Concern is growing in the world of high school sports about nutritional supplement companies’ increasingly aggressive marketing of their products to teen athletes. Among the tactics: One company sponsors a national high school sports association and another a 16-year-old swimmer.

Sports-nutrition supplements, sold online or over the counter and including everything from protein shakes to capsules promising bigger muscles, represent a growing market. They have $6.7 billion in annual sales in the U.S., says Chris Schmidt, senior consumer health analyst for market-research firm Euromonitor. Many athletes, including Olympians, take nutritional supplements to help them fuel or recover from grueling workouts.

Some doctors, school districts and athletic organizations have concerns about marketing nutritional supplements to young athletes because there is little scientific research on how supplements affect growing bodies. Supplement makers don't need approval from the Food and Drug Administration.

The International Olympic Committee published a statement in the British Journal of Sports Medicine in 2015 saying it is “inappropriate and unacceptable to encourage dietary supplements for performance enhancement with youth athletes.” A spokesman for USA Swimming, the sport’s national governing body, says the organization “does not support nor endorse the use of supplements, particularly for athletes under 18 years old.”

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