Vitamin B12 is the most chemically complex of all vitamins.
Triathlon Magazine: Because of vitamin B12’s influence on red blood cells and oxygen transport in the body, it plays a vital role in a triathlete’s energy levels, health, training and performance.
Vitamin B12 is an important nutrient that is involved in many bodily functions, including the production of red blood cells and the metabolism of carbohydrates, fats, and proteins. Some research suggests that vitamin B12 may have specific benefits for endurance athletes, such as:
Improved performance: Some studies have found that vitamin B12 supplements can improve exercise performance in athletes, particularly when taken in combination with other B-vitamins.
Enhanced recovery: Vitamin B12 may help to reduce muscle fatigue and speed up recovery after exercise.
Increased energy production: Vitamin B12 is involved in the production of energy within the body, and may help to increase energy levels in athletes.
Improved oxygen delivery: Vitamin B12 is needed for the production of red blood cells, which carry oxygen to the muscles. By helping to increase the production of red blood cells, vitamin B12 may improve oxygen delivery to the muscles during exercise.
B12 is Found Naturally in Animal Products
Vitamin B12 is found naturally in animal products such as meat, fish, eggs, milk, and poultry. However, many people don't get enough from their diets. For example, according to the National Institutes of Health (NIH), adults between 19 and 50 years old should consume 2.4 micrograms of vitamin B12 per day. Those older than 51 should consume 3.0 micrograms daily.
Harvard.edu: Vitamin B12, or cobalamin, is naturally found in animal foods. It can also be added to foods or supplements. Vitamin B12 is needed to form red blood cells and DNA. It is also a key player in the function and development of brain and nerve cells.
Recommended Dietary Allowance:
Mayo Clinic: While the recommended daily amount of vitamin B-12 for adults is 2.4 micrograms, higher doses have been found to be safe.
Vitamin B12 is found naturally in a variety of animal-derived foods.
Meat: Beef, lamb, pork, and chicken are all good sources of vitamin B12.
Fish and shellfish: Fish such as salmon, trout, and haddock, as well as shellfish such as clams and mussels, are all rich in vitamin B12.
Dairy products: Milk, cheese, and yogurt are all good sources of vitamin B12.
Eggs: Eggs are also a good source of vitamin B12.
Vitamin B12 is not naturally found in plant-based foods. However, many plant-based foods are fortified with vitamin B12, including some types of plant-based milk, breakfast cereals, and nutritional yeast.
Perform Better with B12
If you're looking for ways to improve sports performance, consider taking a supplement of vitamin B12. In fact, some athletes take supplements of vitamin B12 before big games or competitions.
A study published in the Journal of Strength and Conditioning Research suggests that vitamin B12 dietary supplements improves athletic performance.
Researchers gave participants either a placebo or 1,000 mcg of vitamin B12 daily for eight weeks. Afterward, those who took vitamin B12 performed better on tests measuring strength, endurance, and speed.
"Vitamin B12 has been shown to increase muscle mass and reduce fatigue," says Dr. Michael Roizen, M.D., coauthor of You Are What You Eat. "It's important for people who exercise regularly because it helps maintain lean body mass."
Vitamin B12 Deficiency
National Library of Medicine: Indeed, DNA synthesis is impaired by low vitamin B12, which hampers the activation of folate and, as a result, decreases normal red blood cell production, impairing oxygen delivery 
WebMD indicates, if you have vitamin B12 deficiency, you could become anemic. A mild deficiency may cause no symptoms. Below are possible Vitamin B12 deficiency symptoms:
Weakness, tiredness, or lightheadedness
Heart palpitations and shortness of breath
A smooth tongue
Constipation, diarrhea, loss of appetite, or gas
Nerve problems like numbness or tingling, muscle weakness, and problems walking
Mental problems like depression, memory loss, or behavioral changes
You can also get vitamin B12 deficiency if you follow a vegan diet or you are a vegetarian who doesn't eat enough eggs or dairy products to meet your vitamin B12 needs. In both of those cases, you can add fortified foods to your diet or take supplements to meet this need.
“B12 can be found in oral supplementation, such as cyanocobalamin, but other supplements containing B12 include multivitamins/minerals, injections, B-complex, capsules, soft gels and lozenges,” Tiffone Powers-Parker of Banner Health
Vitamin B12 for Weight Loss
Mayo Clinic: There's no solid proof that vitamin B-12 shots, also called injections, help you lose weight.
The Mayo Clinic indicates high doses of vitamin B-12, such as those used to treat a deficiency, might cause:
Nausea and vomiting
Fatigue or weakness
Tingling sensation in hands and feet
Healthline says: In very rare cases, some people can experience side effects caused by allergic reactions or sensitivity
In addition to helping athletes build muscle, vitamin B12 plays a role in maintaining nervous system function and brain health. It also supports bone growth and development, which is especially helpful for those who participate in endurance sports such as running, cycling, swimming, .