This has become an issue lately.
Clients of mine (and of other trainers I speak to) who want to lose weight have been acting like it's a good idea to not eat before they work out, thinking it's a good way to lose weight.
This creates many, many problems.
First of all, under great stress, your body will not burn fat. Instead, it will hold onto that fat.
Secondly, cutting calories too drastically will cause your body to think it's being starved. Your body will slow down your metabolism to a crawl to protect you against the famine. (And that's before your workout.)
More importantly, without having the nutrients in your system, you will not have the strength or stamina to make it through your workout.
Now let's take a look at how that affects your workout. Without the proper energy in your muscles, your heart rate will increase to a dangerous level, your blood pressure will rise substantially, your body temperature will rise to the point where you feel clammy. Your blood sugar will drop and you will feel dizzy, possibly even pass out.
If you faint with weights in your hands, think about where they might land — on you or some innocent passer-by. Imagine losing consciousness while sitting on a stability ball or standing on a BOSU ball. Or while jogging on a treadmill.
You get the picture.
Ideally, the largest meal of your day should be two hours before you work out. If you can't manage that, at least make sure to eat something substantial two hours before your workout.
Include carbs in this meal or snack: carbohydrates, when they break down, turn to glycogen — the fuel source for your muscles. (Remember, carbs include fruits and vegetables.)
So, make sure your vehicle is properly fueled before your workout.