Energy deficiency: the bare bones of RED-S

New RED-S research

To try and understand more about exactly how RED-S affects bone health in elite athletes, a team of German scientists have carried out a study to clarify how RED-S leads to bone deterioration in athletes, and the implications for early detection to prevent injuries, and sustain athletic careers⁽¹⁵⁾. Published in the Journal of Cachexia Sarcopenia and Muscle, this study drew parallels between RED-S and a condition known as ‘cachexia’, which is a wasting syndrome characterized by severe loss of muscle and fat tissue often seen in chronic illnesses like cancer. While elite athletes typically build strength through rigorous exercise, those with RED-S often struggle to do this and instead often experience tissue breakdown similar to that seen in cachexia. This results in poor bone quality, higher injury rates, and reduced performance.

To carry out the study, the researchers analyzed patient records from 82 elite athletes (25 women and 57 men, average age 23.4 years) who were treated at the Department of Osteology and Biomechanics, University of Hamburg-Eppendorf Medical Centre, Germany, between June 2013 and November 2023. The athletes’ elite status was defined as performing at least 21 hours of weekly training or competition at national/international levels. In addition (to add further rigor to the study), athletes were also classified using the McKay system,- a scientifically validated method, which tiers sports by performance level, with all participants being classified at Tier 3 or higher (51% at Tier 4 or 5).

What they did

The first step was to determine how many of the athletes treated over that period had been suffering from RED-S at the time. To diagnose RED-S from this historical data, the researchers looked back at notes made during consultations, the athletes’ documented training history, any recorded performance dips, the athletes’ history of fractures (verified by X-rays or scans), plus the results of routine blood and urine tests. This data was then assessed using a tool called the REDs Clinical Assessment Tool Version 2 (CAT2), which is derived from the 2023 IOC guidelines⁽¹³⁾. This scores risk based on a number of factors, including energy intake, menstrual cycles (in women), bone density, and injuries—categorized as low-risk (green), mild (yellow), moderate-to-high (orange), or very high-to-extreme (red). When this scoring was complete, it turned out that about 24% of the athletes (mostly endurance athletes) had been suffering from RED-S at the time of their injury diagnosis and treatment. Of particular interest for bone metabolism were the blood and urine tests. These included:

·        Tests for bone turnover markers. These gauge how bones build up (formation) and break down (resorption). Formation markers like osteocalcin and P1NP (proteins from bone-building cells called osteoblasts) indicate new bone creation. Resorption markers like deoxypyridinoline (DPD) and calcium in urine indicate bone breakdown by osteoclasts (bone-resorbing cells).

·        Bone density scans at the spine and hips using a very accurate technique known as Dual-Energy X-ray Absorptiometry (DXA). DXA can measure ‘area bone mineral density’ (aBMD), which is the bone mineral content per unit area of bone. This yields ‘Z-scores’, which compare results to age- and sex-matched norms. A score below -2.0 signals low density.

·        Advanced microstructure imaging of the wrist (radius) and ankle (tibia), using high-resolution computed tomography (CT). This yields a detailed scan assessing 3-dimensional bone mineral density per volume (vBMD), and shows the bone micro-architecture, including tiny structures like trabeculae (spongy bone struts) and cortical bone (dense outer shell).

Once all the data was gathered, the stats were crunched to provide a robust comparison between athletes who were suffering from RED-S vs. those who were not. In addition, the researchers looked to see how sport type impacted the findings.

What they found

There were a number of key findings, many of which have profound implications for athletes at risk of RED-S (see figures 1 and 2):

·        Firstly, RED-S was far more prevalent in endurance athletes; 69% of athletes in endurance sports were diagnosed with it compared with just 3.6% of strength athletes.

·        Stress fractures (not full breaks but bone cracks from overuse) were far more common in the athletes diagnosed with RED-S cases compared with non-RED-S athletes (80% vs. 34% respectively). This finding highlights the increased risk of fracture vulnerability in RED-S.

·        RED-S diagnosed athletes were found to have lower levels of hemoglobin (Hb - oxygen-carrying component in red blood cells) and hematocrit (a measure of red blood cell volume), which together results in poorer oxygen delivery to tissues, and thus poorer performance.

·        Bone formation in the RED-S athletes was suppressed compared to the non-RED-S athletes. Markers of bone formation (eg osteocalcin) were 38% lower while markers of breakdown (eg DPD) were 46% higher, with urinary calcium excretion being tripled. Overall, this combination creates a net bone calcium loss – rather like continually withdrawing funds from a bank account faster than deposits are made. The researchers found that this imbalance mimicked the metabolic chaos of cachexia but stemmed from dietary shortfalls in calories and nutrients rather - not disease.

·        DXA scans showed that the RED-S athletes had alarmingly low Z-scores: -1.65 at the spine and -0.94 at the hip. This was in stark contrast to the positive Z-scores of the strength athletes). So low were these scores in the RED-S athletes that 32% of them would have been classified as having osteoporosis-like low density at the spine!

·        Last but not least, the high-res CT scans showed a grim picture of bone quality micro-architecture in the RED-S athletes; bone volume density was reduced by 10-18% at both the radius of the wrist and tibia (shin bone). There were fewer, thinner trabeculae and slimmer cortical shells meaning more fragile bone architecture, prone to cracks under stress.

Figure 1: Indicators of RED-S, and markers of bone formation/loss

Figure 2: Hip and lumbar Z-scores from DXA scans

Implications for athletes

The main take-home lesson from this new study is that RED-S in athletes acts rather like a ‘silent thief’ of bone mineral content and structure. Despite there being no obvious bone symptoms, RED-S promotes bone loss and impairs bone repair regardless of the athlete’s training (something that normally BOOSTS bone health), especially where training involves strength work. The low energy availability in RED-S almost certainly drives these changes via hormonal disruptions such as reduced estrogen/testosterone, which impairs bone formation, while the elevated levels of cortisol promote bone loss. This metabolic imbalance hinders the process of ‘mechanotransduction’ - the process by which mechanical stress from exercise stimulates bone remodelling. This in turn leads to inadequate bone adaptations under training loads, causing vulnerability to stress fracture at weight-bearing sites like the tibia.

In the RED-S athletes who were subsequently treated at the University Medical Centre, most were prescribed vitamin D (80%) and calcium (60%) supplements. That’s because higher calcium intakes combined with vitamin D are known to help improve bone density in otherwise healthy adults⁽¹⁶⁾. RED-S athletes who suffered the lowest levels of BMD were also prescribed bone-building medications such as teriparatide (20% - helps promote bone formation) or bisphosphonates (13% - helps reduce bone breakdown). When these treated athletes were checked over a year later, two thirds of them showed BMD improvements; unfortunately however, this also means that a third of the athletes showed no improvement.

It should be abundantly clear from the above that RED-S is a real threat to athlete health and performance, and should therefore be avoided at all costs. Some athletes stumble into RED-S by simply underestimating their daily energy needs or by over enthusiastic weight management strategies. With other athletes, longer-term eating habits – possibly dysfunctional – may come into play. Regardless, the key to prevention is to be on the lookout, recognize if you may be at risk and then take steps to improve your energy availability. This article by Alicia Filley provides a great starting point to help you understand the potential pitfalls of RED-S, how it arises, and (importantly) how to navigate out of it.

Another recommendation for athletes who may be at risk is to undertake some regular strength training (1-2 times per week). Not only can strength training help reduce the likelihood of bone density loss/degradation, its addition will almost certainly help improve performance! A further way to help maintain bone health is to ensure a plentiful intake of dietary calcium and vitamin D. Here are some tips to help:

·        Emphasize the importance of both calcium-rich and vitamin D-rich foods in the diet. For vitamin D: oily fish such as salmon, mackerel, herring, sardines, pilchards etc are excellent sources; for calcium: milk, yoghurt, cheeses, nuts and seeds should all be consumed regularly.

·        Bear in mind seasonal fluctuations in vitamin D status; athletes living and training at northerly latitudes will probably benefit from routine supplementation from October to March (blood tests are readily available that can identify a sub-optimal vitamin D status – ie below 75ng/L).

·        Encourage some regular sun exposure to boost vitamin D levels; 15-20 minutes per day in strong sunshine without sun cream is ideal (cover up after that though).

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