Interesting article...when to work out, HIIT, fat burn, etc.



A.M. Fat Burn

By Tom Venuto

Even though morning cardio has been embraced by bodybuilders as a "tried and true" fat loss technique, there is definitely not a unanimous agreement about its effectiveness, especially in the scientific community. Most competitive bodybuilders are die-hard advocates of doing cardio first thing in the morning before eating their first meal. They believe it will cause them to mobilize more stored body fat and increase their metabolic rate all day long. There’s quite a bit of scientific literature supporting the a.m. fasted cardio theory, but generally, the exercise physiologists and scientists tend not to buy it. They subscribe to the energy balance hypothesis, which states; as long as you burn more calories than you consume in each 24 hour period, then the time of day you burn them doesn’t matter, nor does whether you burn them from fat or carbohydrate.

If you have even the most rudimentary understanding of human physiology and physics, you have to concede that the timing of your cardio is not the most important factor in fat loss. When you do your cardio won’t make or break you. Simply doing it whenever it’s convenient and following a mildly calorie restricted diet is what’s important. However, there’s a very strong case for doing fasted a.m. cardio and if you want to gain every legal and ethical advantage possible in your quest to get leaner then it’s definitely something you should take a closer look at.

The argument in favor of fasted early morning cardio goes something like this:

1. When you wake up in the morning after an overnight 8-12 hour fast, your body’s stores of glycogen are somewhat depleted. Doing cardio in this state causes your body to mobilize more fat because of the unavailability of glycogen.

2. Eating causes a release of insulin. Insulin interferes with the mobilization of body fat. Less insulin is present in the morning; therefore, more body fat is burned when cardio is done in the morning.

3. There is less carbohydrate (glucose) "floating around" in the bloodstream when you wake up after an overnight fast. With less glucose available, you will burn more fat.

4. If you eat immediately before a workout, you have to burn off what you just ate first before tapping into stored body fat (and insulin is elevated after a meal.)

5. When you do cardio in the morning, your metabolism stays elevated for a period of time after the workout is over. If you do cardio in the evening, you burn calories during the session so you definitely benefit from it, but you fail to take advantage of the "afterburn" effect because your metabolic rate drops dramatically as soon as you go to sleep.

Research supports this theory. A study performed at Kansas State University and published in Medicine and Science in Sports and Exercise showed that a kilogram of fat is burned sooner when exercise is done in the fasted state in the morning than when it’s done later in the day. The researchers measured respiratory gas exchange, caloric expenditure and carbohydrate and fatty acid metabolism, and found that the amount of fat burned during aerobic exercise amounted to 67% of the total energy expenditure in the morning after a 12 hour fast. This is substantially higher than the 50% expenditure achieved when the same exercise was done later in the day or after eating. A similar study from The Journal of Applied Physiology looked at the effects of aerobic exercise on lipid oxidation in fed versus fasted states. The researchers concluded, "our results support the hypothesis that endurance training enhances lipid oxidation in men after a 12 hour overnight fast." Yet another scientific paper, Optimizing Exercise for Fat Loss," reports, "The ability of exercise to selectively promote fat oxidation should be optimized if exercise is done during morning fasted metabolism."

Despite the fact that increased fat burning from morning aerobics seems logical and is backed by research, the majority of scientists and exercise physiologists vehemently deny its effectiveness. They are quick to point out that you can find a study to support almost any theory you want to advocate. Interestingly though, even the most dyed in the wool academics agree that you’ll burn more fat in the fuel mix as compared to sugars. The real controversy lies in whether this fact has any impact on overall fat loss in the long run.

Exercise Physiologist Greg Landry, MS, author of "The Metabolism System for Weight Loss and Fitness," explains, "I agree that you burn a fuel mix that is a little higher in fat if you’re exercising on an empty stomach. However, I think the real question is, does that matter? I believe we have a ‘pool’ of calories stored in different forms in the body (fat, glycogen, etc.), so ‘burned’ calories all come from the same pool. Thus, it really doesn’t matter that the fuel mix has a little more fat in it at a given time. If it’s pulling from fat stores at that time, then it’s pulling less from glycogen stores and thus future consumed calories will be a little more likely to be stored as fat because glycogen stores are a little fuller. So it’s all a wash."

Lyle McDonald, an expert on bodybuilding nutrition and author of "The Ketogenic Diet," agrees. He argues that the body will compensate later in the day and is simply "too smart" for strategies like this to ever work: "All that research says is that you burn a greater proportion of fat this way, which I agree with 100%," says Lyle. "The majority of research shows that as far as real world fat loss goes, it doesn’t really matter what you burn. Rather, 24-hour calorie balance is what matters. Because if you burn glucose during exercise, you tend to burn more fat the rest of the day. If you burn fat during exercise, you burn more glucose during the day. The end result is identical. If that weren’t the case, then athletes like sprinters who never ‘burn fat’ during exercise wouldn’t be shredded. Basically, they burn so many calories that they remain in balance and don’t gain any fat. So, while morning cardio probably provides some psychological benefits to bodybuilders who are programmed to do it that way, I can’t say that I think it will result in greater ‘real world’ fat loss, which is what matters."

When it comes to "real world" fat loss, few people have more experience than Chris Aceto. A successful bodybuilder and nutritionist to some of the top pro bodybuilders in the world, Aceto is a firm believer in morning cardio. He unequivocally states, "The fastest way to tap stored body fat is to do cardio first thing in the morning on an empty stomach."

Aceto believes that looking at calories only in terms of energy in vs. energy out is "limited thinking." He asserts that there are more factors involved in "real world" results than just energy balance. This all comes back to the old argument, are all calories created equal? "Absolutely not!" Aceto declares. "A calorie is not just a calorie and exercise physiologists ‘freak out’ when they hear this."

"These guys are working from the assumption that it’s just a matter of calories in vs. calories out, period," Chris continued. "With that line of reasoning, they’d be forced to say that if I consume nothing but candy bars and Coca-Cola, and take in 100 calories less than maintenance, I’d lose weight. We know it’s not that simple. You also have to account for ratios of carbs, protein, and fat. Then there’s meal frequency too: From real world results we know you put down more muscle mass from 5 or 6 meals a day than from 3 meals a day. There are more things involved than just calories."

Whether or not morning cardio in the fasted state increases "real world" fat loss is still the subject of controversy, but there are many other reasons you might want to consider making it a part of your daily routine. Landry, despite his doubts about whether the fuel source matters, admits, "If I had to pick a single factor I thought was most important in a successful weight loss program, it would have to be to exercise first thing in the morning."

Here are some of the additional benefits of doing cardio early in the morning:

1.It makes you feel great all day by releasing mood-enhancing endorphins.

2. It "energizes" you and "wakes you up."

3. It may help regulate your appetite for the rest of the day.

4. Your body’s circadian rhythm adjusts to your morning routine, making it easier to wake up at the same time every day.

5. You’ll be less likely to "blow off" your workout when it’s out of the way early (like when you’re exhausted after work or when friends ask you to join them at the pub for happy hour).

6. You can always "make time" for exercise by setting your alarm earlier in the morning.

7. It increases your metabolic rate for hours after the session is over.

Of all these benefits, the post-exercise increase in your metabolic rate is one of the most talked about. Scientists call this "afterburn" effect the "excess post-exercise oxygen consumption" or EPOC for short.

Looking only at the number of calories and the type of calories burned during the session doesn’t give you the full picture. You also need to look at the increased number of calories you continue to burn after the workout is over. That’s right - work out in the morning and you burn calories all day long. Imagine burning extra fat as you sit at your desk at work! That’s the good news. The bad news is, the degree of EPOC is not as great as most people think. It’s a myth that your metabolism stays elevated for 24 hours after a regular aerobic workout. That only happens after extremely intense and/or prolonged exercise such as running a marathon.

After low intensity exercise, the magnitude of the EPOC is so small that its impact on fat loss is negligible. Somewhere between 9 and 30 extra calories are burned after exercise at an intensity of less than 60-65% of maximal heart rate. In other words, a casual stroll on the treadmill will do next to nothing to increase your metabolism.

However, EPOC does increase with the intensity (and duration) of the exercise. According to Wilmore and Costill in "Physiology of Sport and Exercise," the EPOC after moderate exercise (75-80%) will amount to approximately .25 kcal/min or 15 kcal/hour. This would provide an additional expenditure of 75 kcal that would not normally be calculated in the total energy expended for that activity. An extra 75 calories is definitely nothing Earth shattering. However, it does add up over time. In a year that would mean (in theory) you would burn an extra 5.2 lbs of fat from the additional calories expended after the workout.

One way to get a significant post exercise "afterburn" is high intensity interval training (HIIT). HIIT is done by alternating brief periods of high intensity work (85% or more) with brief periods of lower intensity work. Studies on the effects of HIIT have demonstrated a much higher EPOC, which can add substantially to the day’s calorie expenditure. In one study, scientists from the University of Alabama compared the effects of two exercise protocols on 24-hour energy expenditure. The first group cycled for 60 minutes at a moderate intensity. The second group performed HIIT, cycling for two minutes at high intensity followed by two minutes at a low intensity. The group that performed the HIIT burned 160 more calories in 24 hours than the low intensity group. That means the HIIT group would burn an extra 11.8 pounds of fat in one year if they did HIIT five days a week instead of conventional training.

Ironically, weight training has a much higher magnitude of EPOC than aerobic training. Studies have shown increases in metabolic rate of as much as 4-7% over a 24-hour period from resistance training. Yes - that means bodybuilding does burn fat – albeit through an indirect mechanism. For someone with an expenditure of 2500 calories per day, that could add up to 100 - 175 extra calories burned after your weight training workout is over. The lesson is simple: Anyone interested in losing body fat who is not lifting weights should first take up a regimen of bodybuilding, then – and only then – start thinking about the morning cardio!

A common concern about doing cardio in the fasted state, especially if it’s done with high intensity, is the possibility of losing muscle. After an overnight fast, glycogen, blood glucose and insulin are all low. As we’ve already concluded, this is an optimum environment for burning fat. Unfortunately, it may also be an optimum environment for burning muscle because carbohydrate fuel sources are low and levels of the catabolic stress hormone cortisol are high. It sounds like morning cardio might be a double-edged sword, but there are ways to avert muscle loss.

All aerobic exercise will have some effect on building muscle, but as long as you don’t overdo it, you shouldn’t worry about losing muscle. It's a fact that muscle proteins are broken down and used for energy during aerobic exercise. But you are constantly breaking down and re-building muscle tissue anyway. This process is called "protein turnover" and it’s a daily fact of life. Your goal is to tip the scales slightly in favor of increasing the anabolic side and reducing the catabolic side just enough so you stay anabolic and you gain or at least maintain muscle.

How do you build up more muscle than you break down? First, avoid excessive cardio. Aceto suggests limiting your cardio on an empty stomach to 30 minutes, and then it would be "highly unlikely that amino acids will be burned as fuel." He also mentions that "a strong cup of coffee should facilitate a shifting to burn more fat and less glycogen. If you can spare glycogen, you’ll ultimately spare protein too." You might also want to consider experimenting with the thermogenic ephedrine-caffeine-aspirin stack (or it’s herbal equivalent).

Second, give your body the proper nutritional support. Losing muscle probably has more to do with inadequate nutrition than with excessive aerobics. Provide yourself with the proper nutritional support for the rest of the day, including adequate meal frequency, protein, carbohydrates and total calories, and it’s not as likely that there will be a net loss of muscle tissue over each 24-hour period.

Third, keep training with heavy weights, even during a fat loss phase. Using light weights and higher reps thinking that it will help you get more "cut" is a mistake: What put the muscle on in the first place is likely to help you keep it there.

Still petrified of losing your hard-earned muscle, but you’d like to take advantage of the fat-burning and metabolism-boosting effects of morning cardio? One strategy many bodybuilders use is to drink a protein shake or eat a protein only meal 30-60 minutes prior to the morning session. The protein without the carbs will minimize the insulin response and allow you to mobilize fat while providing amino acids to prevent muscle breakdown.

In conclusion, it seems that morning cardio has enough indisputable benefits to motivate most people to set their alarms early. But let’s talk bottom line results here: Does it really result in more "real world fat loss" than aerobics performed at other times of the day or after eating? I have to believe it does. Experience, common sense and research all tell me so. Nevertheless, this will obviously continue to be an area of much debate, and clearly, more research is needed. In the meantime, while the scientists are busy in their labs measuring respiratory exchange ratios, caloric expenditures and rates of substrate utilization, I’m going to keep waking up at 6:00 AM every morning to get on my Stairmaster.


1. Aceto, Chris. Everything you need to know about fat loss. Club Creavalle, Inc. (1997).

2. Bahr, R. Excess post-exercise oxygen consumption – Magnitude, Mechanisms and Practical Implications. Acta Physiol Scand. Suppl. (1992) 605. 1-70.

3. Bergman, BC, Brooks, GA. Respiratory gas-exchange ratios during graded exercise in fed and fasted trained and untrained men. Journal of Applied Physiology. (1999) 86: 2.

4. Brehm, B.A., and Gutin, B. Recovery energy expenditure for steady state exercise in runners and non-exercisers. Medicine and Science in Sports and Exercise. (1986) 18: 205,

5. Brybner, BW. The effects of exercise intensity on body composition, weight loss, and dietary composition in women. Journal of American College of Nutrition, (1997) 16: 68-73

6. Landry, Greg. The Metabolism System for Weight Loss. Greg Landry. (2000).

7. Maehlum, S., etc al. Magnitude and duration of post exercise oxygen consumption in healthy young subjects. Metabolism (1986) 35 (5): 425-429.

8. McCarty, MF. Optimizing Exercise for Fat Loss. Medical Hypothesis. (1995) 44: 325-330

9. McDonald, Lyle. The Ketogenic Diet. Morris Publishing, (1998).

10. Melby, C. et al. Effect of acute resistance exercise on post exercise energy expenditure and resting metabolic rate. J Applied Physiology, (1993). 75: 1847-1853

11. Wilmore, Jack, Costill, David. Physiology of Sport and Exercise. (1999) 2nd ed. Human Kinetics

12. Tremblay, A, et al, Impact of exercise intensity on body fatness and skeletal muscle metabolism. Metabolism (1994) 43: 818-818

13. Treuth, M.S., Hunter, G.R., & Williams, M. Effects of exercise intensity on 24-h energy expenditure and substrate oxidation. Medicine and Science in Sports & Exercise. (1996) 28, 1138-1143

14.Wilcox, Harford & Wedel. Medicine and Science in Sports & Exercise, (1985) 17:2


Wow, great article! I am doing something right! Wake up early, have a strong cup of coffee, then a fasted workout.

Thanks for posting this!!


Thanks for posting the article.

However, no matter what anybody says I always feel like s**t in the mornings so no early morning fasting cardio for me, ever.

I maintain that cardio works best for you at the time of day at which you are mentally ready to do it. For me, that will always be afternoon/evening.

But, I am sticking with the heavy weights and the long lasting post-exercise calorie burn, permanently.



I've recently been doing the HIIT cardio first thing in the am following the BFL plan and I MUST say I've had the BEST results EVER in losing fat....of course the clean eating 6 days and weight workouts are helping as puzzle pieces but definitely I AM getting fat loss results MUCH quicker than ever.....

Thanks for posting this article, sister Cheetah.. ;-)



Thanks for the article. I am posting a summary from another article on the subject of morning vs later exercise (doesn't address HIIT or other topics). I think when we read these articles, we need to make sure opinion is backed up by scientific studies and documentation.In the previous article many of the opinions were backed up by clinical studies except for Chris Aceto's opinion , which is just , an opinion gleened from his personal experience and not backed up by clinically collected data and his thoughts on a calorie is not a calorie are just that... thoughts with no scientific evidence to back him up. As far as exercise time of day, in the long run, different strokes for different folks. For me, I do most of my workout later in the day because my energy, strength and felxibility are much higher and I burn more calories. I have used my accitrainer to collect data on the same workouts done AM and PM and I burn more calories in PM on same workout, consistent with my perception of having more energy and giving more intensity to the later in the day workout. I would recommend that if you have a hrm or an accitrainer, you conduct the same experiment for yourself. You can do the same thing to measure strength and endurance at different times of day and before eating and after eating. For ex. compare how many reps at a certain challenging weight you can lift at different times for leg work, arm work etc and see how much weight you can lift, compare strength training workouts at different times and see what time of day, you can lift heavier and/or go longer. In the end, working out at a specific time will be dictated by your schedule and how your performance is at different times of the day. :)

I am pasting a summary of article I found here regarding pros and cons of exercising at different times. For data looking at calorie burn I would recommend only looking at well controlled studies in scientifically peer reviewed articles some of which you can find at EntrezPubMed. Hope you find this useful.

"Research on lung function, body rhythms, temperature, and hormone levels says one thing - to exercise around 6 pm.

Surveys on exercise habits say another - to exercise in the morning before other commitments distract you, or during the day when you have a free period of time. Let's explore the pros and cons of when to exercise.


• The majority of people who exercise consistently do so early in the day. It is easier to form the exercise habit through morning exercise.
• Fewer distractions and schedule interruptions.
• Can make time for exercise by getting up a bit earlier.
• Raises your heart rate and metabolism to burn more calories earlier in the day.
• Gives a feeling of physical energy for hours.
• Improves your mental acuity for hours.
• Cooler temperatures in summer.
• Air pollution is lowest in the morning.
• The body adjusts to your exercise time, so if you are training for a morning walking event, train in the morning.

• Body temperature is at its lowest 1-3 hours before awakening, and lower in the morning after awakening compared to later in day making morning a time of naturally lower energy and blood flow. Lung function is also lower in the morning.
• Cold, stiff muscles may be more prone to injury - be sure to warm up well before doing a higher speed workout, and do gentle stretching.
• If you do not enjoy morning exercise, you won't easily form a walking habit by choosing a morning workout time.
• Because body temperature and hormones are higher in late afternoon, you probably get the same and likely better calorie-burning effects later in the day.


• Can make a habit to walk and exercise at lunch time and break time.
• Can use a walking and exercise partner at work, school, or in your neighborhood.
• Body temperature and hormone levels are higher than first thing in the morning.
• Can help regulate the amount of food you feel like eating for lunch, can help you avoid break-time snacking.
• I mproves blood flow to the brain so you are sharper in the afternoon.
• Stress relief from work, school, or home stresses.

• Time limits may not allow you to get in a full workout - any amount is good, but best if you can walk for 30-60 minutes or more at a stretch
• Distractions and other commitments make keep you from walking and exercising at the appointed times.
• Research shows that lung function is worst at noon.


• For most people, body temperature and hormone levels peak at 6 pm. Exercising 3 hours before or after the peak will give your your best workout for both endurance and building muscle and calorie burn.
• Research shows lung function is best at 4-5 pm.
• Muscles are warm and flexible.
• Perceived exertion is lowest - how hard you feel yourself to be working at exercise. So you may be able to work out harder or faster by doing so in the afternoon.
• Can help regulate the amount of food you feel like eating for dinner.
• Stress relief after a day at work, school, or home.

• Distractions and other commitments make keep you from walking and exercising at the appointed times.


• For most people, body temperature and hormone levels peak at 6 pm. Exercising 3 hours before or after the peak will give your your best workout for both endurance and building muscle.
• Muscles are warm and flexible.
• Perceived exertion is low - how hard you feel yourself to be working at exercise. So you may be able to work out harder or faster.
• Can help regulate the amount of food you feel like eating for dinner.
• Stress relief after a day at work, school, or home.

• Distractions and other commitments make keep you from walking and exercising at the appointed times.
• Need to allow one to three hours to wind down after walking or exercise to be able to fall asleep. "
• If you discover sleeping problems, need to schedule your workout earlier.
• At dark times of the year, be sure to wear reflective gear.


Cathy, thank you for this post! At this stage in my life it is IMPOSSIBLE to exercise in the morning so I usually begin my workouts at 8PM. I go to bed between 11 and 12. Your Pro list of the evening made me feel better after reading about how good AM cardio is for you!

Just want to say again how amazingly helpful everyone is here!


all the research is conflicting (as it is with most things). i think the bottom line is to do what works for you (both schedule-wise and for your body). i just posted the article because i thought it was interesting. i think the point of exercise is to make you feel good and be healthy and it has to be a plan that works for you and that will encourage you to continue.
Thanks for the articles ladies.

My best losses ever were a 30 min cardio in the AM (I am always awake so no problem there) and a half hour is easy to do otherwise I am at work very early and leave too early - others don't know I am putting in a full day.

Then I am much better at weight lifting in the afternoon as Cathy's article pointed out. And with the help of those on this board, am doing really well at getting in my workouts. :7


RE: Interesting article...when to work out, HIIT, fat b...


Thanks for posting the article because it was very interesting reading and knowledge is alwasy power. I did fasted cardio for years (because of my work schedule and nothing else) and my only comment is that I think overall, that fitness and BF are dependent on many elements of a fitness program and not necessarily one aspect; which I think we all know.

I have always had relatively low BF, but not until I changed my diet and other componenets of my weight training, stopped fasted cardio, (just because of the nature of my job) did I really get the results that I wanted.

In the long run, I think that it just depends on building a program that the individual will stick with consistently and that is one with some built in variability and fasted cardio could be a part of that for a period of time. I'm just wondering if like any other exercise, we become acclimated to fasted cardio over a period of time and it looses its impact; just a thought.



RE: Interesting article...when to work out, HIIT, fat b...

my whole problem is that "fasted cardio" is the only kind i am able to do timewise do. i already wake up at 0430 to w/o and can't imagine waking up earler just so i can eat before. i tried working out after work (actually joined a gym, got a trainer) and it just didn't work for me. i hated thinking all day that i had to go w/o, i was always trying to get out of work on time so i didn't skimp on my w/o's (there were times i just couldn't leave and had to miss my w/o's :-( ), and working out hard later in the evening seems to interfere with my ability to sleep (say, if i did a long run or HIIT). besides, i love my quiet morning time. i have more than enough time to get in a long workout if i want and no one bothers me and there is nothing going on. it gives me time to reflect on the day before and i feel like it prepares me for the day ahead. i can really tell the days i don't workout. one thing i like doing is working out twice a day if i can and feel like it. my evening workout will usually be weights or something light but my primary work out is in the morning.

>"it just depends on building a program that the individual will stick with consistently"-to me, that's really the bottom line right there.

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