Why do we do sports? Why do we ride road bikes? First and foremost, cycling is fun, whether in everyday life, on the way to work, together with others or against others in competition. There are many other reasons for cycling. A very important reason for the majority of cyclists is to maintain their health. Walking with a cane or needing a walker in old age is a daunting prospect. But in return, could there be health risks that are promoted by this sport? In recent years, there has been an increase in medical research suggesting that a large proportion of ambitious amateur and elite cyclists, with a correspondingly high level of training, have reduced bone density and may suffer from bone loss in old age (1, 3, 14, 31). Should perhaps even cycling be partly to blame for this? What could be possible causes of low bone density in cyclists? What can be done to best maintain bone density?
Bone loss
Osteopenia is the term for reduced bone density, it is a precursor to osteoporosis, bone loss. The term osteoporosis is derived from the Greek and means “porous bone”. In Germany, osteoporosis is now considered a widespread disease. In a 2014/2015 EHIS (European Health Interview Survey) study, nearly 8% of women and 2% of men reported having suffered from osteoporosis in the past 12 months (7). The fact that more women than men suffer from bone loss reflects an important factor in the development of reduced bone density: the influence of hormones on bone metabolism. With increasing age, the hormone balance of men and women changes and osteoporosis rates increase. A telephone survey by the Robert Koch Institute (RKI) found that nearly 12 percent (men: 5.2%; women: 17.6%) of respondents over the age of 50 had already been diagnosed with bone loss by a physician (16).
What are the correlations? Hormones play a decisive role in bone formation. In particular, sex hormones such as estrogen and testosterone, the thyroid hormones T3 (triiodothyronine) and T4 (thyroxine), and the hormone TSH of the pituitary gland regulate bone metabolism. With advancing age, hormone status changes in both women and men, and cases of bone loss occur more frequently. Now, at first glance, one might dismiss the problem of osteoporosis as an “old people’s problem.” But this is a mistake! Bone density can increase up to the age of 35. After that, there is an increasing decrease in bone substance and only a preservation of the previous bone mass is still possible. The foundation for osteoporosis is laid at a young age and this is where cycling comes into play. This is because intensive cycling at a young age could increase the risk of osteoporosis (3).
Bone fractures in sports
Philipp Lahm, Sabrina Mockenhaupt or Jan Frodeno, they have all had it at one time or another: diagnosis of fatigue fracture. In sports where running plays a central role, the phenomenon of fatigue fractures, so-called stress fractures, is well known. A fracture in the tibia or metatarsus is nothing unusual in running, soccer or triathlon. The risk factors have been scientifically studied here for a long time. If the athlete’s bone metabolism functions smoothly, fatigue fractures quickly heal completely without complications. In some cases, the affected athletes show existing osteopenia. In cycling, damage to the bones and especially fractures still occur mainly in accidents and falls. Especially in professional cycling, this fact is succinctly referred to by some as an “occupational hazard”. In recent years, more and more importance has been given to the safety of riders. Apart from measures such as changing the choice of routes and race regulations, or banning the “super tuck” position, it now seems that more and more cycling is becoming aware of the risk posed to athletes by reduced bone density. In short, bones that are strong and elastic are less likely to break in crashes.
Bone density measurement
There are various diagnostic methods for diagnosing osteopenia or osteoporosis. Nowadays, bone density is most frequently measured using the so-called DXA measurement (Dual-Energy X-Ray Absorptiometry). Other methods for determining bone density are quantitative computed tomography (QCT) and quantitative ultrasound examination (QUS). The basis for DXA is an ordinary X-ray examination, but two X-ray tubes with different power are used. The lumbar spine or the hip is usually examined. The radiation used is absorbed to different degrees by tissue of varying density. Strictly speaking, DXA does not provide density values in the physical sense. By definition, density is mass per volume. In the case of DXA, the examination provides an area-projected mass (areal density). In this case, bone density is approximated indirectly by the calcium and hydroxyapatite content of the bones. For this reason, DXA is also criticized by some physicians for not providing sufficiently reliable values – especially for the diagnosis of osteopenia. However, radiation exposure is lower than with three-dimensional quantitative computed tomography (QCT). QCT provides a three-dimensional image of the bone and it is also possible to visualize the bone in the outer region (cortical bone) and the center (trabecular meshwork). In QUS, sound waves penetrate the tissue, usually examining the calcaneus, radius or phalanges. This method also provides information about the density of the bone and does not result in radiation exposure, but is considered too less reliable. The costs of bone densitometry are only covered by health insurance in justified cases of suspicion or risk. For early detection, bone densitometry is a private service (IGeL) and must be paid by the patient.
Factors for osteopenia
But for whom is a bone density measurement useful at a young age? Who is at risk as an athlete? There are numerous diseases that are associated with reduced bone density. Some of these diseases are hardly ever found in athletes, while others should definitely be looked at more closely. Kidney disease, diabetes, rheumatism, hyperthyroidism, rheumatoid arthritis, anorexia or epilepsy are conditions that increase the risk of reduced bone density. The consumption of some stimulants also increases the risk: cigarettes, alcohol and soft drinks. In the case of nicotine, the exact mechanism is not yet fully understood. It is assumed that it has a similar effect to alcohol consumption: the balance in bone renewal is disturbed. In the case of soft drinks, it is the phosphates from phosphoric acid that inhibit calcium absorption in the blood. Calcium is an important building material for our bones. For a long time, coffee was also on the list of culprits. In 2013, coffee was cleared of suspicion because a large-scale Swedish study of more than 60,000 women over an observation period of 14 years on average could not demonstrate a link between coffee consumption and osteoporosis (6).
Human bone is not a constant, rigid mass, but is undergoing constant remodeling. This remodeling is to ensure the stability of the skeleton. In everyday life, small structural damages (micro cracks) occur which have to be repaired. If small damages are not repaired immediately and at the same time the load on bones continues, the aforementioned fatigue fractures can occur. In running or soccer, for example, one or two rest days per week are not necessarily enough to give the bones time to regenerate. Osteoporosis results from a decrease in bone density due to a breakdown of bone tissue that exceeds the build-up. This imbalance is also the cause of osteopenia. 10 percent of the bone substance is renewed annually. In 10 years at the most, the entire bone mass is completely resynthesized once through remodeling. In 95 percent of the cases in which bone loss occurs, it is a so-called primary osteoporosis, which in the vast majority of cases occurs in (old) age. Only 5 percent of cases with bone loss are secondary osteoporosis, in which the bone loss is caused by another disease or its (drug) treatment. Osteopenia does not cause any symptoms. Similarly, existing osteoporosis, due to the lack of obvious symptoms, remains undetected for a long time. Pain only occurs at a late stage. Figuratively speaking, three reasons for reduced bone density can be identified as the cause: a lack/lack of building materials, a lack/error of the building plan, and errors/disruptions in the building process.
Stress on the bones
“Form follows function” is especially true for our bones. Wolff’s law states: if a bone is loaded, it builds up, and if a bone is not loaded, it degrades. So, in order for bone formation to take place, it needs, among other things, a stimulus, a load. To prevent a reduction in bone density and thus osteoporosis in old age, the catchword is often: regular exercise. The fact that this advice is very simplified can be seen if one takes a closer look at the matter.
For a long time, few people had cycling on their radar when examining osteopenia or osteoporosis. In general, sports were long considered across-the-board sufficient to stimulate bones to grow. Then, disciplines such as swimming or long-distance running were initially scrutinized and it was determined that the load of these sports was insufficient to ensure adequate bone density. With technological advances, bone density could be better and more cost-effectively imaged. Testing of athletes for bone density became more frequent, and cycling increasingly became the focus of scientific studies as a high-risk sport for reduced bone density. Professional cyclists in particular spend a lot of time – often around 30 hours per week – on their bikes. This involves exerting little or no force on a large part of the bones. The remaining time is then mainly used for regeneration. However, similar activity distributions can also occur in hobby cyclists. At risk are then above all “office jockeys”, whose activities then often take place at the desk, on the bicycle and asleep in bed. Road cyclists are particularly at risk from osteopenia, while mountain biking already involves other muscles to improve bone density (12).
An extreme case of “non-exertion” is represented by astronauts who float through space in complete weightlessness on space stations or rockets. There are now numerous studies showing the devastating effects of weightlessness on bones. To counteract bone loss, astronauts must perform regular strength training in space. Despite an extensive training program, astronauts show a significant reduction in bone density (17,18). Currently, an improvement of the training program is being discussed for astronauts – in addition to the administration of medication. This shows the importance of loading the bones.
Athletic triad
Loading is a crucial factor, but it is not sufficient on its own. As mentioned earlier, hormones have a major impact on the formation and breakdown of bone substance. Apart from age-related hormonal changes, changes in hormonal balance at a young age are also a cause of osteopenia. Hormonal disturbances are more easily recognizable in women than in men. Cycle disturbances or complete absence of menstruation (amenorrhea) is a clear sign that something is wrong with the hormonal balance. For women, the term “female triad” had been established, referring to the three points of energy availability, menstrual status and bone health in a mutual interaction. The cause of menstrual disorders and reduced bone density is a relative energy deficit (energy requirements from basal metabolic rate and power metabolic rate are not fully covered). Often the relative energy deficit is caused by an eating disorder. But even athletes without a diagnosed eating disorder sometimes consume too little energy and run the risk of jeopardizing their bone health.
Anorexia in the sports scene is well known. Nevertheless, it is addressed far too little. In cycling, it’s mainly watts per kilogram of body weight that count. The ex-professional cyclist Dominik Nerz speaks openly about the subject in his book “Gestürzt”. This triad can also be observed in men. This is why the term “athletic triad” has now become established. Recently, there has also been talk of the phenomenon RED-S, the Relative Energy Deficiency in Sport, whereby here the cause – the relative energy deficit – is clearly named (24). In contrast to women, the problem often remains undetected for a long time in male athletes. In many sports, the aim is to achieve a low body fat percentage and thus low body weight. A low calorie intake affects the metabolism. The basal metabolic rate is reduced and the hormonal balance is disturbed. However, it is not weight loss alone that causes the potential danger. An athlete with a constantly low body weight and a constant but low energy intake has a greatly increased risk of harming his health. For example, no one would think of calling a model who maintains weight on 800 kilocalories a day healthy. A significantly reduced basal metabolic rate makes it possible for models and also athletes to maintain their body weight despite a low energy intake. The effects on hormone balance are devastating. Various studies show that not only is an adequate proportion of lean body mass important for healthy bone metabolism, but a good portion of body fat is also necessary for functioning hormone metabolism (13, 42). Overweight people, on the other hand, should lose body fat. In men, the Athletic Triad is manifested, among other things, by low testosterone levels. Inadequate energy intake causes testosterone levels to drop in men, which has a negative effect on bone metabolism (22). In this context, low bone density may also be present despite normal testosterone levels in cyclists (1). Especially in ambitious amateur and professional athletes, the athletic load with release of the stress hormone cortisol in combination with a calorie deficit thus contribute to an increased risk of low bone density. However, the training load or the associated cortisol is not the actual cause of the Athletic Triad, but the existing energy deficit. A large-scale Norwegian study of elite female athletes showed that cycle disturbances occurred in sports where low body weight was of importance to the elite athletes for athletic success (20). In sports in which hard training with high volumes but less low body weight was critical, the proportion of female athletes affected by cycle disorders was only half compared with sports focused on body weight. A 2020 study published in the journal Frontiers in Endocrinology examined the effects of an intense four-week training block (mesocycle) – commonly used to enhance performance in cycling – on male cyclists (12). The subject of the study was how the increase in performance in interaction with other parameters such as hormones affects the RED-S. As expected, maximal power, VO2max, and FTP were increased by the training block. Body weight and composition, among other parameters, were monitored with a DXA scan and remained constant during the period of the study. After the four weeks, reductions in resting metabolic rate and thyroid hormone T3 were shown, among others. Thus, even short blocks of exercise with inadequate energy intake can depress metabolic rate and fuel the althletic triad. Hormone replacement therapy for an existing Athletic Triad is not a solution, and feeding anabolic steroids is prohibited in competitive sports. Hormones should only be administered to athletes in cases of hardship. Usually, when an athlete is diagnosed with athletic triad, a healthy balance is attempted with sport reduction, increased energy intake, and strength training. The administration of hormones has side effects. Significant and sometimes life-threatening effects can occur from taking and after discontinuing exogenous hormones such as testosterone (23). The list is long. Effects include worsened blood flow properties, calcification of blood vessels, disease of the heart muscle leading to thrombosis, stroke, heart failure, cardiac arrhythmia or heart attack. Infertility or psychological problems such as depression can also result from the hormone change.
Nutrition for stable bones
Both the quantity and quality of food are essential for maintaining bone density. Even with adequate energy intake, osteoporosis can occur with malnutrition. This is because our body needs the right building materials for stable bones. Our bones are an organ consisting of different types of tissue with different tasks. The basic bone substance is responsible for the supporting function of the bones. It contains organic and inorganic components. The organic parts of the bone ground substance consist of 95 percent collagen and make up one third of the bone tissue. A quarter of the bone tissue is water and just under half consists of inorganic material such as mineral compounds.
Even as children, we learn from commercials from candy manufacturers that calcium is important for bone growth. With an “extra portion of milk,” children’s bones are supposed to become strong and firm. In fact, bones are made up of a large percentage of calcium and at the same time, bones are our body’s calcium store. Almost all the calcium present in the body is found in bones and teeth. The calcium mass weighs a good kilo. Although calcium is an important building substance, it fulfills numerous other functions in the body and is released from the bone store as needed. For example, calcium is important for blood clotting, cell division, for the excitation of muscles and nerves, for a functioning glycogen metabolism or for the activation of hormones and enzymes. The German Nutrition Society DGE recommends a daily intake of 1000 milligrams of calcium for a middle-aged person. As with many other minerals, the absorption of calcium is coupled with intake and demand. Only about one-third of the calcium ingested with food is absorbed in the intestine. In addition to children, pregnant women and senior citizens, athletes also have an increased requirement. Resorption is inhibited by the classic “calcium predators” phosphate, oxalic and phytic acid, which are found in soft cheese, soft drinks, spinach, beetroot, whole grains and legumes, among other things. The consumption of table salt, sulfur-containing amino acids, coffee and alcohol increases calcium excretion via the urine. Athletes additionally lose calcium through sweat. Foods with a high calcium content include poppy seeds, sesame seeds, almonds, kale, hard cheese or mineral water. However, to consider calcium intake and calcium balance on their own would be a big mistake! Calcium is bound to phosphate in bone. Hydroxyapatite is formed, one of the strongest substances in our body. Our tooth enamel consists almost entirely of hydroxyapatite and is thus even harder than any bone. Calcium metabolism should therefore always be considered together with phosphate metabolism. Our body keeps the ratio of calcium and phosphate constant. We hardly lack phosphate – the compound of phosphorus and oxygen – with our modern industrial food. On the contrary, our food often contains a very high proportion of phosphorus. As mentioned before, a high intake of phosphorus (acid), as in soft drinks, can even lead to calcium deficiency and is a risk factor for osteopenia. However, calcium is not deposited in bone as a motionless mass. There is a constant flow of the mineral from the bone into the bloodstream and vice versa. A calcium deficiency can therefore not necessarily be determined by the blood calcium level, because our body immediately deposits calcium from the bones in the event of a calcium deficiency and thus keeps the blood calcium level approximately constant. It is only when the hormones regulating calcium levels become unbalanced that the blood calcium level also becomes unbalanced, resulting in muscle cramps or disease. However, taking calcium does not reliably protect against decreased bone density. In a study of elite cyclists, bone density did not improve even with increased calcium consumption (2). Calcium intake may well have positive effects on bone growth and is essential as a building material, but is not a guarantor of preventing fractures or low bone density (19).
There are three hormones in particular that are crucial for bone or calcium metabolism. Two of them, in short, are responsible for the formation or breakdown of bone substance. The third hormone (calcitriol) is the active form of cholecalciferol, which is better known as vitamin D3. Vitamin D3 is actually not a vitamin, but a hormone. It leads to increased absorption of calcium in the intestine as well as increased reabsorption of calcium in the kidney. Vitamin D3 is important for bone formation. In old age, the body’s own production of the hormone decreases. Substitution through dietary supplementation is possible and recommended for older people and for the winter months. Vitamin D3 is produced by our body with the help of UVB-rich solar radiation from cholesterol that hits the skin. Marine animals, eggs, cheese, milk and mushrooms also contain vitamin D (plant foods often contain the less effective vitamin D2). In mid-latitudes, like ours in Central Europe, effective UVB radiation is largely weakened in winter as it makes its longer journey through the atmosphere to the ground, and our skin no longer produces vitamin D3 (29). In late fall and spring, long clothing prevents the sun’s rays from reaching the skin when cycling. Thus, substitution with dietary supplements is well justified. However, very high doses of vitamin D do not confer a bone health benefit (30). Here, as with greatly increased intakes of calcium, the disadvantages may outweigh the benefits. Very high doses of dietary supplements containing calcium (especially calcium carbonate) and/or vitamin D have been recorded, sometimes with severe side effects of kidney stones, heart attacks, strokes, and deaths (26, 27, 28, 33). In addition to calcium, other minerals such as potassium, phosphorus, and magnesium are important for bone formation. Vitamin D ensures improved absorption of calcium. In addition, vitamin K plays a role in bone incorporation (35). None of these alone can prevent osteopenia, but all of the building blocks are important.
A major (contentious) issue, especially among athletes, is the topic of vegan/vegetarian diets. Proponents of the (predominantly) plant-based diet argue, among other things, that animal protein contributes to hyperacidity of the body and thus also promotes osteopenia. Numerous studies have examined the effects of protein on bone health, and there are both results showing that a vegan diet is beneficial for bone health and those showing that a vegan diet is detrimental to bone health (36, 37). In some cases, (thyroid) hormones rather than protein consumption were actually behind the reduced bone density (40). In a 2016 study published in the Journal of Nutrition, Health & Aging, more than 500 women of retirement age were studied over 3 years (8). The subject of the study was the effects of protein consumption on bone density as well as bone mineral content. Not only did the level of protein intake play a role. A distinction was also made between vegetable and animal protein. Furthermore, the influence of BMI (Bodymass Index) and physical activity in connection with protein intake was investigated. A control group received no supplements, while the women studied received calcium and vitamin D3 at the internationally recommended daily allowance. The study showed the complex relationships. Animal protein can have a negative effect on bones, while no effect of vegetable protein was demonstrated. High protein consumption led to lower bone densities and mineral contents of bones in less active women. This effect was even increased in slim women. However, in active individuals even the opposite was shown: a higher mineralization and density of the bones. An analysis of several studies confirmed that there is no clear relationship between protein consumption and bone density (34). Some studies even suggest that low protein intake leads to low bone density, especially in older individuals (32). Nevertheless, the idea of linking body acidity to bone density has merit. Minerals such as sodium, potassium, calcium, or phosphate buffer protons and serve to maintain the physiological pH norm in blood and tissues. However, there is a lot of charlatanism associated with the topic of acidosis or hyperacidity. However, human physiology is very complex and simple and quick remedies should always be taken with caution.
Last but not least, this also applies to the organic building blocks of bones. As mentioned, an essential bone substance is collagen. It is also part of the connective tissue of cartilage, tendons, teeth, ligaments and the skin. It provides structure and strength. However, collagen does not have to be supplied with food like calcium or other minerals. Collagen is produced via biosynthesis, which takes place in certain types of cells. Collagen is a so-called structural protein, which makes up about one third of the total mass of all proteins in the body and is therefore the most abundant protein. Provided there is a sufficient intake of food, a healthy body can produce collagen itself without any problems. In certain diseases and with age, the ability to produce decreases and supplementation may be useful for improving bone density (41).
How can cyclists protect themselves from low bone density?
Building bone with high bone density is of immense importance, especially since in old age bone loss due to hormonal changes cannot necessarily be prevented even with an extensive training program (11, 21). Cycling gives thick thighs, but to prevent osteoporosis it is also necessary to strengthen other muscles that are not or hardly challenged on the bike with strength training. The load provides the blueprint for our body. A balanced diet is a pillar of health and therefore also a pillar of a healthy bone metabolism. This is possible with different nutritional concepts. Targeted nutritional supplementation with minerals and vitamins can be particularly useful for athletes with increased requirements. This provides the necessary building blocks for bone formation. Adequate energy intake is particularly important. Ambitious competitive athletes aiming for a low body weight should avoid hunger phases, which lead to a reduced basal metabolic rate and throttled hormone production. Only with a healthy hormone balance can the smooth building of our bones function.
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This article is an excerpt from the original article published in the RennRad 11-12/2021 issue of the German RennRad magazine. The issue can be ordered here.*
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