This is taken from my book, Bread Science. I didn’t know how long it would take me to write up a new post, so I decided just to post this:
Sugar affects the rate of fermentation reactions. A little sugar, up to three percent, speeds up fermentation. The yeast processes the added sugar first, saving the time it would take to break down starch into sugar. With over three percent sugar, however, the fermentation rate no longer increases.  Above six percent, sugar actually decreases the rate. This is because the sugar begins to dehydrate the yeast cells. This effect, called crenation, was described in the “Salt and fermentation” section (copied below).
Salt slows fermentation reactions by dehydrating the yeast and bacteria cells. Without the nutrients they need, these cells cannot perform fermentation like usual.
Basically, water molecules are able to pass in and out of cells, a process called osmosis (top left). When there is salty water outside of the cells, the salt interferes with the movement of the outside water molecules (top right). They pass into the cells more slowly. The inside water molecules are unaffected and pass out of the cell at their usual rate. Thus the net movement of water is out of the cells, an effect called crenation. This results in dehydrated yeast cells (bottom).
 Barham, H. N., Jr. and J. A. Johnson. “The influence of various sugars on dough and bread properties.” Cereal Chemistry 28 (1951) 463-473.