Woman Working With Blood Samples In Laboratory, Closeup

How Coaches Can Use Blood Testing For Better Endurance Training

BY Phil White

Competitive athletes can find significant value in having annual bloodwork to maintain good health and performance. However, knowing which essential markers to focus on helps coaches find the deficiencies to address 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 with web-based services from the likes of InsideTracker and WellnessFX, blood testing has become easier and more accessible than ever before. Let’s look at which nutrition-related markers your clients might want to consider testing and what the results could mean.

Finding Ferritin and Iron Deficiency

To get an expert opinion on which blood tests will be worthwhile for your athletes, I reached out to Meghann Featherstone, a Certified Specialist in Sports Dietetics who is also an Ironman competitor and posted a 2:50 time in this year’s Chicago Marathon. She told me that iron is one of the most common minerals that runners, cyclists, triathletes and other 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,” Featherstone said. “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 25 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 your athletes who don’t live or train at altitude.

“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 for your clients to get a panel that includes other iron level indicators,” Featherstone said. 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 that a dietician like Featherstone or a sports-specific online service outlines, then supplementation might be advisable.

 Your 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 your clients would need to eat a lot of these, as this form of iron is less readily absorbed. Adding a supplement to get their iron stores topped up may be required.

Targeting Two Vitamins

In addition to checking their iron status, your athletes may want to look into the levels of specific vitamins. While they could go to town with a comprehensive panel, Featherstone advises a more targeted approach to most of her clients. “Vitamin D is often chronically low for many people and can impact both health and performance,” she said.

Vitamin D deficiency can create a whole host of problems. As I shared in a recent TrainingPeaks article, 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. Featherstone pointed out that among athletes, getting inadequate D for an extended period also makes your 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,” she said. “So if someone has a history of bone-related issues, they should keep an eye on their vitamin D status.”

As with iron and ferritin, one of your clients could get a blood test that returns a vitamin D score their 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. The current wintry weather makes this difficult, and Featherstone pointed out that she has some clients in sunny states like Florida and California whose blood tests have identified a vitamin D deficiency. That’s why she recommends taking a daily supplement containing 2,000 IUs of vitamin D3, though more may be needed depending on body size and the extent of the deficiency.

There’s another common vitamin deficiency that might be holding your athletes back. “I’ve seen a lot of runners and triathletes who aren’t getting enough B12,” Featherstone said. This micronutrient helps regulate nervous system function and is involved in red blood cell formation, DNA synthesis and cell metabolism. Signs that a client 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] Featherstone has found that supplementation can yield rapid improvements in vitamin B levels compared to certain other micronutrient deficiencies. “As it’s water soluble, we can see a meaningful change in B12 in just a few days, with athletes starting to feel and perform better,” she said. “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 your athletes’ performance and durability. Featherstone told me that blood testing can uncover underlying health issues that might compromise your clients’ overall well-being. She recommended zeroing in on LDL cholesterol and blood glucose, particularly if they 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, we talk about their frequency of eating to ensure they’re not putting too many carbs in at once but are spreading them throughout the day,” Featherstone said. “Complex carbohydrates are going to keep blood sugar from getting as high as simple ones. They 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

Once they identify some initial lab tests to order, it can be tricky for your athletes to know what to do next after the results come back. If you have a background in nutrition, you might be able to provide some insights, but if not, it would be worth putting them in touch with a dietician or nutritionist in your network.

InsideTracker and WellnessFX help determine ideal ranges and create personalized recommendations that include foods to eat more of to improve micronutrient levels and health markers. But suppose you team up with a nutrition professional as well. In that case, you’ll be able to create a comprehensive strategy that includes their training and injury history, diet and supplementation, programming, lifestyle and other factors. This isn’t an either/or decision, as many dieticians like Featherstone use online tools as part of their services.

As with paces, splits and other metrics, your clients will likely want to know-how their most recent test results change over time. So how often should they repeat the process? 

“A general guideline for anyone competing, training, and trying to be their best is yearly bloodwork,” Featherstone said. “If they have a history of issues or their last test showed deficiencies, every three to six months might be more suitable. Another reason to get lab work done more often is if they’re significantly ramping up their training, working toward major races, or making a big change in their 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 prompt me to recommend bloodwork too.” Even if your athletes don’t get the same regular, extensive blood draws that elite competitors like Kristian Blummenfelt and Gustav Iden do, they can still benefit from periodic testing. This 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.


[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.

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About Phil White
Phil White is an Emmy-nominated writer and the co-author of The 17 Hour Fast with Dr. Frank Merritt, Waterman 2.0 with Kelly Starrettand Unplugged with Andy Galpin and Brian Mackenzie. Learn more at www.philwhitebooks.com and follow Phil on Instagram @philwhitebooks.

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