nutrients you may be missing out on, and supplement if needed to make sure that your body gets what it needs. If you are not sure consult with your doctor who will help set out a plan for your needs.