It’s not the sugar leading to obesity as it causes the release of the hormone leptin and you even burn fat. The problem is in combination with a high fat and high protein intake. If you keep those under 10% of your calories and work out moderately, you won’t be able to stuff enough into your body to gain weight as long as it’s fruit and vegetables.
Protein and calories are also far overrated to build muscles, as Nun Amen-Ra shows, setting a deadlift world record with a body weight of 82.5kg only consuming 1500 calories a day and 0.5 g protein per kg bodyweight.
I did a couple of months Dr. Doug Grahams 80/10/10 diet and was thin like a beanpole,
But on the Amen diet of Nun Amen-Ra I gained a good amount of muscle, despite taking in less calories, which also stimulates human growth hormone(HGH) release.