A hard workout is supposed to feel hard. But there is a difference between the fatigue that comes from challenging your body and consistently feeling as though the tank is empty before the work really begins. Nutrition is one piece of that equation. Vitamins do not create fitness on their own, but several play important roles in energy metabolism, red blood cell formation, nervous system function, bone health, and other processes that support physical activity. When a true deficiency develops, the effects may show up in training as unusual fatigue, reduced capacity, slower progress, or difficulty recovering.

For endurance athletes in particular, small problems can become noticeable when they are repeated across long sessions and weeks of training. Greg Schaefer’s experience across endurance sports, business, family, and adversity reflects a larger principle: performance depends on the systems underneath the visible effort. Paying attention to nutrition is part of taking those systems seriously.

Why can vitamin deficiencies affect workout performance?

Exercise places a coordinated demand on nearly every major system in the body. Muscles need fuel. Oxygen has to reach working tissue. Nerves have to communicate with muscles. Bones and connective tissues have to tolerate repeated loading. After the session, the body needs resources to repair and adapt.

Vitamins participate in many of the chemical reactions that make those processes possible. They are not stimulants, and having more than the body needs generally does not mean more speed, strength, or endurance. The more useful distinction is between adequate nutrition and a genuine deficiency. If an essential nutrient becomes too limited, one or more normal physiological processes may become less efficient or impaired.

That distinction matters because supplement marketing can make vitamins sound like performance enhancers. For most people, the goal is not to create unusually high vitamin levels. It is to provide the body with what it needs and investigate situations in which intake, absorption, health conditions, medications, or other factors may be contributing to inadequate status.

Vitamin B12: oxygen delivery, blood cells, and fatigue

Vitamin B12 helps support healthy red blood cell formation and neurological function. According to the National Institutes of Health Office of Dietary Supplements, vitamin B12 deficiency can cause megaloblastic anemia as well as symptoms that include fatigue and neurological changes.

For an athlete, that connection is important. Red blood cells help transport oxygen, and endurance exercise depends heavily on the body’s ability to deliver and use oxygen over time. A significant B12 deficiency that contributes to anemia can therefore become much more than a nutrition footnote. It may affect how demanding sustained work feels.

B12 status also deserves attention because deficiency is not always simply the result of eating too little of the vitamin. Some people have difficulty absorbing B12 from food, and certain medications can affect B12 absorption or concentrations. Someone experiencing persistent fatigue should not assume that taking a supplement will solve the problem. A clinician can help determine whether B12, another nutrient, or something entirely different deserves investigation.

Folate: another vitamin tied to healthy blood cell production

Folate, also known as vitamin B9, is involved in DNA synthesis, amino acid metabolism, and other fundamental cellular processes. Like vitamin B12 deficiency, folate deficiency can contribute to megaloblastic anemia.

This is a good example of why workout performance cannot always be separated neatly from general health. When the body does not have what it needs to support normal blood cell production, the effect may become noticeable during activities that place a sustained demand on oxygen delivery.

It is also a reason not to diagnose nutritional problems based on a single symptom. Fatigue during exercise is common and nonspecific. Training load, sleep, illness, stress, inadequate calorie intake, hydration, iron status, medications, and many other factors may contribute. The useful question is not, ‘Which vitamin should I take?’ It is, ‘What is actually limiting my ability to perform and recover normally?’

Vitamin D: muscle and bone health matter to training

Vitamin D has an important role in bone health and normal physiological function, and inadequate vitamin D status can have musculoskeletal consequences. Severe deficiency can lead to osteomalacia in adults, a condition involving weakened bones.

For athletes, particularly those completing high volumes of running or other weight-bearing exercise, bone health is part of performance even when it is not measured on a watch or scoreboard. A training plan cannot deliver its intended benefit if the body’s structural foundation is struggling to tolerate the workload.

There is also an important caution here. The relationship between vitamin D and performance is sometimes oversimplified. Correcting a genuine deficiency and taking extra vitamin D in hopes of becoming faster or stronger are not the same thing. Evidence does not support treating every athlete as though more vitamin D automatically produces better performance. Vitamin D status can be evaluated when medically appropriate, and supplementation decisions should be based on individual needs rather than performance hype.

Thiamin and the machinery of energy metabolism

Thiamin, or vitamin B1, is one of the B vitamins involved in energy metabolism. The body uses thiamin-dependent enzymes in processes involving glucose, amino acids, and fats. Those pathways matter because exercise depends on converting available fuel into usable energy.

This does not mean a thiamin supplement functions like an energy drink. If thiamin status is adequate, consuming more does not simply turn up the body’s power output. The point is more fundamental: essential vitamins help normal metabolic machinery work as intended, and significant deficiencies can interfere with that machinery.

What athletes often miss about nutrient deficiencies

The most important clues are not always dramatic. A person can be highly disciplined, follow a detailed training plan, and still overlook changes in eating patterns or health that affect nutritional status.

• Training more while eating less: Increasing weekly volume without increasing overall food intake can narrow the margin for meeting nutritional needs.
• Eliminating entire food groups: Dietary choices can be perfectly workable, but restrictive patterns may require more deliberate planning to obtain certain nutrients.
• Assuming every bad workout is a fitness problem: Repeatedly pushing harder does not address an underlying medical or nutritional issue.
• Assuming supplements are harmless insurance: More is not always better, and supplements can produce side effects or interact with medications.
• Ignoring absorption: Adequate intake does not always equal adequate nutrient status. Digestive conditions, surgery, medications, and other factors can influence absorption of particular nutrients.

Performance problems are often about patterns, not one workout

One slow run, difficult ride, or disappointing strength session tells you very little by itself. Bodies fluctuate. Heat, hydration, stress, poor sleep, accumulated training load, illness, and simple day-to-day variation can all change how exercise feels.

A persistent pattern deserves more attention. If normal training suddenly feels disproportionately difficult for an extended period, recovery changes significantly, or fatigue begins affecting life outside the gym, repeatedly adding intensity may not be the smartest response.

That mindset applies beyond sport. Strong performance in athletics, leadership, and life often depends on noticing what the system is telling you rather than forcing the same strategy harder. Greg brings that grounded approach to his speaking work with organizations and teams: resilience is not pretending nothing has changed. It is responding intelligently and continuing forward.

What to do if you are concerned about a deficiency

Start with the basics rather than trying to identify a deficiency from an online symptom list. Look at your overall eating pattern, training volume, recovery, sleep, recent illnesses, medications, and any major dietary changes. Those details can provide useful context when speaking with a healthcare professional.

If fatigue, weakness, neurological symptoms, major performance changes, or other concerns persist, a qualified healthcare professional can determine whether medical evaluation or laboratory testing is appropriate. Testing matters because different deficiencies and health conditions can produce overlapping symptoms.

It is also worth resisting the temptation to begin several high-dose supplements at once. Doing so can complicate the picture and may create unnecessary risk. A targeted approach based on actual nutritional needs is more useful than trying to cover every possibility.

Frequently asked questions

Can a vitamin deficiency make workouts feel harder?

Yes, some vitamin deficiencies may contribute to problems such as fatigue, anemia, neurological changes, or impaired musculoskeletal health that can affect exercise. However, these symptoms can have many causes, so performance changes alone cannot diagnose a deficiency.

Which vitamin deficiencies are most relevant to exercise?

Vitamin B12, folate, vitamin D, and B vitamins involved in energy metabolism can all be relevant, depending on the individual and the severity of the deficiency. Minerals such as iron are also important to exercise physiology, although iron is not a vitamin.

Will taking a multivitamin improve athletic performance?

Not necessarily. Supplements do not automatically improve performance when nutritional status is already adequate. Their usefulness depends on a person’s diet, health, laboratory findings, and individual needs.

Should athletes get vitamin levels tested regularly?

Routine testing is not automatically necessary for every healthy athlete. A healthcare professional can help determine whether testing is appropriate based on symptoms, diet, medical history, medications, risk factors, and training demands.

Can I tell if I have a vitamin deficiency from fatigue alone?

No. Fatigue is nonspecific and can be related to training load, sleep, stress, illness, inadequate energy intake, anemia, medication effects, and many other factors. A clinician can help evaluate persistent or concerning fatigue.

This article is for educational purposes only and is not medical advice. For diagnosis, treatment, or personalized medical guidance, please speak with a qualified healthcare professional.

Sources & further reading

• National Institutes of Health Office of Dietary Supplements: Vitamin B12 Fact Sheet for Health Professionals
• National Institutes of Health Office of Dietary Supplements: Vitamin D Fact Sheet for Health Professionals
• National Institutes of Health Office of Dietary Supplements: Folate Fact Sheet for Health Professionals
• National Institutes of Health Office of Dietary Supplements: Thiamin Fact Sheet for Health Professionals