Man running on treadmill 1200 x 800

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!

Dov Michaeli, MD, PhD
Dov Michaeli, MD, PhD loves to write about the brain and human behavior as well as translate complicated basic science concepts into entertainment for the rest of us. He was a professor at the University of California San Francisco before leaving to enter the world of biotech. He served as the Chief Medical Officer of biotech companies, including Aphton Corporation. He also founded and served as the CEO of Madah Medica, an early stage biotech company developing products to improve post-surgical pain control. He is now retired and enjoys working out, following the stock market, travelling the world, and, of course, writing for TDWI.


  1. “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.”

    You have the information in the paragraph above backwards. It should read: During exercise at HIGHER percentages of VO2 max there is less oxygen available for fat metabolism and the energy source shifts towards blood sugar and glycogen instead of fat. Breaking down fat completely requires MORE oxygen than breaking down glucose. Above VO2 max the fat metabolism is incomplete, leading to lactic acid accumulation and an ‘oxygen debt’ At low levels of intensity fat comprises a higher percentage of the calories burned, decreasing as the intensity increases. As long as the subject consumes less calories than are expended the key to weight reduction is the difference between the total energy expended and the total energy absorbed over time.

  2. New research suggests that exercising in the morning raises blood pressure due to the natural body clock. What would your take on this be?

  3. I am 50 year old woman. But people think that I am rond 30. It is due to the Mongolian soup and tibetian exercise. The most people in Tibet and Mongolian have less wrinkles than western people. It is related to their eating patron, according to me. My dutch friend is now 65 years old. He was the most healthy person at his age rond others. People asked the reason. He answered that he was eating last 10 years only mongolian soup. He never have sugar problem. Actually he consumes also sugar. But he says the sugar level is stabilized by the mongolian soup.

  4. I have my highest blood sugar count first thing in the morning. I am a Type 2 Diabetic and I manage this through diet and exercise as I don’t take any medications.

    I don’t understand your statement on low Blood Sugar in the morning, as mine is high.

  5. Working out in the evenings always wound me up and made sleep more difficult. So for me morning workouts are extremely beneficial. I experience greater energy, better mood, and improved mental sharpness. I always just had water with lemon to hydrate before because anything else would make me feel nauseous. It is easier for me to eat a healthy breakfast after and feel happy that the biggest challenge of the day is behind you. I would also agree that you need to ease into things and be gentle with yourself. Even moderate exercise can yield excellent benefits. The biggest hurdle is just getting up. After that it is easy. If you can find a workout buddy who doesn’t think you’re crazy that helps.

  6. I do a 6 km power walk every alternate morning, walking very fast/strenuously (time is under 60 min, including one long hill). Sometimes I just drink a medium glass of water, or sometimes I drink a very small glass of juice and snack on a few bites of some leftovers half an hour before heading out. Either way I have noticed no difference in my time results in my or stamina, but on the snack days I sometimes feel emotionally tired during the last kilometre! (and on the snack mornings I sometime get a mild “stitch” during the downhill) . Either way my legs do hurt later in the day, and the next day legs and hips are fairly stiff. Wondering if I should be having more days off in between? I am female, age 53, 5ft2, 130 lbs.

  7. If you have a glass of orange juice you do bave breakfast. There is the same amount of carbs in it as is in for example a sandwich or a banana, and it is just enough to refill your glycogeen storage. It is not about fasting and excercise. I think the reason you dont gain nor loose weight is simple: your in- and output is just in balance!

  8. I will actually struggle to do exercise after breakfast, I need to eat at least breakfast, lunch and preferably also my second lunch before I will do my 1 hour and 20 minutes of running. But this is strenuous exercise, I will burn about 1200 Kcal. I eat about 4000 Kcal per day and I weigh about 53 kg. After exercise I will eat a big 1700 Kcal dinner and a few hours later another meal to make sure I can sleep for 8 hours without feeling hungry. But after I wake up I really must eat first, there is no way I could do my daily running exercise without eating.

  9. I currently ride a bicycle for weight loss and its many other benefits, and so far a lot of riders fuel up an hour or two before riding. In my case I like to eat 1 cup of oatmeal with honey, Karbolyn which they claim pulls more nutrients into the muscle and lasts up to 2 hours per serving which in my case is perfect. My concern is bonking with your approach. Do you have any advice to help me get the most out of cycling and the weight burning process?

  10. Great article which encourage me to go for my spinning class first thing in the morning. Indeed I really apreciate it. Is it right to eat 20 almonds while spinning?

    • Twenty, or more, almonds while spinning is OK. Just be careful not to get some almond fragments in your windpipe while breathing hard. You may want to wash it down with some water.

  11. I am an exercise physiologist, and although I agree with exercising before breakfast once in a while if you had a very heavy dinner, I really don’t recommend this for most people. It all has to do with what your goals are. If you want to lose weight, yes, go for it unless you start feeling dizzy. But if you’re trying to gain muscle mass, not just tone, but thicker, more muscular legs, arms, butt… This is really not the way to go. Particularly not for a woman. Men are able to build muscle much more easily than women because of their higher levels of testosterone. But if you are a girl, and like me, you like a stronger, curvier look, this exercise regime is not for you. I prefer a light breakfast with some fruit, yoghurt, nuts and some grains along with a nice cup of coffee (helps mobilize fats and increases metabolism) about 1hr before working out (weight-lifting and light cardio as a warm up). After this I like some lean protein to help my muscles get stronger :)

    • Thanks for you thoughtful comment.
      I can’t argue with you exercise regimen, especially if your main goal is to build up muscle tone vs. weight loss. Having said that, the common wisdom is that to build up muscle mass you need to increase the weights you lift, and to increase tone you increase the number of reps of lifting lighter weights.

  12. Nice article, noticed one small mistake:
    Glucose (C6H12O6) requires less oxygen (not more) for its complete oxidation than a molecule of triglyceride (at least 40 CH2), because glucose contains already more oxygen than fat. That is also the reason why a gram of fat provides more energy than a gram of glucose. Oxygen is not limiting at low intensity (= low VO2 max).

    • This is right, as far as it goes. But you didn’t go far enough. A molecule of glucose indeed requires less oxygen than than a molecule of fat. But, glucose has only 6 carbons, and fats have a much higher number. for instance, stearic acid has 18, and higher fats have even more. So, per carbon, glucose requires more, not less, oxygen for a complete burn. Here is another aspect of the same thing: the respiratory quotient of glucose is 1.0, and of fat only 0.66.

  13. Doc, please answer this. How about us people who work on the night time? When is the right time in a day to do exercise because i usually hit the gym in the evening before my shift starts. I appreciate the answers and i love this article you’ve created.

    • Unfortunately I don’t have a very optimistic answer to your query. Night shift work ‘screws up’ the circadian rhythm and interferes with a variety of metabolic processes. Consequently, I don’t know if it would be better to exercise after you get off work (morning) or after you wake up (before dinner). One thing I do know: regardless when you exercise, it will do you good.

  14. Just be aware if you have back issues (herniated or protruding discs), your discs contain the most hydration upon waking. If you do exercise incorrectly or with poor form you risk creating greater issues, as the fuller discs can extrude into the nerve creating inflammation or risk of greater herniation. Avoid spinal flexion under load (sit-ups, bending over to put on shoes, etc), and compression activities like running if you suffer these problems. Also recent studies (Harvard) show anaerobic activity is superior to aerobic for reducing waistline (area of visceral fat, best measurement of fat loss or gain).

    • I do suffer from bilateral herniations of the discs, but I deal with it by stretching before starting my elliptical exercise. I do a cat/cow type stretch, and forward bends (very carefully and slowly, especially coming out of it). I also do twists, again, very carefully.
      You are right about anaerobic activity being more effective that aerobic for weight loss. It also makes perfect metabolic sense: anaerobic oxidation of one mole of glucose yields only 6 moles of ATP, whereas aerobic oxidation yields 36-38 moles of ATP. Therefore, to get the amount of energy (in the form of ATP) that intense exercise requires you burn a lot more glucose, about 6 times as much as in aerobic exercise. This leads to very rapid shift to utilization of fat as a source of energy. But bear in mind: this form of exercise is intense, and most people aren’t up to it, especially if they are untrained, and if they suffer from cardiac or coronary disease -it may even be catastrophic.

    • You are in the enviable position of not having to worry about weight loss. You can exercise any time you choose, just make sure to replenish your body reserves with plenty of food. Eat well, and in your case, a lot.

    • Unless you deplete your glucose, glycogen and fat stores -requiring super long and intense exercise- you don’t have to worry about muscle protein breakdown. The muscle pain you feel is not due to catabolism of muscle protein but due to micro injuries. And these are good for you; the muscle reaction of repair is what causes muscle mass buildup.

    • I too have a glass of orange juice before exercise, to avoid hypoglycemia. The amount of sugar in the juice is quite small and is readily available to the muscle.By the the time it is consumed muscle glycogen starts mobilizing, replenishing the consumed glucose. Overall, blood glucose levels do drop during exercise, but normally not to dangerous hypoglycemic levels, assuming you insulin functions normally.

      • As a doctor you should research the damage sugar does to your body.

        OJ has more sugar that soda and is not a natural drink unless you squeeze your own and eat the pulp.

        Please read Dr. Perlmutter’s books and then come back here as it is not healthy in any way to drink sugar laden liquid. Eat protein. Please read the book. What you used to think was right has been proven WRONG.

    • You have a Masters yet , you didn’t even read the first sentance of the article ?
      Yes , he precedes his workout pretty breakfast with , yep , juice .
      Me personally, I give the literature a read before my opinion. ♡


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