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‘Non-Ketogenic’ Low-Carb is Still Too Low

So at higher levels of VO2max, zero-carb diets clearly destroy physical performance in both untrained and trained subjects. But what about less-restrictive carbohydrate-reduced diets – what effect do they have in normal-weight individuals undergoing strenuous exercise?

A study answering this very question was conducted by Helge and colleagues at the University of Copenhagen. During the first seven weeks of this study, one group ate a high-carb diet, the other a low-carb/high fat diet. During the eighth week, both groups ate the high-carb diet. The subjects trained three times a week during the first 4 weeks, then 4 times a week during the last 4 weeks on stationary cycles, at a controlled exercise intensity that ranged between 60 and 85 percent. The workouts lasted 60-75 minutes.

During the first seven weeks, the low-carb/high fat group ate 177 grams of carbs and 217 grams of fat per day. The high-carb group ate 546 grams of carbs and 75 grams of fat per day. Both groups ate similar amounts of protein. While 177 grams of carbs may not typically be considered ‘low-carb’, keep in mind that exercise increases the carbohydrate threshold required to reach ketosis. Indeed, blood ketones rose markedly during the study in the low-carbohydrate group, and were far higher at seven weeks than in the high-carb group.

When time to exhaustion at 81% VO2max was tested at 7 weeks, the high-carb group improved their time by 191 percent, compared to only 68 percent in the low-carb group – a hefty 280 percent difference! After the eighth week, when both groups ate the high-carb diet, the low-carb group managed to improve their time by another 18 percent, but their average time to exhaustion was still much shorter than in the high-carbers[7].

The bottom line is that both zero-carb and low-carb diets are a disaster for those engaged in regular strenuous exercise. And for anyone with a sound knowledge of the biochemistry of energy production, this is no big surprise.