How Blood Testing Can Be Used For Better Endurance Training

Competitive athletes can find significant value in having annual bloodwork to maintain good health and performance. Knowing which essential markers to focus on helps find the deficiencies that could be hampering training.

It used to be that blood tests were almost exclusively conducted by elite competitors who had the resources to find an inside edge in their physiology. The amateur athlete would only get lab work done if their doctor suspected a particular sickness or needed more information to make a diagnosis. But blood testing has become easier and more accessible than ever before. Let’s look at which nutrition-related markers you might want to consider testing and what the results could mean.

Finding Ferritin and Iron Deficiency

Iron is one of the most common minerals that endurance athletes could be low in.

Iron is used to form hemoglobin, which is responsible for shuttling oxygen to your muscles and other tissues via the bloodstream. Iron is also essential for regulating myoglobin, which releases oxygen into muscles and is involved in energy metabolism, immune function, hormonal balance, and many other vital processes.

As training intensity, volume or overall load increases, your body utilizes a greater amount of iron because it needs more hemoglobin to keep up with the elevated demand for oxygenated blood flow. Iron status can also be negatively impacted by frequent use of NSAID painkillers, heavy sweating and GI tract bleeding. Plus, runners are most prone to foot strike hemolysis, where the impact of your feet on the ground damages or even bursts blood vessels.

Ferritin is one of the most common markers in blood testing that assesses iron levels. It’s the primary storage form of iron in humans and amounts to around a quarter of the total iron someone has in their body at any given time. Female athletes can be low on ferritin because of iron loss during their menstrual cycle, which is compounded if they’re following a vegan or vegetarian diet that excludes animal products which are the primary source of heme iron – the most readily absorbed kind.

The tricky thing with ferritin test results is that the so-called “normal” range is so broad from a clinical perspective: 10 – 120 ng/mL for females and 20 – 250 ng/mL for males. Luckily, the scientific literature offers some clues as to what’s more optimal for your athletes. Studies have found performance limitations in athletes whose ferritin is below 30 ng/mL or mcg/l.

At the same time, one paper published in Swiss Medical Weekly stated that “a ferritin value of 50 mcg/l should be attained in athletes before altitude training, as iron demands in these situations are increased.”[i] Somewhere between 30 and 40 ng/mL should be sufficient for those who don’t live or train at altitude. Above 40 ng/mL is recommended for athletes living at altitudes above 5,000 feet.

As ferritin values can vary so dramatically and be artificially high if an athlete is overtrained, sick, stressed or under-recovered, it would be beneficial to get a panel that includes other iron level indicators. These include total iron-binding capacity (how well blood cells attach themselves to iron), serum iron (the amount of iron in the bloodstream), and transferrin (a protein that ferries iron through the body).

In addition to painting a fuller picture of iron status, a complete blood count panel can also provide a view into how efficiently the body transports oxygen by showing red blood cell count and the hemoglobin and hematocrit levels (proportion of red blood cells). If ferritin is below 30 and values for these other indicators are also lower than the desirable ranges, then supplementation might be advisable.

Athletes could also top up on foods containing heme iron, like beef, bison, lamb, shellfish and eggs. Plant-based foods like leafy greens, pumpkin seeds, and sweet potatoes all contain non-heme iron, but athletes need to eat a lot of these, as this form of iron is less readily absorbed. Adding a supplement to get iron stores topped up may be required.

Targeting Two Vitamins

In addition to checking their iron status, athletes may want to look into the levels of specific vitamins. While you could go to town with a comprehensive panel, a more targeted approach may do. Vitamin D is often chronically low for many people and can impact both health and performance.

Vitamin D deficiency can create a whole host of problems. It can increase your chances of catching a cold, COVID or the flu, as it plays a crucial role in both innate and adaptive immunity. Getting inadequate D for an extended period also makes athletes susceptible to certain kinds of injuries. We know that when our vitamin D is low, we have an increased risk of stress fracture, or if we have a bone injury, it’s going to delay healing. If someone has a history of bone-related issues, they should keep an eye on their vitamin D status.

As with iron and ferritin, a blood test could return a vitamin D score a doctor says is medically acceptable, but it’s still sub-optimal for performance. Researchers from the University of Arizona found that when college athletes’ vitamin D was below 20 mg/nL, they were 12% more likely to suffer a stress fracture than those whose level was already at 40 mg/nL or above or was raised to this level through supplementation.[ii]

The easiest way to address low vitamin D is to get out in direct sunlight with arms, legs and other areas exposed and without sunscreen for at least 20 minutes. Three times a week is a minimum, and daily would be ideal. Wintry weather makes this difficult, and some athletes in sunny states like Florida and California have a vitamin D deficiency. A daily supplement containing 2,000 IUs of vitamin D is recommended,, though more may be needed depending on body size and the extent of the deficiency.

There’s another common vitamin deficiency that might hold athletes back. A lot of runners and triathletes don’t get enough B12. This micronutrient helps regulate nervous system function and is involved in red blood cell formation, DNA synthesis and cell metabolism. Signs that an athlete might be low in B12 include persistent fatigue, yellowish skin, cramps, tingling and a loss of appetite.

The range of a B12 test result could be quite broad. A study by Polish researchers published in Nutrients concluded that athletes “should regularly monitor vitamin B-12 concentration and maintain the range of 400-700 pg/mL as it may improve red blood cell parameters…Special attention is required in athletes with a vitamin B-12 concentration below 400 pg/mL.”[iii]  Supplementation can yield rapid improvements in vitamin B levels compared to certain other micronutrient deficiencies. As it’s water soluble, a meaningful change in B12 can occur in just a few days, with athletes starting to feel and perform better. Whereas with iron, vitamin D, and some other vitamins and minerals, it can take longer to make a difference.

Investigating Health Indicators

Vitamin D, iron and vitamin B-12 can directly impact an athlete's performance and durability. Blood testing can uncover underlying health issues that might compromise athletic overall well-being. Zero in on LDL cholesterol and blood glucose, particularly if you have a family history of high cholesterol or type two diabetes.

While the kind of at-home finger prick method you can use to take your own blood glucose level only offers a snapshot, a hemoglobin A1C test looks at the median over two to three months to provide a more useful result. A level of 5.7 to 6.4 suggests someone in the prediabetic range, while 6.4 or more is considered diabetic. Some simple nutritional changes can help an athlete get their blood glucose under control.

If someone has high blood sugar, they may want to increase their frequency of eating to ensure they’re not putting too many carbs in at once but are spreading them throughout the day. Complex carbohydrates are going to keep blood sugar from getting as high as simple ones. Athletes could reserve simple sugars for before and during their runs to use as fuel, and then take in complex carbs post-run to help manage their blood sugar.

While a high overall cholesterol score can be cause for concern, a big HDL number can skew it higher, and it’s the LDL kind that’s most closely correlated with cardiovascular disease. That’s why it’s preferable for athletes to keep their LDL cholesterol under 100. If it’s higher than that, the athlete can reduce the amount of saturated fat they’re getting from dairy, palm oil, and coconut, and fit more fiber into their diet via fruit, vegetables and whole grains.

Testing Frequency and Next Steps

Athletes will likely want to know how their test results change over time. So how often should you repeat the process? 

A general guideline for anyone competing, training, and trying to be their best is yearly bloodwork. If you have a history of issues or your last test showed deficiencies, every three to six months might be more suitable. Another reason to get lab work done more often is if you are significantly ramping up your training, working toward major races, or making a big change in your environment, like going into high altitude. Unexplained symptoms like sleep disruption, trouble reaching and maintaining paces, higher resting and working heart rate, and increased perception of fatigue might be reasons to get bloodwork, too. Periodic testing can identify any micronutrient deficiencies, find underlying causes for frustrating symptoms and inform better overall health and performance going forward, particularly if you partner with a qualified professional to interpret the results and provide a holistic action plan.

References

[i] German Clénin et al., “Iron Deficiency in Sports – Definition, Influence on Performance and Therapy,” Swiss Medical Weekly, October 2015, available online at https://pubmed.ncbi.nlm.nih.gov/26512429.

[ii] David Millward et al., “Association of Serum Vitamin D Levels and Stress Fractures in Collegiate Athletes,” Orthopedic Journal of Sports Medicine, December 2020, available online at https://pubmed.ncbi.nlm.nih.gov/33816638.

[iii] Jarosław Krzywański et al., “Vitamin B12 Status and Optimal Range for Hemoglobin Formation in Elite Athletes,” Nutrients, April 2020, available online at https://pubmed.ncbi.nlm.nih.gov/32283824.