By Dr. Mercola
It's long been said that you should avoid eating shortly before exercise as it can lead to a spike in blood sugar followed by a decline that could harm your performance.
The New York Times recently featured an article busting this fitness dogma as a myth, as newer research shows that eating before a workout doesn't necessarily impact performance.1
The author cited one study, in particular, in which cyclists who drank sugary drinks prior to a workout were able to complete a strenuous 20-minute ride with no problems.2 It was also noted that research has shown eating easily digestible carbohydrates before exercise may enable you to work out longer.
That said, there's actually plenty of research, and reason, that strongly supports skipping eating before exercise… especially if you're interested in maximizing your fat-burning potential.
Why Exercising While Fasting Is Beneficial
If you're already devoting the time to working out, you're probably interested in making the most of that time and getting in the most possible benefit in the shortest amount of time … and one way to boost your return on your exercise "investment" may be to do your workout while fasting.
When you exercise while intermittent fasting, it essentially forces your body to shed fat, as your body's fat burning processes are controlled by your sympathetic nervous system (SNS), and your SNS is activated by exercise and lack of food.
The combination of fasting and exercising maximizes the impact of cellular factors and catalysts (cyclic AMP and AMP Kinases), which force the breakdown of fat and glycogen for energy.
One study found, for example, that fasting before aerobic training leads to reductions in both body weight and body fat, while eating before a workout decreases only body weight.3
Exercising While Fasting May Be a Relative Fountain of Youth
Exercise and fasting together also yields acute oxidative stress, which actually benefits your muscle. According to fitness expert Ori Hofmekler, acute states of oxidative stress are:
" … essential for keeping your muscle machinery tuned. Technically, acute oxidative stress makes your muscle increasingly resilient to oxidative stress; it stimulates glutathione and SOD [superoxide dismutase, the first antioxidant mobilized by your cells for defense] production in your mitochondria along with increased muscular capacity to utilize energy, generate force and resist fatigue.
Hence, exercise and fasting help counteract all the main determinants of muscle aging. But there is something else about exercise and fasting. When combined, they trigger a mechanism that recycles and rejuvenates your brain and muscle tissues."
The mechanism he refers to is triggering genes and growth factors, including brain-derived neurotropic factor (BDNF) and muscle regulatory factors (MRFs), which signal brain stem cells and muscle satellite cells to convert into new neurons and new muscle cells, respectively.
This means that exercise while fasting may actually help to keep your brain, neuro-motors and muscle fibers biologically young. The combined effect of both intermittent fasting and short intense exercise may go way beyond helping you to burn more fat and lose weight; it may help you to:
Turn back the biological clock in your muscle and brain
Boost growth hormone
Improve body composition
Boost cognitive function
You Can Start by Exercising Before Breakfast
Intermittent fasting is not about binge eating followed by starvation, or any other extreme form of dieting. Rather what we're talking about here involves timing your meals to allow for regular periods of fasting. I prefer daily intermittent fasting, but you could also fast a couple of days a week if you prefer, or every other day. There are many different variations.
To be effective, in the case of daily intermittent fasting, the length of your fast should be targeted to 16 to 18 hours. This means eating only between the hours of 11 am and 7 pm, as an example. Essentially, this equates to simply skipping breakfast and making lunch your first meal of the day instead.
You can get many of the same benefits of fasting and exercise by exercising first thing in the morning, when your stomach is empty. This is because eating a full meal, particularly carbohydrates, before your workout will inhibit your sympathetic nervous system and reduce the fat burning effect of your exercise. Instead, eating lots of carbs activates your parasympathetic nervous system, (which promotes energy storage—the complete opposite of what you're aiming for).
As mentioned earlier, training on an empty stomach will effectively force your body to burn fat, while also offering additional benefits. For instance, in one study those who fasted before exercise had increased levels of a certain muscle protein that plays a pivotal role in insulin sensitivity. As I've explained in many articles, insulin resistance is the root cause of most chronic disease, making maintaining proper insulin regulation a primary factor of good health.
If You're Doing Heavy Lifting, Eating After Exercise is Important
The exception to exercising while fasting is if you're doing heavy lifting. In that case, it's important to eat within 30 minutes after your workout, and your meal should include fast-assimilating protein. Whey protein is a useful option here. Furthermore, some people do have a hard time exercising without eating something first, and for these people whey protein can also be a beneficial pre-workout meal.
Typically these people are more sensitive to changes in their blood sugar levels, which can decline during the first 15-25 minutes of their workout. It is this decline in blood sugar that causes dizziness, faintness, nausea or lightheadedness. This is especially true if you exercise first thing in the morning.
Of course, a number of individual factors can also play a role in whether it's appropriate to exercise while fasting, such as your age, when you last ate, whether or not you're pregnant, taking medications, your medical history, level of fitness, and the type of workout you engage in. I have also been playing with having some fruit immediately prior to lifting. You will easily burn it during the workout so it should not impair insulin sensitivity and it also provides you with some more fuel to work out harder. I typically have a serving of whey protein 30 minutes after a strength workout.
Always Use Common Sense When Deciding Whether to Eat Before or After Exercise
I believe the best approach is to use some common sense and listen to your body. If you feel weak or nauseous while exercising on an empty stomach, you may want to eat a small meal, such as a high-quality whey protein shake, as mentioned, before your workout.
A study published in the journal Medicine and Science in Sports & Exercise demonstrated that consuming whey protein (20 grams protein/serving) 30 minutes before resistance training boosts your body's metabolism for as much as 24 hours after your workout.4 It appears as though the amino acids found in high-quality whey protein activate certain cellular mechanisms (mTORC-1), which in turn promote muscle protein synthesis, boost thyroid, and also protect against declining testosterone levels after exercise.
In practical terms, consuming 20 grams of whey protein before exercise and another serving afterward may yield the double benefit of increasing both fat burning and muscle build-up at the same time. You can play with the dose as that is an average. If you are a small woman you would need half the dose and if you are 250 pounds you might need 50-75% more. Again, not everyone will need to eat something prior to exercise, but if you do, a high-quality whey protein is one of your best bets. It'll curb your hunger while still optimizing fat burning.