My experience over the years observing folks here is that people who like low carb because it controls their blood sugar swings have more energy doing endurance sports while low carbing/high fatting. Those, like me, who have very good insulin response and normal blood sugars, feel like heck. My husband who has hypoglycemia and whose fasting blood sugars run too can run and do sports on zero carb - if he ate carbs, he would just crash into a stupor.