Bone health in athletes: can we learn from blood tests?

Bone loading and sport type

Physical inactivity is a major risk factor for developing osteoporosis because vigorous ‘bone-loading’ during physical activity is very effective at stimulating the uptake of calcium into bones, thereby helping to build bone mass in earlier years, and reducing the loss of bone mass in later years⁽⁸⁾. However, physical activity with minimal bone loading is a different kettle of fish altogether. In short, the higher the muscular and impact loading (gravitational) forces, the higher the BMD produced. For example, gymnasts whose sport requires high loadings and impacts tend to have higher BMDs than endurance runners⁽⁹⁾. Also at (possibly greater) risk are those who participate in sports with plenty of smooth muscular motion, but without substantial impact loading (eg swimming and cycling). These athletes do not achieve the high BMDs of sports with higher loading despite training vigorously – (see the case study below)⁽¹⁰⁾.

Because of the accelerated age-related bone losses during the menopause, all women are more vulnerable to developing osteoporosis in later years. However, research has shown that high-volume endurance training is also strongly linked to an increased risk of reduced bone density. This occurs because high training volumes disrupt reproductive hormone secretions such as follicle-stimulating hormone (FSH), estradiol and others, which are important for bone synthesis and repair; disruption of these hormones results in impaired bone remodeling and increased fragility, particularly in areas like the lumbar spine^(11,12) This combination of factors means that female endurance athletes – especially senior athletes training at elite level - may be at particular risk of this health condition.

Case study: woes of a female British Cycling team member

Because of my work as a research in sports nutrition, athletes I knew would sometimes approach me for guidance. One such athlete who approached me during the early 2000s was a personal friend of mine, and member of the British Cycling Women’s Mountain Bike Team. She was highly committed to the sport and mentally tough, accumulating high training mileages in addition to her competition schedule, later competing successfully in road cycling. She did however struggle with her personal day-to-day nutrition and, and when I screened her diet, it was clear that her calorie and nutrient intake often did not meet her training and health demands. Part of this was psychological – a fear (unfounded) that consuming more calories and nutrients - particularly protein) might result in weight gain and a lower power-to-weight ratio, and thus poorer performance.

Over the following 20 years, we kept in touch and while this cyclist has since retired from international competition, she is still actively racing and training. However, after suffering a number of unexpected fractures following quite minor falls in recent years, DXA bone scans revealed significant osteoporosis present despite this athlete still being relatively young (in her late 40s). She is now on long-term medical treatment to try and stem further age-related bone losses and has to be extra careful to avoid falls from the bike or any other significant impacts as she is at a very high risk of further fractures.

Screening for bone health

Give the risks faced by (particularly female) endurance athletes, is there a simple and easy way to monitor bone health? The gold standard screening procedure is DXA, which stands for ‘Dual X-ray Absorptiometry’. DXA is widely regarded as the gold-standard for measuring bone mineral density because of its precision, safety, and ability to predict fracture risk before a fracture actually occurs. DXA allows for an extremely accurate measurement of bone density in every region of the body, but can especially focus on the lumbar spine and hip, which are the most critical sites for assessing osteoporotic risk.

Importantly, regular DXA scans can detect as little as a 1% change in bone mass, which means it can detect small changes in BMD and flag up when someone starts to approach the threshold for osteoporosis. The big downside of course is that since DXA scanning requires expensive machinery and a qualified technician, repeat DXA scans are not cheap. And even if you have access to a national healthcare system such as the NHS, you have to firstly get referred by a GP and then you may find yourself on the end of a long waiting list!

Blood testing for bone health?

Since athletes frequently undergo blood testing, is it possible to screen the blood for key hormone levels in order to flag up an increased risk of sub-optimal bone health? If that was possible, monitoring bone health would be much simpler since many athletes routinely undergo blood testing each season. It’s known that suboptimal blood iron levels are common in female athletes undergoing high-volume training, which can compromise collagen synthesis – needed to form connective tissue around various skeletal structures. Meanwhile, low triiodothyronine (T3) levels (often observed in athletes undergoing heavy training and who have an insufficient calorie intake) can inhibit optimum levels of bone-building activity⁽¹³⁾. Despite this, there’s very little data available examining the relationship of common blood markers and hormone levels with bone health in athletes. But now, new research provides some answers to this question.

New research

Published in the journal ‘Healthcare (Basel)’, a team of Spanish researchers set out to identify which blood-based markers are the strongest predictors of bone mineral density (BMD) and bone mineral content (BMC) in elite female trail runners⁽¹⁴⁾. The researchers specifically looked at female mountain trail runners because this sport is characterized by high levels of mechanical stress from downhill and uneven surface running, and elevated metabolic demands due to high volumes of endurance training (which can frequently lead to an insufficient energy intake - more technically known as low energy availability or LEA). The goal of this research was to see if athletes at risk of bone stress injuries could be better identified just by blood tests and then targeted for nutritional or recovery interventions.

What they did

Thirty-five elite female trail runners who were members of the Spanish national team were recruited for this study. The average age of the runners was 34 years, and the runners’ elite status was verified by their International Trail Running Association performance scores, which averaged over 600 points, placing them in the top tiers of the sport globally.

All the runners underwent pre-season blood testing to determine various blood marker and hormone levels, along with DXA scanning to determine the runners’ BMD levels. All the testing procedures were carried out at the same time of day (8:00am to 10.00am) in order to standardize for daily fluctuations circadian rhythm-related factors (see this article). In addition, all the tests were conducted in a climate-controlled laboratory maintained at 24°C and 50% relative humidity. Furthermore, to avoid hormonal fluctuations related to food intake patterns and exercise, all the runners were tested in a fasted state (at least eight hours since the last meal), and having strictly refrained from all physical exercise for the previous 48 hours.

What they tested

BMD was measured using DXA scanning across multiple sites: the overall BMD of the whole body, BMD of the lumbar spine (L1–L4 the thickest and widest vertebrae of the spine due to their role in supporting body weight), and the proximal (top end) of the femur (thigh bone), including the femoral neck, which separates the head of the femur (the ‘ball’ that fits in the socket of the hip joint) from the main femur region (see figure 2). The femoral neck is an important region to analyze for BMD as it is prone to fracture in osteoporotic bone, resulting in a very debilitating injury.

Figure 2: Location of the femoral head and neck of the hip joint

The blood testing was comprehensive and a total of 44 variables were analyzed, including:

·        Complete blood count - red and white blood cells, hemoglobin, and platelets.

·        Iron status: Ferritin (an iron storage protein), blood iron, and transferrin (a measure of how ‘hungry’ the body is for more iron).

·        Hormonal Profile – follicle stimulating hormone (FSH), leuteinizing hormone (LH), estradiol, all of which are involved in the menstrual cycle.

·        The thyroid hormones TSH, T4, T3, which are involved the rate of tickover metabolism and energy output – akin to setting the body’s ‘thermostat’.

·        The nutrients calcium, magnesium, phosphorus, and vitamin D, all of which are needed for bone synthesis and health.

·        Metabolic and liver function markers such as glucose, cholesterol, and enzymes like creatine kinase –essential for energy production in muscles.

Once all the testing was completed, the researchers analyzed the findings by exploring any links between the blood test results and the runner’s levels of BMD/bone health.

What they found

The key findings were as follows:

*BMD – the DXA results revealed a significant difference in bone health depending on the location of the bone tissue. The femoral neck region showed good levels of BMD, almost certainly due to the bone-loading forces transmitted through the hip and femur during running. However, the BMD findings in the lumbar spine area were much poorer; the runners showed significantly lower than normal levels of BMD with a high prevalence of osteopenia (the transition phase between normal bone density and osteoporosis). In short, the runners showed an age-related vulnerability to developing osteoporosis in the lumbar region as the years progress.

*Magnesium – one of the most striking findings was that magnesium levels were strongly negatively associated with bone density and bone mineral content regardless of location. In simpler terms, athletes with higher blood magnesium often had lower bone density in the spine and whole body. This is not because higher magnesium intakes inhibit bone formation – quite the contrary. Instead, the researchers hypothesized that under conditions of chronic physical stress and metabolic demand, the runners were likely mobilizing magnesium from the bone matrix into the bloodstream to maintain muscle function and energy production. That’s because magnesium is needed to activate a key enzyme known as ATPase, which is hugely active during exercise thanks to its ability to regenerate ATP – the body’s energy currency (see this article). Therefore, high blood magnesium in these athletes could actually be a red flag for bone breakdown and loss rather than a sign of high magnesium intakes!

*Calcium – as with magnesium, blood calcium levels must be kept within a very tight range. If an athlete’s diet is insufficient, the body can ‘borrow’ essential calcium from the bones to keep blood levels stable. Like magnesium, higher than normal blood calcium seemed to be correlated to reduced bone density - however unlike magnesium, this effect was not seen across all the bone locations. Either way, this finding could mean that blood testing for calcium without some kind of bone health measurement could be masking a calcium shortfall in the diet.

*Hormones – the findings from hormone tests suggested that in women, a healthy, functioning reproductive system confers protection in terms of bone health. In particular, higher levels of FSH within the normal range) correlated with better bone BMD scores. Meanwhile, higher levels (but within normal range) of thyroxine (T4) was also linked to better bone health. Since thyroid hormones regulate the speed of bone turnover, it’s likely that even slight depressions in these hormones (common where there’s an energy deficit in an athlete) can lead to a reduction in the rate of bone synthesis and repair.

Practical implications

The first thing to say is that while blood testing can yield some useful information about bone metabolism, it is no substitute for a bone scan. For example, a high or normal level of blood magnesium or calcium could indicate an ample intake of these minerals and therefore helpful for bone health. But this data shows it could more likely be an indicator of insufficiency, indicating a breakdown of bone tissue to top up blood levels. In this respect, testing should also include measurements of FSH; a low FSH likely indicates that a higher level of magnesium could be as a result of magnesium withdrawals from the ‘bone bank’! More generally, a comprehensive set of blood tests that includes ferritin, T3, and FSH will be more informative for female athletes than ‘standard’ health-check blood tests.

Regarding magnesium and calcium status, combining blood levels of magnesium/calcium and FSH with an additional dietary analysis is likely to be much more useful. That’s because from such an analysis, athletes and their coaches can see whether the daily magnesium/calcium intakes of an athlete are insufficient, sufficient or ample. Regarding intakes, current findings suggest that endurance athletes, or those in high-intensity sports, should aim for an intake 10–20% higher than the standard RDA, which for men equates to around 450–500mg/day and women 350-400mg/day of magnesium, especially when training in hot conditions, where sweat and urinary losses are increased.

For athletes who might be at risk of poor bone health (eg female endurance athletes performing high volumes of training), DXA scanning is advised. If a scan is performed, it should not be just a ‘whole body’ scan, as this will not be able to differentiate the weaker areas of bone from stronger areas. It should be one that assesses different bone locations. Athletes such as distance runners for example, are likely to have better hip bone scores but poorer lumbar spine scores. Finally, all athletes and their coaches should remember the golden rule: if that athlete is in a chronic calorie deficit and short of bone-building nutrients such as calcium, magnesium and vitamin D, no amount of impact loading will arrest the decline in bone density. It’s really important to ensure that energy (calorie) intake matches the energy demands of training and competition. This is where the knowledge and skills of a professional sports nutritionist can be invaluable!

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