Appeal to Nature. That something is natural means it is good, or better, or that deviation from it is inherently wrong.

Similar are the Appeal to Novelty (that something is new means it is better) and Appeal to Tradition (that something was done this way before makes it better).

What makes you think that mankind naturally evolved to bypass his endurance limits by hacking his body in such ways as artificially injecting nutrients into it? Juices and sports drinks are made so you can wind out beyond what your body's supposed to handle and then inject pure sugar and salts in an aqueous solution right into the system by dumping them, absorption-ready, into your stomach. No need to digest anything, just suck this straight through.

The natural path would involve simply easing back when you reach your limits. We're talking about people who are either trying to become more comfortable within their limits or trying to bypass their limits artificially. Consumption of fruit is irrelevant here; and as "natural" as it is in comparison to fruit juice or sports drink, your body isn't designed to intake fruit in that manner for that purpose.

In other words, eating fruit is "more natural" than drinking gatorade; and it's also not as good for the purpose.