Building Better Bones





As the mom of a swimmer and a childhood nutritionist with a special focus on young athletes, I’ve often wondered about the bone health of young athletes. Certainly, a healthy diet and exercise contributes to healthy bones… but is it enough?

Childhood and adolescence are when bones are built. If bone health is not established during this time, the risk of future osteoporosis (weak, brittle bones) is real. In fact, it is estimated that by 2020, 50% of Americans will be at risk for bone fractures related to osteoporosis, according to the National Osteoporosis Foundation.

What Swimmers Eat

Diet is one area that impacts bone health, especially the nutrients, calcium and vitamin D. Unfortunately, consumption rates of these nutrients among children and teens fall short of requirements, according to the 2015 Dietary Guidelines for Americans.

The average calcium intake by teen girls is 876 mg per day (67% of the Recommended Dietary Allowance (RDA)), and less than 15% of teen girls actually meet the RDA. Boys have slightly better intake levels, but remain less than desirable.

As a refresher, children aged 9-18 years need 1300 mg calcium every day (and 600 IU of vitamin D daily).

Complicating the matter, milk consumption in teens has decreased over the years in favor of soda consumption and other beverages. Although milk substitutes such as almond milk or rice milk are fortified, bioavailable calcium (calcium that is active and available for the body to use) may be reduced in these products. Plant-based calcium foods are an option, but quantities may be too much for some young athletes to consume and others may be uninterested in these food options.

Also important to bone health is vitamin D, which is required for the absorption of calcium. Without it, only 10-15% of calcium from food is absorbed.

Swimmers can get vitamin D from animal-based food (fish), plant-based food (mushrooms), fortified food (milk, orange juice, eggs and cereal) and from activated vitamin D in the skin through sun exposure.

Sun exposure is a primary source of vitamin D, but this depends on skin color (darker skin colors absorb less vitamin D), time of day, latitude and use of sunscreen. Sunscreen with an SPF of 8 or greater will effectively block the synthesis of vitamin D in the skin. On the other hand, according to a 2014 report from the American Academy of Pediatrics, exposure of arms and legs to the sun without sunscreen for 5 to 15 minutes, 2 to 3 times per week can generate up to 3000 IU of vitamin D.

Meeting calcium requirements from food requires thought and planning. Here’s an example of what it looks like for a 9 to 18 year old to meet daily requirements:

  • Two 8 ounce glasses of milk, 1 cup of yogurt and 1 cup of calcium-fortified orange juice; or 1 cup yogurt, ½ cup calcium-fortified orange juice, 1 cup calcium-fortified cereal, 1 slice of American cheese, ½ cup of cottage cheese, and an ounce of chia seeds.

For the dairy-free athlete: 

  • One cup of almond milk and ½ cup of tofu (prepared with calcium sulfate); or 1 cup chocolate soymilk, 1 cup of spinach, two slices of calcium-fortified white wheat bread, ¼ cup almonds, 1 cup Bok Choy, and ½ cup V-8 juice.

How Swimmers Exercise
Other aspects encourage the healthy foundation and creation of strong bones in young athletes. Exercise is one of them.

Weight-bearing exercise seems to have the most impact on the development of bone structure, density and strength.

So what does that mean for a swimmer? Especially when the primary exercise in which he participates is non-weight-bearing?

I looked to a 2016 meta-analysis for insight. In this analysis, the effect of swimming on bone mineral density (BMD) was evaluated. Child and teen swimmers were compared to non-athletic sedentary peers, and were also compared to young athletes competing in high bone-building sports such as gymnastics. 

Researchers found that swimmers had similar bone mineral density as the non-athlete sedentary group, and lower bone mineral density than the athlete peers participating in high bone-building sports. Furthermore, as the swimmers aged, the difference in bone mineral density grew with significantly lowered BMD over time compared to their athlete peers playing bone-building sports. 

The authors concluded that swimming had a neutral effect on bone mineral density. In other words, it was not an effective sport for improving bone mineral density. 

They suggested young swimmers add other bone-building exercises to their workout routine, such as running, jumping, brisk stair climbing and speed walking. This appears to be particularly important for the younger swimmer who may not be cross-training outside of the pool. Weight training also appears to help, and for younger swimmers, using one’s own body weight is effective and considered safe.

Jill Castle, MS, RDN is a registered dietitian, childhood nutritionist, and youth sports nutrition expert. She is the author of Eat Like a Champion: Performance Nutrition for Your Young Athlete. Learn more about Jill and check out her free list of 70 Awesome Pre-Workout Snacks for Kids here.