Sugar is generally considered to be a great multiplier of yeast in the system, as it is yeast’s favorite food. Some sugars, I will caveat that, are much more beneficial than others, with refined sugar and high fructose corn syrup supposedly the worst for you.
The Atkins diet consists of not consuming sugars or breads, and one person I know who actually does in fact have a thyroid problem goes on this plan to lose weight effectively. The breads contain yeast that will feed on the sugars and increase your body fat.
Other than that, sugar is burned by muscles when they contract, as a fuel. It is not used by them to add mass. Several articles I have read say that protein powder does not do as much for you as a simple glass of low fat milk, even chocolate milk, after a workout, because of the way the body assimilates the protein. I have known guys who went with creatine powder to help build muscle mass while working out, and it did well for them; however, when they stopped taking it and working out, the extra mass gained was lost very quickly, and within 3 months or so they were back to normal. Heavy weight lifters in the magazines work out several times a day, take massive amounts of supplements, and have very strict diets, just to maintain their mass.
Each individual has a certain body type that he or she is genetically engineered through their DNA to fit into. You can be stronger or weaker, fatter or skinnier than this type through your lifestyle, but each alteration of your original type must be maintained for a sustained period of time through that lifestyle to permanently change you. My grandfather was limber as hell and strong as an ox at 70 years old, because of the way he lived his life. His wife, my grandmother, was fat and sat around knitting all day, just like she did most of her life. He died 15 years ago, she is still hanging on.
The point being that you can try to change who you are, physically, but keep in mind the continued commitment you will have to make to maintain it, and sometimes it just doesn’t matter, if you happen to have a thyroid problem. I have another friend that works out for a minimum of two hours a day, running three hours straight sometimes, and he just looks really fit, not all that muscular or anything. He doesn’t use supplements, just has paranoia about getting fat.
NO, sugars will not slow down muscle gain, they really have nothing to do with it, unless you count the energy needed to move them around while they are straining at something. They will, however, increase your body fat content.
Muscles strain, they rip and tear, then heal back. When they heal back, with copious amounts of protein and other nutrients, they heal back stronger than they would have without the added bonuses, but sugar is not a building block for muscle tissue. Protein is.