I do just fine on keto. No lack of energy at all in fact I have way more. I dont do "no carb" but some people do and they are fine. Humans are omnivores, which means we can derive energy from almost anything. Your body will actually turn protien into glucose in the absence of carbs, so they are just not necessary. Fruit is highly overrated in the nutrition world, and fructose is probably the worst sugar there is. You can derive most of those same nutrients from a vegetable, or even some animal fats. There are indigenous people in the harshest cold climates, where nothing can grow, they dont eat a single vegetable or fruit and yet survive. I hate this pervasive idea that the only feasible source of energy there is is carbs, and that we will die if we dont get our 5 a day. There are essential protiens, essential fatty acids, but no essential carbs in human biology. You store energy as fat, and that says to me that it is what your body prefers to run off of.