Gulping a glass of OJ might seem like a good idea before your morning sweat session, but you'll question your decision halfway through spin class, when climbing that hill seems impossible. What gives? You are better off eating the orange instead, Batayneh says. Juice (even 100-percent fresh-squeezed) has a lot of sugar, and no fiber. While it still offers some vitamins, it will go right through your system, causing a blood sugar crash later.