After Luffy ate food he instantly reverted to Gear 5 which again doesn’t get weaker as Luffy gets more hurt or drained. Luffys just at 100 or 0
The marines saw luffy eating and he was on the ground and he was not in G5 and he look normal .
We also see luffy go in and out of G5 and look normal like when he went into it to hit topman and came out and he was normal looking .
What happen with Kizaru was stamina issues not a damage one .
Luffy has always been one of the most durable character in the series G5 just added to that .