Repeated exposure to certain things cause an allergic reaction. Acne, fatigue, headaches, allergies come in many forms. I can't say for sure it was the sugar. All I can tell you is, I gave up sugar. (And other things, keep reading.) You come up with an educated guess and test it. In the case of the food allergist, he generally knew what foods caused acne problems for other people. Basically, you're using the process of elimination combined with an educated guess. If you completely eliminate sugar from your diet and you still have acne--then you know sugar isn't the problem. (I went to a dermatologist first, tried a number of prescriptions--no effect.) The food allergist worked. The food allergist explained, it's not the sugar by itself. The sugar is one component of a reaction, creating conditions for the acne. At the same time as giving up sugar, I also gave up dairy and yeast/bread. Maybe extreme but it worked. I went about two years without dairy! No ice cream, no cheese! Lots of salads and oatmeal LOL.

It doesn't, and they're not. But it's possible he lowered his intake of things like soybean oil by not eating sugar.

I break out in pimples any time I eat sugary, carb-rich foods. A few weeks without these foods, and I have no issues at all.