After a night of sleep, your spinal discs are filled with fluid, which makes them more likely to rupture if they're put under pressure first thing in the morning. Other joints and connective tissues could be more vulnerable in the morning because it takes longer to warm up and get the body's natural protective mechanisms working.
And as for fat loss here are some examples of research showing that fasted workouts really don't burn more fat: One study that examined the claim directly in 1995 found that a group of people did burn more calories from fat on days when they exercised on an empty stomach than on days when they had a small breakfast first. But the researchers found that the difference was negligible, and other studies have shown that fewer calories are burned in the long run because the workouts are shorter.
A study published in 1999 in the journal Medicine and Science in Sports and Exercise had a group of people ride an exercise bike on two mornings: on one day after a small breakfast, and the other after eating nothing. The researchers found that when the subjects ate nothing, they became fatigued faster and stopped exercising about 30 minutes earlier.
Dr. David Prince, an assistant professor of rehabilitation medicine at the Albert Einstein College of Medicine of Yeshiva University in New York, said that when you exercise on an empty tank, your body burns through stored carbohydrates first, then protein, before it finally moves on to fat. In the meantime, he said, "you lower your blood sugar, causing ravenous hunger that in most people would lead them to eat much more than they would otherwise."
Depending on the person's body type, fasted workouts can also burn through muscle mass. Also, people don't necessarily have to have diabetes to get low blood sugar or feel lightheaded and be at risk for fainting and becoming injured.
Eating even a small piece of fruit or a handful of nuts is better than nothing.
Sarah