Should You Train on an Empty Stomach?
I often get asked whether it would be better to eat your first meal before a morning workout or wait until afterwards. The answer to this really depends on the overall goal you hope to achieve.
If you are primarily doing strength training, I absolutely would recommend eating something with protein and carbohydrates, or at least taking an amino acid supplement with a low-fat granola bar (or something similar). Because you will be breaking down muscle tissue as you lift, you want to have some amino acids available from the get-go to promote muscle repair as soon as possible.
Adding in some carbohydrates will raise your blood sugar level (which will likely be low after sleeping all night), giving you more energy and hopefully making your workout feel a little easier.
Empty Stomach Training and Weight Loss
If you are planning to do cardiovascular activity, the answer varies. If your main purpose of doing aerobic activity is simply to lose weight and burn off calories, an empty stomach is the best way to go. This will force your body to use fat for fuel during the workout since no carbohydrates are available from food and your stored carbohydrates in the liver will be low (because these stores support normal body functions while you sleep).
However, if your cardio endeavors are performance-based, such as a running or cycling race, you MUST eat a source of carbohydrates beforehand. Without immediately available carbohydrates, your brain will tell your heart, lungs, and muscles to slow down and conserve energy (because your brain is greedy and wants to make sure it has enough glucose at the expense of other organs).
This is a protective mechanism of our body that kicks in long before we would ever truly run out of a fuel source.
So unless fat burning is your only goal, try to at least drink a sports beverage, eat ½ a cup of fruit or low-fat yogurt, or grab a handful of jellybeans (really!), even if your stomach is queasy in the morning or if you aren’t hungry when you wake up. Even just 15 grams or so of carbohydrates is enough to boost blood sugar and thus alertness, energy, and overall performance.