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| Indoor-based training has become increasingly popular for endurance athletes who live in areas where winter weather is a challenge. |
Indoor-based training has become an increasingly popular option for endurance athletes during winter months. This includes social media-based technology platforms like Zwift for cyclists and the treadmill for runners. These options provide endurance athletes with safe, predictable, and comfortable environments in which to carry out winter training workouts. This may be particularly important for athletes with targeted events in the spring for whom quality training units are critical. There may, however, be unintended consequences to these predominantly indoor-based training plans that have the potential to impact health and athletic performance. The lack of significant exposure to the sun during the winter months may negatively effect levels of Vitamin D. These seasonal variations in Vitamin D levels have the potential to impact bones, immune system, and mood.
Vitamin D
Vitamin D is a fat-soluble substance which is naturally present in very few foods, added to others, and available as a dietary supplement. It is commonly produced when UVB light (from sunshine) comes into contact with exposed skin, is absorbed, and triggers Vitamin D synthesis. Within the body Vitamin D is converted to calcitriol, a hormone, which directs cells to produce certain proteins. These proteins then in turn play important roles in the bones, immune system, and potentially within the brain.
Insufficiency vs. Deficiency
It is fairly well established that residents in areas with harsh winter conditions demonstrate annual variations in Vitamin D levels as a result of less available daylight and less time spent outdoors. That is to say that Vitamin D levels tend to be lower in the winter months when compared to spring, summer, and fall. Although clinical Vitamin D deficiency is uncommon in endurance athletes in these geographical areas, sub-clinical relative insufficiency may be present. And because the optimal function of bone, immune system, and the brain is particularly important for these athletes, the maintenance of adequate Vitamin D levels year round may be essential to both optimal health and athletic performance.
Indoor training reduces the time spent outdoors. In a modern life spent predominantly inside, this location of training potentially impacts the production of Vitamin D through less exposure to natural sunlight. The impact of seasonal variations in exposure to this UVB light has consequences to the endurance athlete in three principle areas.
The Bones
Vitamin D is essential for building and maintaining healthy bones. Calcium, the principle constituent of bone, is absorbed when Vitamin D is present. Running has the potential to both positively impact bone density and negatively impact the risk of stress-related processes. First, it may help strengthen running-specific bones such as those about the hip. These are particularly vulnerable to fall-related fractures in older adults. Optimizing bone density about the hip earlier in life through both adequate Vitamin D levels and bone building impact activities such as running, may reduce the likelihood of fractures later in adulthood. However, in vulnerable athletes who participate in certain training practices, running may increase the likelihood of stress-related bony pathologies such as stress reactions and fractures. Once these pathologies are diagnosed, adequate levels of available calcium along with modifications in training practices help the athlete to recover.
The Immune System
A properly functioning immune system helps endurance athletes stay healthy. When health is maintained, training remains consistent. And consistent training leads to fitness improvements. T-cells are an important part of our immune defense. Vitamin D (via calcitriol) signals the formation of these cells and strengthens our immunity. Endurance athletes are subject to the same seasonal illnesses as non-athletes. However, a combination of heavy endurance training (which may reduce immune function) and indoor gym-based exercise (which subjects athletes to large crowds) may significantly increase the likelihood of exercise-limiting illnesses. Therefore, adequate levels of Vitamin D may assure that sufficient levels of T-cells are available to fight these illnesses.
The Brain
Most endurance athletes during winter months travel to work in the dark, work inside all day, and then travel home in the dark resulting in very little time spent exposed to natural outdoor light. Seasonal affective disorder is a mood disorder that is common during winter months and characterized by symptoms such as low energy levels, trouble concentrating, fatigue, increased desire for isolation, sleep disturbances, and carbohydrate cravings which may lead to weight gain. While the exact causes are unknown, scientists suspect the mood disorder may be linked indirectly to less exposure to sunlight in the fall and early winter. Vitamin D (via calcitriol) is involved in the synthesis of serotonin and dopamine in the brain. Both of these substances are linked to depression.
Recommendations
It is possible to balance the desire for quality training units and the investment in indoor training technology with the well established health benefits of Vitamin D. There are two possible ways that endurance athletes who wish to primarily train indoors may take advantage. The first is to dedicate as much time as possible in the winter to being outside on the weekends between the hours of 11:30 am - 1:30 pm during which UVB reaches peak levels. The more skin exposed during these times the greater the amount of Vitamin D production. Lastly, it would seem prudent to supplement with 600 IU's of Vitamin D3 daily during the winter months to optimize the benefits.
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