Does adding sugar to caffeinated drinks affect your respiration or heart rate? The answer may depend on your personal physiology.

New Zealand researchers found wide variations in how individuals responded to sugar, caffeine, or a combination of the two. Caffeine alone slowed the heart rate of some participants and did not affect others. Sugar alone either increased the heart rate or did not change it. And caffeine with sugar decreased some participants’ heart rates, increased others, and had no effect on some. After all three treatments, carbon dioxide production (a measure of respiration) increased in some participants and remained the same in others.

The researchers noted that further study is needed.