Exercise Intensity and Fuel Selection - Carbohydrates vs Fats 

Thank you! I hope your students find these videos helpful.

I'm glad you found it helpful.

Thank you!

You are very welcome.

Best of luck.

@@VivoPhys 1 question tho why is fats the dominant substrate for Low to moderate intensity exercises?

It's plentiful. Our bodies must conserve carbohydrates for higher intensities. We have a lot less carbohydrate stores in the body.

@@VivoPhys thanks Dr appreciate it

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Thank you!

You are welcome.

Thank you!

Thank you for contributing your comment. The main point being made at the moment you cited in the video is about exercise intensity and weight loss for the purpose of fat loss. I was not attempting to discuss diet or weight loss unrelated to body fat. Admittedly, the video is a simplification. All undergraduate content is a simplified and direct to the point understanding without the nuance learned by reading research articles. If you have more good resources you think people should read please reply with another comment. Thank you!

@@VivoPhys Indeed, fat loss is what I'm interested in. I'm not interested in losing carbohydrate and water weight due to depleted carbohydrate stores and I'm certainly not interested in forgoing meals when I'm hungry (fasting). I am also interested in knowing how many assimilated grams of carbohydrates are needed to support long bicycle rides at a given intensity, which is why I watched this video. I'm more concerned with under-eating carbohydrates than overeating them. Moreover, contrary to the commonly held dread of kwashiorkor, I'm not personally concerned with protein deficiency. Mother's milk is acknowledged to be the ideal food for infants and it contains only 5% calories from protein which is used to support growth during the time in the human lifecycle when growth is most rapid. There is also a study that was published nearly a century ago indicating that nitrogen balanced can be attained in adults even when the practically sole source of protein is from potatoes -- hardly a high protein food. (see: The Value of Whole Potato in Human Nutrition (1928) ) My guess is that this can, at least partially, be explained by the protein sparing effect of carbohydrates.

Good question. It will vary by person and textbook resource. Training will likely effect it to.

Hi @Starchaser63. Carbohydrate intake is generally considered critically important for high intensity exercise in both training and competition.

Hi antsworn. That is a good question. It likely has a complex answer, that is a bit out of my area of specialty, but here is my best answer. Hopefully someone who greater expertise will eventually answer too. Generally speaking ketogenic diets either have no effect or a negative effect on both aerobic and anaerobic exercise performance/training outcomes. However, some studies do show improved performance at low aerobic exercise intensities. There are both positive and negative health outcomes that you should be aware of before beginning a ketogenic diet. Importantly some studies show a decrease in bone mass and increased risk of kidney stones. Source: www.ncbi.nlm.nih.gov/pmc/articles/PMC6835497/ You would likely be in a constant state of gluconeogenesis if you were in a ketogenic state. That is regardless if you were exercising or at rest. It may be enough for short duration anaerobic exercise, but, I doubt it would keep up to fuel longer duration or multiple effort exercise like most sports.

@@VivoPhys Awesome! Thank you for your response! That has been on my mind for a while now! Also, thank you for that link! It was a very interesting read!

Hi, great content, thank you for taking the time! Here we see what happens when we start from resting, but what happens if I warm up for 2 min vs 15 min at a "conversational pace" (60% of vo2max)? I noticed that if I warm up for a long time, I don't have this sensation of very low energy ("hitting the wall") after a few minutes of intense effort. For example : 2000m rowing before crossfit all out is much smoother than if I don't do the 2000m row beforehand.

Increasing intensity after a warmup still requires an increase in aerobic metabolism to reach a new steady state. It might not be as pronounced but the process is similar.

@@VivoPhys thank you, very clear as always! I'm learning a lot! Cheers from Switzerland!

Low intensity exercise is generally only recommended for people who cannot do moderate or vigorous exercise. The higher the intensity of exercise the greater the fat loss will be once the body has time to replenish carb stores after exercise. It does not matter if you burn carbs or fat during exercise. The body can convert fuels to fat and in some cases vice-versa.

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Exercise is well known to improve physical health outcomes. To list just a few, it reduces blood pressure, cholesterol, body fat, and insulin resistance which decreases the likelihood or severity of many cardiometabolic diseases. However, exercise also improves mental health. Fewer studies have been conducted evaluating mental health outcomes, but the results strongly suggest exercise improves both anxiety and depression in people with and without anxiety and depression disorders. This means it will reduce anxiety and depression in everyone regardless of the severity of your symptoms. Though exercise as a therapy for depression is most effective in people with clinical level depression. In fact, cardiorespiratory exercise has proven to be as effective as psychotherapy or pharmacotherapy in treating depression. Exercise can also be used in conjunction with other mental health treatments to improve anxiety and depression. All adults, including those hoping to improve mental health, should accumulate 150 minutes of moderate intensity or 75 minutes of vigorous intensity cardiorespiratory exercise per week. Performing cardiorespiratory exercise 3-5 days per week and at the higher intensities (e.g., 60-90% of maximal heart rate) appear best for reducing anxiety. A bout of exercise as short as 1 minute will reduce feelings of anxiety immediately, but 30 minutes or more is better. For improving depression symptoms cardiorespiratory exercise training may require at least 13 or more days to show consistent and lasting results, but depression symptoms are temporarily improved immediately after each bout of exercise. People without depressive disorders should perform cardiorespiratory exercise for at least 20 minutes per bout to reduce feelings of depression. Those with depressive disorders are recommended to perform cardiorespiratory exercise for 45 minutes. Resistance exercise has also been shown to reduce anxiety and depression symptoms. Therefore, people attempting to reduce anxiety and depression symptoms should also do resistance exercise engaging all major muscle groups at least twice per week. This is especially important for reducing depression symptoms as doing both cardiorespiratory and resistance exercise is more effective in reducing symptoms than either by itself. This is a summary of the information in the most recent ACSM Guidelines textbook. While it is mostly moderate intensity exercise, vigorous intensity exercise generally just provides even more benefit.

@@VivoPhys Thanks Dr. Matt for the information. I am a living proof. It works for me with out generalizing.

Please consider doing a short presentation and video on this subject because majority of Americans might not know this valuable prescription and information. I would assume the information would have a great educational value and benefit to the public, also impact people in a positive way. The information may help patients who are on a psychiatric medication, people who have mild symptoms, and for those who are health but would like to experience Nirvana in a natural and healthy way. Thanks for all that you do to educate Americans.

Hi Oscar Zárate. If you do a workout that would be expected to burn 80kcal in the morning and another worth 60kcal in the evening, then yes you will have burned 140kcal with exercise that day. It would be equivalent to if you did all of the activity all at once. As far as elevated metabolic burn post exercise, that may be more complicated, and I do not know the answer.

@@VivoPhys hey, thanks for the answer and just to be clear those hypotheticall 140 kcal wold be fat?

That would depend on things like the exercise intensity and your body's energy available. However, it doesn't really matter for weight loss. Energy deficit is all that really matters, not energy source.

@@VivoPhys thanks again Sr., this course is helping me a lot. Salute to you.

For a marathon runner doing some long slow distance training sessions like what you describe makes sense. I do believe you can increase your fat usage with training, but high intensity exercise will still predominately burn carbohydrates.

Hi aroundandround. Good question. First a general comment. These lectures are designed for undergraduate level knowledge. So I do not go into the fine details, and I often simplify the reality of the situation. Learning details of any physiological concept requires reading the body of peer reviewed research on the topic. No one can be an expert on everything. With that said read below. Simply put, fat metabolism does not increase at a linear rate with increased exercise intensity. It will eventually decrease with increased intensity. Read the facts below with the sources provided for a bit more information. Even before the body shifts from largely aerobic to largely anaerobic energy sources the body is mostly using carbohydrates not fat or protein. The reasons being that carbohydrate energy production is 2x faster than fat or protein energy production. We also create almost 6% more energy per liter of oxygen used with carbohydrate metabolism than fat metabolism. Prolonged duration submaximal intensity exercise will gradually increase fat utilization as carbohydrate stores decrease. Exercise training also increases fat utilization and decreases carbohydrate utilization at submaximal intensities. Source of these facts: Exercise Physiology, Nutrition, Energy, and Human Performance, Seventh Edition- William D. McArdle Muscle glycogen and plasma glucose oxidation rates increased with every increment in exercise intensity. Whole-body fat oxidation increased to 32 ± 2 kJ min−1 at 55 % Wmax, but declined at 75 % Wmax (19 ± 2 kJ min−1). This decline involved a decrease in the oxidation rate of both plasma FFA and triacylglycerol fat sources (sum of intramuscular plus lipoprotein-derived triacylglycerol), and was accompanied by increases in muscle pyruvate dehydrogenase complex activation and acetylation of the carnitine pool, resulting in a decline in muscle free carnitine concentration. We conclude that the most likely mechanism for the reduction in fat oxidation during high-intensity exercise is a downregulation of carnitine palmitoyltransferase I, either by this marked decline in free carnitine availability or by a decrease in intracellular pH. Learn more about how exercise intensity effects fuel substrate: www.ncbi.nlm.nih.gov/pmc/articles/PMC2278845/ Good luck with your studying!

@@VivoPhys Much thanks for the response and the paper reference that is very interesting indeed. What the paper’s findings - specifically the lower fat oxidation rate at 75% intensity - suggest to me is that there is indeed such a thing as the “fat burning zone”, so that commonly seen label on exercise machines is not a myth after all. In particular, exercising for 30 minutes at 55% intensity is likely to burn more total fat than the same time spent at 75% intensity, yes? Or is that just the case during exercise and the total fat burned including that to replenish lost carbs post-exercise end up being higher even at the higher 75% intensity?

There is a fat burning zone, but it doesn't really matter. That is the myth. Attempting to stay in the fat burning zone will lead to fewer calories burned unless exercise duration is very long. For weight management, calories burned regardless of the micronutrient substrate is what matters. If you burn carbs during exercise, then fewer of the carbs eaten will convert to fat because they will be used to replenish glycogen stores. If you burn fat during exercise your excess carbs eaten (because glycogen stores are still high) will just be converted to fat and stored. You can go through a similar logic with fat or protein consumption. In the end it is just calories in vs calories out that matters.

@@VivoPhys Thank you, I’m still confused on the “myth” aspect and would appreciate clarification. In your video at around 5:00, you compare 20% and 50% VO2Max to say that the latter burns more absolute fat even if the relative percentage is smaller, which is well and good. But the paper you referred says that at 75% VO2Max, the absolute fat burn rate decreases to 19 Kj/min from 32 Kj/min at 55% VO2Max, so no matter which way I see it, it seems to suggest that exercising at 55% intensity burns more total fat than at 75% intensity, *both for the same duration*. What am I missing?

Also, I do understand that whatever store - fat or glycogen - is depleted more will be replenished accordingly later, so ultimately calories in/out is what matters most, but based on the numbers in the paper, it still seems like if one creates a caloric deficit exclusively with 85% max intensity exercise, they will hardly impact fat reserves, but if they create the same caloric deficit with say 55% intensity, they will lose some fat over time. So the optimal approach would be to “burn as many calories as you can at moderate intensity”, not simply “burn as many calories as you can” (oblivious to intensity).

Carbs help replenish muscle glycogen so you can exercise again tomorrow.

We can convert some products from fat metabolism into carbohydrates or other glycolic intermediary molecules. Our bodies will also adjust when possible to utilize the most abundant and freely available fuels. So future metabolism is impacted by diet.

Hi @tommyfischer1. Please see below for the definitions of calorie. Please let me know what you mean by missing pieces. Thank you. A “small calorie” (cal, gram calorie) is the amount of energy required to raise the temperature of 1 gram (g) of water by 1º Celsius (º C). A “large calorie” (Cal [with upper case C], food calorie, kilocalorie, kcal) is the amount of energy required to raise 1 kilogram (kg) of water by 1º C. It is also known as a kilocalorie.