The Role of Vitamin D in stress fractures

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Many people suffer from fatigue, reduced mental activity, sleep disorders, muscle cramps, depression and an increased susceptibility to infection. A decreased level of Vitamin D due to lack of adequate exposure to sunshine is often held responsible for many of these symptoms.

New findings have even uncovered a direct association of this essential vitamin with stress fractures in active individuals! However, before discussing the impact of this recently published research data, we have to look at this vitamin and its biochemistry in the human body in more detail, to understand how sunlight can influence the concentration of Vitamin D.

In winter the skin is often covered with clothes and not adequately exposed to sunlight as in warm and sunny summer. The intensity of ultraviolet B radiation is lower compared to summer.

What is more important is the fact that more and more people are increasingly remaining inside protected buildings!

The group of D vitamins (or calciferols) biochemically belongs to steroids: the two most important calciferols are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Since our body is able to synthesise the precursors or pro-vitamin by itself, calciferols are not strictly vitamins but hormones.

But they can also be taken up by nutrition — seafood, especially cod liver oil is highly enriched. Dairy products and mushrooms are other sources containing a lot of Vitamin D.

 

Understanding It

The first step of the conversion of the inactive pro-vitamins into the active form takes place in the skin. The exposure of pro-vitamin D3 with ultraviolet light leads to the synthesis of cholecalciferol or Vitamin D3. Afterwards, Vitamin D3 enters the liver via the bloodstream and is once more modified.

Lastly it is transported to the kidneys, where it reaches its biologically active form (calcitriol) by a last bio-chemical modification. Now one can understand how the exposure to sunlight directly influences the concentration of Vitamin D in the body.

But what are the functions of calciferolin in the body? How does a vitamin deficiency (so-called hypo-vitaminosis) have such severe symptoms like bone fractures?

The most important function of vitamin D is the regulation of calcium concentration in the blood plasma – the calcium homeostasis. Simplified Vitamin D raises the calcium plasma concentration by an increased re-absorption of this mineral in the intestine.

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Furthermore, it decreases the loss of calcium via urine since it triggers calcium re-absorption in the kidneys. It also mobilises calcium from the bones.

The calcium concentration in the plasma is kept within very narrow ranges and is measured by receptors residing in the para-thyroid glands. When the concentration goes below the limit, para-thyroid hormone (PTH) is released, which stimulates the renal re-absorption of calcium and the production of enzymes for the bio-synthesis of calcitriol, the active Vitamin D.

 

Stress Fractures

Bones are the major storage compartments for calcium ions in the human body. If the concentration of calcium in the plasma is lowered, the concentration of calcitriol will be instantly heightened. Calcitriol can now act on cells, the osteoclasts, which break down bone substance and free the calcium.

On the one hand the body uses this process to make sure that the plasma concentration stays on a constant level, but on the other hand it also leads to a loss of bone mass density. We now see that Vitamin D has an important role in the modulation of bone mass density and bone health.

Hypo-vitaminosis D is reported to lead to osteoporosis, osteomalacia, a decreased bone mass density, and to a higher risk of fractures. But so far no consensus of an adequate plasma level of Vitamin D has been reached, especially for patients who participate in high-impact activities.

In a recently published study in the Journal of Foot And Ankle Surgery, scientists examined the impact of Vitamin D plasma concentration in patients with confirmed stress fractures. Patients who were thought to have a stress fracture were initially examined by X-ray radiography. They were then sent for magnetic resonance imaging (MRI) for a confirmatory diagnosis.

Different muscoskeletal radiologists independently analyzed the MRI scans and confirmed the diagnosis. More than 120 patients with acute stress fractures were included in the study and bio-medical data was collected. The plasma Vitamin D concentration was monitored within three month of diagnosis.

The most common bones with stress fractureswere the second and third metatarsal. The plasma Vitamin D concentrations were almost similar for males and females, and the mean was approx. 31 ng/ml. Compared to the current guidelines set by the Vitamin D Council and the Endocrine Society, 83% and 53% of the patients would have been classified as having a Vitamin D insufficiency or even deficiency.

 

Common Guidelines

Vitamin D deficiency directly leads to a decrease of absorption of dietary calcium to just 15% of normal levels. The low plasma calcium concentration is detected by the calcium receptors, followed by an increased release of para-thyroid hormone. PTH stimulates osteoclasts to mature, which in turn mobilise calcium from the bone substance.

Thus, bone mineralisation and structural integrity is successively decreased.

Since the common guidelines are just considered estimates and no minimum Vitamin D level protecting from stress fractures has ever been defined, some scientists propose a plasma concentration of at least 40ng/ml. This threshold value should be recommended for active patients with moderate or high physical stress.

Patients with diagnosed stress fractures should supplement Vitamin D at high dosage to further support healing and for prophylaxis. Likewise, the Endocrine Society advocates a Vitamin D supplementation for adults with Vitamin D deficiency at least once a week for two months to achieve plasma levels of up to 30 ng/ml.

To prevent recrudescence and to hold up basal levels the patients should continue to supplement Vitamin D at lower concentrations. Although no harmful effect of Vitamin D supplementation in moderate dosages has been reported so far, physicians will still have to supervise the consequences of long-term intake.

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