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Medical Report: Avoiding Exercise-Related Fatigue

Rush, Shelby M.D., FACSM

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ACSM's Health & Fitness Journal: March-April 2005 - Volume 9 - Issue 2 - p 30-32
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In Brief

Many factors can cause fatigue as clients begin an exercise regimen or increase the intensity or duration of their current program. Appropriate screening of clients involves a thorough review of calorie intake, fluid and electrolyte intake, and rest and recovery periods. A few simple questions can help identify errors of training and nutritional deficits. This is especially important when working with clients who are dieting to lose weight because they may be restricting calorie intake and avoiding energy foods like complex carbohydrates.

Once all nutritional deficits and training errors have been addressed, ask clients about prior or current medical conditions and current medications. There are many medications that commonly cause fatigue as a side effect (Table 1). This side effect may be exaggerated with exercise. Once a client recognizes that the medication is impacting his or her exercise training, he or she can discuss this with his or her physician. As for medical conditions, if a client has untreated or undertreated asthma, heart disease, infection, vascular disease, or one of many other illnesses, he or she needs to discuss beginning an exercise regimen with his or her physician. Only people with severe lung disease and unstable cardiac problems should be excluded from exercise (Table 2). All other clients can exercise safely but should see their doctor if they are feeling fatigued during exercise.

Table 1
Table 1:
Classes of Medications that Cause Fatigue
Table 2
Table 2:
Contraindications to Exercise

Some illnesses that can cause fatigue are high blood pressure, thyroid disease, congestive heart failure, asthma, mononucleosis, and hepatitis. Depression can cause fatigue but usually in the morning and not temporally associated with exercise. Most illnesses and medical conditions cause fatigue that worsens throughout the day. Mild or subclinical anemia can cause fatigue only upon exercising.


Anemia is very prevalent. Approximately 15% to 20% of female athletes have low hemoglobin levels (<12.6 g/dl). Anemia is more common in women than men and occurs frequently in people who do not eat red meat, like vegetarians. Anemia also is common in people who skip breakfast and those who are cutting calories. Diets high in carbohydrates can lead to anemia, as well. If your client is fatigued with no obvious explanation, make sure he or she has a complete evaluation with his or her doctor, including blood tests like a complete blood cell count, liver function tests, a basic metabolic panel, total iron, and a ferritin level. In the face of normal hemoglobin levels but persistent fatigue upon exertion, make sure clients have their ferritin levels checked. Ferritin is normally checked in patients with anemia to ensure the anemia is caused by iron deficiency and not a different medical problem. "Normal" hemoglobin levels are based upon local standards, not upon what level of iron- or oxygen-carrying capacity is normal for that client. Some people have normal hemoglobin levels based upon their age and gender but still feel fatigued with exercise. This may be the result of a relative anemia or a low hemoglobin level for that person.

Some recent studies looked at anemia in young women. One study analyzed blood in 25,000 people, and it found that 10% of young women have a low ferritin level, but only 3% have low hemoglobin levels. In response, another study was done placing women with normal hemoglobin levels (>12 g/dl) but low ferritin levels (<16 mg) on iron versus placebo for 6 weeks. The subjects then underwent a supervised exercised regimen for 4 weeks. They performed a 15-km cycling time trial at the beginning and end of the study. The subjects who took iron cycled faster and had an improvement in O2max. The noted improvements were linked with an increase in hemoglobin. Even though all of the subjects started with "normal" hemoglobin levels, the subjects' exercise ability improved with iron supplementation compared with control subjects. This phenomenon is known as "functional," or relative, anemia.

A similar study looked at the hemoglobin and ferritin levels of 30 normal women. All of the subjects had normal hemoglobin levels (Hgb > 12.0 g/dl), but half of them had low ferritin levels (average = 4). The women with low ferritin had lower O2max tests compared with the women with normal ferritin values (average = 31). The women with the higher ferritin values also had higher hemoglobin levels (average = 14.5 vs. 13.6).

There was one small study done on the effect of low ferritin levels in men. Nine men underwent phlebotomy, or having blood removed, to the point of anemia. They were maintained at a hemoglobin level of 11 g/dl and a ferritin level of 7 for 1 month. At the end of the month, they were transfused to normal hemoglobin levels, but their ferritin levels were still low. They all then raced at the same performance levels as before the phlebotomy.

Ferritin has been blamed for athlete fatigue in the past. However, in the previously mentioned studies, the ferritin level was merely a marker of a relative anemia. Iron is the key to oxygen-carrying capacity in the blood. In muscle, the oxygen is transferred to myoglobin molecules. This transfer of oxygen depends upon iron molecules at the center of protein complexes. Iron also is a key component in mitochondrial electron transport proteins that function in the production of adenosine triphosphate (ATP), the key energy-transporting molecule in the body. Ferritin is the marker of iron stores within the body. Iron is stored in muscle, the liver, and in bones. If the body stores are low, the ferritin level goes down, reflecting the decrease in iron transport. Therefore, as the body's stores are replenished, the ferritin level will rise.

One theory of why an athlete might have low ferritin or low iron is hemodilution. Although expansion of blood plasma volume is a normal response to exercise, it does not explain relative anemia. Dilutional anemia results in a decrease of blood hemoglobin from 1.0 to 1.5 g/dl, but it is generally beneficial to exercise and does not result in fatigue with exercise. The largest expansion of blood volume is seen in endurance athletes. The expanded blood volume maintains cardiac output and proper fluid and electrolyte transport.

Other theories include blood losses through heavy menses, sweat losses, heel strike hemolysis, and losses through the gastrointestinal tract with exercise. All of these can result in blood losses but not usually enough to cause symptomatic anemia. Of course, recurrent heavy menstrual losses or gastrointestinal losses not caused by exercise can cause severe anemia; that is why it is important to refer clients with fatigue for medical evaluation. Blood losses associated with exercise can easily be replaced. Men lose more iron in sweat than women, and women can lose up to 6% of absorbed daily iron with heavy exercise. Gastrointestinal losses are more dependent upon the health of the athlete, hydration status, and intensity of exercise. Extreme exercise can result in shunting of the blood flow away from the gastrointestinal tract, resulting in bowel ischemia and frank blood.

Relative anemia is easy to prevent and treat. The best way to prevent anemia is by eating lean red meat a couple of times a week. There is sufficient heme protein in a piece of meat the size of a deck of playing cards. Caffeine interferes with iron absorption, so decreasing coffee intake and not drinking coffee with meals can help prevent anemia. Iron absorption increases with vitamin C, so a small glass of orange juice at breakfast helps. Using cast iron skillets increases absorbable iron in meals, especially meals with high acidity like tomato sauces.

If there is a question of relative anemia, try treating with iron supplementation or small amounts of red meat. Repeat hemoglobin levels after 4 to 6 weeks. A rise of 1 g/dl or more in the hemoglobin level indicates a positive response. The best supplement is ferrous sulfate, usually taken once or twice daily with food and vitamin C. Higher iron supplementation can cause constipation and stomach upset. For athletes with normal ferritin levels (20 to 40), they can usually obtain enough iron in their diet. If they wish to take a supplement, it is important to recommend a daily vitamin without too much iron, like children's chewable tablets or once-a-day vitamins.


1. Eichner, E.R. Anemia and blood doping. Essentials of Sports Medicine. Sallis, R.E., and F. Massimino (Editors), St. Louis: Mosby, 1997, pp. 35-38.
    2. Eichner, E.R., and W.A. Scott. Exercise as disease detector. The Physician and Sportsmedicine 26:41-52, 1998.
      3. Selby, G. When does an athlete need iron? The Physician and Sportsmedicine 19:96-102, 1998.
      © 2005 American College of Sports Medicine