The more glycogen you have in your body, the less cortisol your system releases when you work out, says Dick. So it follows that by depriving yourself of carbs, you’re also depleting your muscles’ glycogen stores, which can cause chronically high cortisol levels.
As a result, you may have increased risks of depression, muscle soreness, insomnia, as well as reduced sex drive, concentration, and immunity, Dick says. Adds Lauren DeLuca, Tier X coach and certified sports nutrition specialist at Gold Coast in Chicago: In women, high cortisol levels can contribute to irregular cycles or missed periods since the spike can impair the pituitary glands that keep hormones in balance.
As mentioned, restricting carbs forces you to use fat for fuel, DeLuca says. When you’re sidelined, the sudden change can delay recovery as your body tries to adapt. That makes it especially important to stick to 40 percent when you’re injured.
Until you've recovered, dial back your total caloric intake slightly while still eating healthy carbs from bananas, pineapple, mixed berries, broccoli, carrots, and whole grains (brown rice, quinoa) to help your body repair itself, DeLuca says.
For every gram of carbs you eat, three grams of water get drawn into your cells, Dick says. (Fat and protein, on the other hand, don’t make you retain water.) That H2O transfer hydrates the muscles and tissues so you can get through intense strength and cardio routines. When you go low-carb, the resulting dehydration can make you feel tired, keep you from training your hardest, and lead to muscle cramps or GI issues, he notes.
Your body needs glucose to release the gonadotropin-releasing hormone (GnRH), a precursor to testosterone, DeLuca says. “It’s an anabolic hormone, meaning it increases muscle and bone strength and size,” she says. When carb intake (and thus, glucose levels) are low, testosterone levels drop, too. In turn, you could lose strength and fatigue more quickly—just one more reason carbs are essential for fitness.