I start my day by rolling out of bed and drinking a tall glass of orange juice before I go to my home gym. I then do about 30 minutes or so of moderate aerobic exercise, followed by yoga stretches, and then strength training on alternate mornings (dumbbells, push-ups, and pull-ups). [Here is video of my exercise routine.]
Only after I have finished my workout, do I sit down for a hearty breakfast. I found this regimen is perfect for outwitting my brain’s infinite capacity to find excuses to skip exercising, “just today”. It also gives me a sense of well-being that lasts the whole day. And, I also found out something that is a subject of debate among exercisers and sport physiologists: I could easily lose excess weight when my bathroom scale warned me that the previous night’s dinner was too indulgent.
This regimen has worked great for me over the 15 years that I have been following it. I have never gained, or lost, more than 2-3 pounds from my preferred weight of 145 lbs. I know, this is an N of 1. But, trust me, it is based on solid science.
How do we burn calories with exercise?
The two most important sources of energy used by muscles when exercising are carbohydrates and fats. Protein can also be a source of energy for exercise, but its contribution is minor (about 6%), and it is used only if the carbs and fats are getting depleted or when the exercise is extreme.
Now, here is a key point. When we get up in the morning, we are basically in a fasting state because we haven’t eaten all night—muscle glycogen and blood sugar levels are low. Insulin levels are low as well. During the day, as we feed, we elevate our blood glucose level, insulin levels rise, and muscle glycogen stores are replenished. Further, insulin works as a brake on fat mobilization from adipose tissue and subsequent breakdown into fatty acids. That means, during the fed state, muscles preferentially use glucose for energy. As a consequence, glycogen can supply energy for as long as two hours before the muscle has to resort to fatty acids for energy supply.
So what happens if we exercise before we eat breakfast? With glycogen stores depleted overnight and fat mobilization facilitated by the low insulin levels, muscles are forced to utilize fatty acids as a source of energy.
Well, you can see where I am going. If you want to get rid of more fat, it makes sense to exercise before you replenish your muscle glycogen stores by eating.
But wait, there is more.
You don’t have to kill yourself
Trained athletes are a class by themselves. Their exercise regimen is high-intensity; most of the rest of us exercise moderately or lightly.
Exercise intensity can be expressed as a percentage of VO2 max (maximum oxygen consumption). Low intensity such as fast walking would be 30-50% of VO2 max. Jogging can demand 50-80% of VO2 max depending on the intensity, and sprints can require from 85-150% of VO2 max (with the added 50% coming from short-term anaerobic energy production).
This difference in intensity has profound metabolic consequences. At low to moderate intensity, muscles prefer fatty acids over glucose as a source of fuel. Why? Because at lower VO2 max, there is less oxygen available for oxidation. It so happens that a molecule of glucose requires more oxygen for its complete oxidation than a molecule of triglyceride (fat). Add to that the fact that gram of fat provides 9 calories of energy and a gram of glucose only 4 calories. You can see then why, in order to maintain the low to moderate level of exercise, it would be more efficient to break down fat.
So if you exercise before breakfast, you are getting a twofer: low levels of insulin, low blood sugar, and facilitated mobilization of fat. How can you pass up such a deal?
Makes a lot of sense. But as we know in biology and medicine, what makes sense is not necessarily true. We need an experiment.
Finally, a real experiment
Gretchen Reynolds reported in the NYT on a 2010 study, in which researchers in Belgium persuaded young, healthy men to stuff themselves for six weeks with a diet consisting of 30% more calories and 50% more fat than the men had been eating previously. Some of the volunteers remained sedentary while gorging. Others began a strenuous, mid-morning exercise routine after they had eaten breakfast. The third group followed the same workout regimen, but before they had eaten anything.
At the end of the six weeks, the sedentary group predictably was supersized and unhealthy, having gained about six pounds each. They had also developed insulin resistance and larded their muscles with new fat cells. The men who exercised after breakfast had also packed on pounds, about three pounds each, and developed insulin problems. But the men who had exercised first thing in the morning, before eating anything, had gained almost no weight and retained healthy insulin levels. Their bodies were also burning more fat throughout the day than were the other men.
Of course, the early-morning exercise prevented weight gain, which is not the same thing as inducing weight loss. Obviously, we would have been happier if the experiment was designed to demonstrate weight loss rather than prevention of gain, but the hormonal and metabolic mechanisms at play here are the same: low insulin, low blood sugar, and preferential utilization of fat as a source of fuel.
So whether you just like to walk, or jog, or swim, or even lift weights—do it first thing in the morning. You’ll get a lot more out of it. I guarantee it!