Not consuming food throughout the day can negatively effect your energy levels and your overall capacity to perform certain tasks. Plus, you'll be cranky. Whether or not it's dangerous, however, depends on how long your fast is.
There's actually a lot of research that supports skipping eating before exercise to maximize your fat-burning potential. This is called "fasted cardio" and is usually done first thing in the morning before you've had anything to eat. It is thought to be so beneficial for fat burning because the glycogen stores are depleted from fasting while you sleep and the body is forced to burn fat as energy. When you are done, eat a breakfast full of protein to rebuild your muscle and carbs to replenish your fuel levels so you aren't slugging through the rest of your day.
There's a big difference between fasted workouts, however, and working out while you are on a fast.
For any adult who is completely healthy, fasting for a period of 24 to 72 hours may not have much of an effect on muscle contractions. A 1987 study showed that during a 72-hour fast, a healthy adult can continue to perform high-intensity exercise for brief periods of time (30 minutes or less). Moderate intensity exercises can be performed for a longer duration (30-45 minutes). However, if your fast restricts water, you should just rest, as exercising while dehydrated is dangerous.
When you're fasting for longer periods of time, over 72 hours, exercise should be avoided. You do not have enough fuel in your body to power through the workout. Even if you feel like you're up for it, any positive benefit from your workout will be severely diminished. You're body is in starvation mode, which means all your body systems have been slowed down, so you won't burn as many calories as you would if you were properly fueled and any results from strength training you do will be greatly reduced, as you aren't consuming the protein necessary afterward to rebuild that muscle tissue.
So if you're under 72 hours and feel good, go for it. You know your body better than anyone. But if you are on a fast, don't feel like you need to force yourself to workout. Rest, relax, and then hit it hard when you have a full belly again.
Kelly Turner is a fitness writer and contributor, personal trainer and social media and marketing consultant. If she's not in the gym or behind her computer, she's lost, so please call the police. Contact her at firstname.lastname@example.org or follow her on Twitter @KellyTurnerFit.