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How Strong are Your Bones?

By Susan Kitchen | Dec. 06, 2016, 6:33 p.m. (ET)

cereal and milk

As the days get shorter and the temps cooler it’s officially post-season for triathletes who live and train in the northern hemisphere. During these months, we flock to indoor pools, indoor trainer sessions, and camp out on treadmills. Despite following a balanced and varied diet, the prevalence of low vitamin D, deficient calcium consumption combined with reduced sunlight exposure can be a recipe for disaster among athletes of all ages. Both calcium and vitamin D play a critical role in bone health, and their role in exercise-related inflammation and prevention of chronic disease is vastly accumulating.

We all know that calcium is good for overall bone health but calcium can only reach its full bone-building potential if your body has adequate vitamin D. Calcium and vitamin D work together to protect bones. Calcium helps build and maintain bone density and strength while vitamin D assists in the absorption of calcium. So, even if you are taking in adequate calcium, it is not effective if you are deficient in vitamin D.

Role of Calcium

Calcium is the most abundant mineral in the body and 98 percent is in our bones while the remaining 2 percent circulates in the bloodstream and in our teeth. While many athletes take in enough calcium in their diet, women on average tend to be deficient in calcium intake. This can be partially blamed for the replacement of beverages such as coffee, soda and alcohol. Furthermore, as endurance athletes, we lose calcium (and sodium) via sweat during strenuous exercise and unlike vitamin D, we require more dietary calcium than our sedentary counterpart. From this calcium loss and possible inadequate calcium intake, we are more susceptible to bone loss, osteoporosis (loss of bone mass) and bone fractures. Additionally, after the age of 35, the body’s natural bone building process begins to decline so it becomes even more important to prioritize maintaining bone mass.

Sources of Calcium

Food is the best source of calcium and is largely found in dairy products such as milk, yogurt, cottage cheese and cheese. Other sources include greens veggies such as collard greens, broccoli, spinach and kale as well as in fortified juices, cereals and enriched breads and grains.

Supplementation of Calcium

Depending on how much calcium you get from food determines the amount of calcium supplementation you need. We only need supplementation to make up for the shortfall of calcium from foods. There is no added benefit from taking more calcium than you need, and, in fact, it comes with some risks.

Reading food labels can be tricky when it comes to calcium content. Food labels list calcium as a percentage of daily value (DV), which is based on 1,000 mg/day. Thirty percent DV equals 300 mg calcium, 20% DV equals 200 mg calcium and 15% DV equals 150 mg calcium, respectively.

When choosing a supplement, look for the USP (United States Pharmacopeia) symbol, which means the USP has tested and found the supplement to meet certain purity and quality standards. Determine the amount of elemental calcium, which is the actual amount of calcium in the supplement. On the label, pay attention to dosage and amount per serving. Amounts of 500 mg or less are best absorbed. Ideally, take in smaller amounts throughout the day than all at once. Take with a meal unless it is in the form of calcium citrate in which case it can be absorbed with or without food.

Role of Vitamin D

Commonly known as the “sunshine vitamin,” vitamin D is a hormone made by the body when the skin is exposed to sunlight. But, why is vitamin D important? Well, for starters, vitamin D is essential for calcium absorption, and it plays a key role in maintaining bone health. Without adequate vitamin D, we absorb only 10-15 percent of the calcium consumed — compared to the standard 30 percent. Over two-thirds of the U.S. population is at risk of vitamin D deficiency and likely may not even know it. Athletes most at risk are those with the following:

  • history of stress fracture, bone or joint injury,
  • muscle pain or weakness,
  • reduced sun exposure (from indoor training, heavy use of sunscreen or sun protective clothing, persons with history of skin cancer and dark complexions),
  • elimination of dairy products.

Sources of Vitamin D

To improve the body’s absorption levels, it’s best to get vitamin D from food sources and sunlight as opposed from a supplement. Bear in mind, vitamin D is NOT found in a wide variety of food groups. The best food sources are: fatty fish (cod, mackerel, salmon and sardines), egg yolks and fortified dairy products such as low-fat or whole milk, soymilk, yogurt, butter, margarine and fortified cereals. Some orange juices are fortified with calcium and vitamin D.

The body regulates how much vitamin D is made when exposed to the sun based on time of day, the season, how far north you live, skin pigmentation (darker skin makes less D), how much of your skin is exposed to the sun and your age (older people produce less). So, as you can see, relying on vitamin D from the sun can be complicated especially if you have or are at risk of skin cancer.

However, recommendations include:

  • Sun exposure without sunscreen — 10 minutes for lighter skin and 20-30 minutes for darker complexions at least three times per week during the middle of the day.
  • The more skin exposed, the more you absorb (ideally bare arms and legs).
  • Making vitamin D from sunlight is effective when the skin turns slightly pink from exposure. 

Supplementation of Vitamin D

If you don’t get adequate sun exposure during the winter months and are not a big fan of dairy products or fatty fish, it would be wise to have your vitamin D blood levels tested. Ideally a range of 32-50 ng/ml is optimal for athletic training-induced adaptation. Blood levels less than 30 ng/ml are considered insufficient.

In reading food labels, the daily value (DV) is based on a total daily intake of 400 IU (yes, below the recommended daily amount of 600-800 IU) of vitamin D.

The Female Athlete and Bone Health

Although male athletes are not exempt from decreased bone mass or stress factures, it is more prevalent in female athletes. The primary contributing factors of low bone-mineral density in females include low energy availability or inadequate energy intake (defined as dietary intake minus energy expenditure less than 30 cal/kg of fat free body mass), amenorrhea, menstrual dysfunction and low estrogen levels. Inadequate calcium intake further increases the risk.

Recommendations for Female Athletes at Risk

  • Consult with a sports dietitian (RD, CSSD) for effective nutrition strategies to improve bone health.
  • Engage in weight bearing activities such as running and weight training to improve skeletal calcium absorption, increase bone mass and offset the bone loss rate as we age. Note: swimming and biking are not bone-strengthening activities.

Calcium and Vitamin D Daily Requirements*


14-18 years old  1,300mg (male/female)  
19-50 years old  1,000mg (male/female) 
51-70 years old  1,000mg (men); 1,200mg (women)
71+ years old  1,200mg (men/women) 

If the calcium supplement is calcium carbonate, it should be taken with meals to improve the absorption rate otherwise if calcium citrate, it can be taken anytime.

Vitamin D 

9-70 years old  600-800 IU 
71+ years old 800+ IU  

From a daily multivitamin that contains 600-800 IU vit D as D3 or cholecalciferol 
*Source: National Institutes of Health

Susan Kitchen, MPH, RD, CSSD, is a USA Triathlon Certified Coach and owner of Race Smart, LLC, a nutrition and coaching practice. Personally, she competes in endurance sports from the marathon to full Ironman. To contact Susan, email or visit and on Facebook at Race Smart.

To get a customized metabolic efficient nutrition plan, contact Susan Kitchen, MPH, RD, CSSD at

The views expressed in this article are the opinion of the author and not necessarily the practices of USA Triathlon. Before starting any new diet or exercise program, you should check with your physician and/or coach.