If you're looking to burn the most fat, try exercising first thing in the morning without eating.
In grad school I did a lot of research on exercise and energy metabolism, and learned that for the most fat burn, it is best to exercise on an empty stomach. Yet another recent study out of the UK confirms this school of thought (2). This study followed 12 physically active men and found that the men burned 20% more calories from fat in a fasted state compared to when they ate breakfast. Furthermore, they did not overcompensate by eating more calories later in the day nor had an increased appetite compared to when they ate prior to exercise. One of the researchers was quoted as saying: "Our results show that exercise does not increase your appetite, hunger or food consumption later in the day and to get the most out of your session it may be optimal to perform this after an overnight fast” (1).
My recommendations: I am hesitant to recommend exercising without eating to people who have never tried for fear they might faint. If weight loss and fat burning is your main motivation for exercising, you may want to try working out first thing in the morning. However, I advise you to bring a glucose-containing drink or piece of fruit with you in case you start to feel light headed. Some people may also find that they have no energy when working out hungry which could negate the benefit of increased fat burning. If you have less energy, your workout will be weaker, so you will end up burning fewer calories than if you had eaten (and therefore, fewer calories from fat).
Furthermore, I never recommend exercising without eating first for diabetics or those with blood-sugar issues. For those athletes who are most concerned about getting a good workout in- as opposed to just burning calories and fat- always eat or consume something with glucose before a workout to get the most out of it. Personally, I would rather have enough energy and a good workout than be feeling hungry and tired when I'm exercising. If I'm exercising early in the morning, I am always sure to eat at least a piece of fruit like a banana beforehand (see post below on the benefits of bananas!), but usually also coffee made with a protein-containing milk (soy or cow) and a hearty meal containing carbs and protein afterwards.
Product Shout out!: This past weekend I gave out CLICK espresso protein drink samples at our open house. For most of my clients, I do not advocate the need for protein shakes and would rather have them make their own. However, this stuff is delicious and is a quick and convenient way to get in caffeine, some carbs, and protein before an early-morning workout when time to prepare food is limited. So, if you are looking for a good pre-workout protein drink, I recommend this product. I tried the vanilla latte flavor before I did kettlebell and spinning on two different days and had an amazing workouts both times- tons of energy and felt great! Another benefit is CLICK is a milk-based protein (casein) so it can actually be heated and not chunk up like whey protein based supplements.
Note: I do not receive any monetary compensation from CLICK, and this is an actual recommendation based on my experience with the product.
2. Javier T. Gonzalez, Rachel C. Veasey, Penny L. S. Rumbold, Emma J. Stevenson. Breakfast and exercise contingently affect postprandial metabolism and energy balance in physically active males. British Journal of Nutrition, 2013