- The former NFL quarterback Alex Smith said he used intermittent fasting during most of his career.
- But he said he had to give up intermittent fasting to heal a leg injury.
- A dietitian said fasting could lead to undereating, which could cause the body to take nutrients from muscles for nourishment.
Alex Smith said intermittent fasting helped his performance during his NFL career, but he said he had to stop doing it after breaking the tibia and fibula in his right leg in 2018.
The 37-year-old former San Francisco 49ers, Kansas City Chiefs, and Washington quarterback told Insider he started using intermittent fasting when he joined the Chiefs in 2013. He said he adopted it because he believed fasting would help him make mental decisions quicker and improve his reaction time, as well as help his brain resist the damage from hits on the field.
A small 2016 study that tested amateur weight lifters after a 48-hour fast found that those who fasted had improved "mental flexibility," which the researchers defined as participants' ability to quickly and efficiently switch between tasks, Insider previously reported.
Evidence that intermittent fasting improves cognitive health and function in humans is limited, and most studies supporting its benefits are based on animal trials, according to WebMD.
Smith said he felt a noticeable difference in his mental quickness after he stopped intermittent fasting. He added that his doctors said it was necessary for him to stop the meal scheduling so he could recover from his injuries and avoid amputation.
Smith said he quit fasting because he was told it would hinder his healing process 
Smith's doctors told him they considered amputating his right leg because the injury caused him to develop a deadly necrotizing-fasciitis infection in the leg, ESPN reported.
But doctors were able to save Smith and avoid amputation by performing multiple surgeries in the days following the injury. Then, Smith began his long road to recovery, which required multiple lifestyle changes, including eating more meals throughout the day.
Smith said he was instructed to be as anabolic as possible, which meant following a meal plan that would help nourish him enough to rebuild the damaged muscles in his leg. Smith said the canned protein shake UCan was what he consumed most during recovery, which helped him recover and rebuild strength in his leg.
"I was all of a sudden trying to eat my weight in grams of protein every day, but I was trying to do it with five or six bowls throughout the day," Smith said. "I completely flipped. Fasting was not what I wanted to be doing."
He added: "It was hard when I was hurt. Eating became cumbersome, and I didn't always have an appetite."
Alix Turoff, a registered dietitian, told Insider intermittent fasting could restrict someone from being able to get enough calories, and undereating could leave someone more susceptible to injury. This is because when the body is not fed what it needs, it will take from other muscles for nourishment, according to WebMD.
Smith returned to football in 2020 and won the Associated Press NFL Comeback Player of the Year Award after leading Washington back to the playoffs. He said he hadn't gone back to intermittent fasting after making his recovery or retiring following the 2020 season because he was still in the process of rebuilding the strength in his leg, which he said would be a lifelong process.