Amazingly, six-time Ironman winner Dave Scott’s best competitive years coincided with his worst nutritionally. At age 64, Scott certainly looks like a guy who takes care of himself. Cruising the Ironman expo in Kona this past October, golden-skinned and lean, wearing a sponsor-emblazoned black polo shirt, he looked ready to win something, if not the Ironman itself, perhaps the award for best-preserved IM winner in the race’s 40-year history.

Yet there were moments on and off the race course during his winning days that told a different story. For instance, was Scott as tired as he looked sometimes at the finish line–not tired in a sense of being fatigued, but actually tired–as in can’t-keep-your-eyes-open sleepy? That seemed impossible–the guy was a shark in competition. Sleepy?

In the early 1980s—a time when Scott won almost every Hawaii Ironman—he came under the influence of low-fat diet guru Nathan Pritikin, who was “enamored” with Scott for performing at such a high level on a diet consisting of about 10-percent total fat. It was Pritikin who suggested that Scott rinse his 2-percent milkfat cottage cheese to reduce the fat content to a level with the 10-percent baseline.

At the time, there was general skepticism within the medical community about the efficacy of a low-fat, high-carb diet for endurance athletes. The concern was not so much that it wouldn’t work (few thought it would), but rather that it might be harmful in the long run, negatively impacting various organs and systems, including cardiovascular function. For Dave Scott, the nearly 10-year experiment was a daily struggle to balance the mind and drive of a fierce competitor against the harsh constraints of flawed dietary theory.

“It was a crazy, crazy period,” Scott says. “I had massive fluctuations in my psychological state. I had massive fluctuations in performance. Some days I was so lethargic I could barely move. I didn’t recognize how I was poisoning myself with the volume of carbohydrates I was eating.”

It wasn’t until the tail end of his career that Scott began to shift the balance of fat versus carbs in his diet. As of three or so years ago, he flipped the model on its head. He is an advocate now of a ketogenic diet, emphasizing the intake of healthy fats over carb levels as low as 5 percent of total calories.

“A low-fat diet is not sustainable, and is very, very, very unhealthy,” Scott believes. He admits that the dire predictions of long-term harm from his radical approach to diet in the 1980s may have been accurate after all. He’s had heart issues of late that have reduced his workout capacity by 30 percent. “It feels like my heart has a governor on it,” he says.

It’s impossible not to wonder what the great Dave Scott might have achieved had he been eating properly during the height of his career. The “What if” question is unavoidable—he laughed when the topic came up: “That’s the irony, isn’t it?”