Stress fractures are among the top 10 injuries sustained by runners, with women more likely to suffer a stress fracture than men. The tibia bone is the most common area for stress fractures in female runners.
Whilst biomechanics and training load can play a significant role in the development of a stress fracture, dietary factors such as low intake of key nutrients and/or insufficient calorie intake can significantly increase your risk of a stress fracture.
The female athlete triad is a combination of factors that can lead to reduced bone mass which increases the risk of stress fractures. The sequence of events usually begins with low energy availability and under nutrition due to mismatch of nutrition intake and exercise expenditure.
A prolonged period of calorie deficit results in the athlete becoming underweight which can then lead to menstrual disturbances. This in turn leads to estrogen deficiency, and the dysfunction of other hormones required for bone health resulting in impaired bone health and a decrease in bone density.
However you don’t need to be suffering from the female athlete triad to be putting your bones at risk. Insufficient intake of key bone building nutrients and excessive intake of bone stealing foods can also make you more susceptible to stress fractures.
Key nutrients that help prevent stress fractures
Calcium
Ninety-nine percent of total body calcium is stored in skeletal bone. According to the most recent Australian Nutrition Survey data, about 90 per cent of women do not achieve the recommended dietary intake (RDI) for calcium.
A diet that is low in calcium can put you at risk for developing bone-related injuries. If the body notices that not enough calcium is circulating in the blood calcium will eventually be taken from the bones. If your dietary intake of calcium is constantly low, your body will eventually remove so much calcium from the skeleton that your bones will become weak and brittle.
Adult women of childbearing age need 1000mg of calcium daily ideally from food sources.
Be mindful of using calcium supplements as many commercial brands are inferior and contain forms of calcium that are not well absorbed. Too much calcium (2,000 mg or more) from supplements may cause other health problems.
Good dietary sources of calcium include:
Milk and milk products – milk, yoghurt, cheese and buttermilk. One cup of milk or a 200 g tub of yoghurt provides around 300 mg calcium. Calcium-fortified milks can provide larger amounts of calcium in a smaller volume of milk – ranging from 280 mg to 400 mg per 200 ml milk.
Leafy green vegetables – broccoli, kale, bok choy, Chinese cabbage and spinach. One cup of cooked spinach contains 100 mg, although only five per cent of this may be absorbed. This is due to the high concentration of oxalate, a compound in spinach that reduces calcium absorption. By contrast, one cup of cooked broccoli contains about 45 mg of calcium, but the absorption from broccoli is much higher at around 50–60 per cent
Soy and tofu – tofu (depending on type) or tempeh and calcium fortified soy drinks
Fish – sardines and salmon (with bones). Half a cup of canned salmon contains 402 mg of calcium providing you eat the bones.
Nuts and seeds – brazil nuts, almonds and sesame seed paste (tahini). Fifteen almonds contain about 40 mg of calcium.
Vitamin D
Whilst most people are aware of the importance of calcium for strong bones many are unaware of the crucial role that vitamin D plays in bone health. Vitamin D is needed to assist calcium absorption and keep bones strong. Vitamin D also keeps your immune system functioning well.
A 2012 study concluded that there was a relatively greater importance of vitamin D over calcium for fracture prevention in young female athletes.
Ultraviolet (UV) radiation from the sun is necessary for the production of vitamin D in the skin and is the best natural source of vitamin D. However your bare skin needs to be exposed to the sun without the use of sun screen. To avoid skin cancer risks it’s best to exposure your skin to the sun during the earlier part of the day or late afternoon for no more than 10 minutes (less if you have fairer skin or during the summer months) several times per week.
If you are concerned about your vitamin D levels ask your health practitioner for a blood test known as a 25-hydroxyvitamin D test, also called a 25(OH)D. If your levels are especially low you may need to consider supplemental vitamin D.
Healthy Fats
Dietary fats have a strong influence on hormonal activity in the body. When women adopt low-fat diets, their estrogen levels drop noticeably in a very short time.
Estrogen is essential for bone growth and preventing bone breakdown. This is why postmenopausal women are at increased risk of osteoporosis as their estrogen levels drop considerably after menopause.
Fat intake also stimulates the release of bile into the small intestine, which helps the digestion of fat and the absorption of fat soluble vitamins, including vitamin D.
Aim for 20% - 30% of your daily calories to come from fat. Healthy fats include olive oil, avocado, nuts and seeds.
Magnesium
Magnesium is essential for absorption and metabolism of calcium. Magnesium and calcium function together, so deficiency of one markedly affects the metabolism of the other. In fact, increasing calcium supplementation without increasing magnesium supplementation can actually increase magnesium loss.
Magnesium also stimulates the production of calcitonin, a bone-preserving hormone and is necessary for the conversion of vitamin D into its active form.
Good sources are almonds, bananas, avocados, dried beans, lentils, nuts and whole grains. The recommended daily intake for adults is 400mg, however athletes often require a higher intake due to losses through urine and sweat.
Vitamin K
While vitamin K is best known for its role in blood clotting, this nutrient also plays an important part in the maintenance of healthy bones. It is required for the production of osteocalcin, which provides structure to bone tissue and is a key nutrient in bone formation.
Importantly vitamin K plays a key role in fracture healing.
Vitamin K comes in two forms: K1 & K2:
Good sources of K1 are dark green leafy vegetables like kale, collard greens and spinach.
K2 is produced from bacteria in fermented foods, such as ripe cheese and yogurt, and from bacteria in the intestinal tract.
Foods that are bad for bone health
Phosphoric Acid
Phosphoric acid is a cheap, widely available food additive used to acidify foods and beverages, colas in particular. Phosphorous and calcium compete with one another for absorption in the body and it has been suggested that the phosphoric acid in cola leaches calcium out of the bone.
Regular and diet sodas are currently the main dietary source of phosphoric acid on the market. A study published in the Journal of Clinical Nutrition found, “The mean BMD (bone mass density) of those with daily cola intake was 3.7% less than of those who consumed less than one serving cola per month”.
Whilst there is debate over the significance of phosphoric acid on bone health, reducing your intake of soft drinks as much as possible provides a number of other health benefits too such as reduced sugar intake.
Sodium
There is still a tendency for runners to consume too much sodium either through diet or from taking salt tablets for endurance events. High sodium levels can cause calcium to be excreted in urine and sweat.
Every 500 mg of sodium leaches 10 mg of calcium from your bones. Try and limit your sodium intake to 2–3 grams daily.
Coffee
Caffeine also prevents calcium absorption and it is suggested that each cup of coffee pulls out an additional 40 mg of calcium from your bones. Green tea is the better alternative, with many health advantages over coffee, including the fact that it increases bone mineral density.
This does not mean you have to give up your daily coffee hit, just limit yourself to a maximum of 3 cups per day.
Sugar
Studies have shown that high blood sugar levels increase the risk of fractures. Most of the protein inside of bone is collagen, higher blood glucose levels lead to collagen becoming attached to glucose molecules (known as glycation), which may weaken bone structure.
Aim to limit your intake of added sugar to 12 teaspoons per day to preserve collagen not only in your bones but also your skin. Yes that’s right a high sugar diet also gives you wrinkles!
Prevention is better than cure.
At the end of the day there can be many reasons why stress fractures occur, but by ensuring you eat a healthy, well balanced diet that contains sufficient calories you can reduce your risk and build strong healthy bones for today and into the future.
Whilst preventing stress fractures is an important consideration for female athletes it is also crucial to consider the long term consequences of poor bone health. Low bone density and weakened bones are risk factors for osteoporosis later in life. Dietary choices made today can have repercussions well beyond your running years.
If you are concerned about your bone health or current diet it is recommend that you speak to a health professional.
Jennifer Moulin is a degree qualified Nutritionist. She has a Bachelor of Health Science in Nutritional Medicine and has a passion for Sports Nutrition. She offers face to face and skype nutrition consultations through Proactive Nutrition based in Sydney.
References
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