Where does energy come from during a fast?


Hi folks. I was looking for information as to where the body gets energy during a fast in a fat adapted individual. Assuming that this person is also weight lifting and expends more energy than the body can liberate from fat stores alone.

From what I read there is a limit as to how much of ones body fat can be converted to energy per day - (290±25) kJ/kg day or (31.5053±2.72) kCal/lb day:

So assuming based on the formula above, that for a person that has 63.5 lbs of body fat that 2,000 Calories of energy can be used for fuel. But what if this same person needs 3,000 Calories a day due to activity/exercise? Where does the delta come from?

Autophagy? - I would hope that this would be the case, but I can’t see the human body being able to quickly figure out where to find 250g (1000 Calories / 4 Calories per gram of protein) of old protein to destroy a day to continue to fuel demand. I hope that I’m wrong.

Glycogen? - I would have thought that’s near depleted since starting keto. I figured that the first 10+ lbs or so of weight loss when starting keto was glycogen and water. So I wouldn’t suspect that this would be a large source of energy to fill that delta.

Burning muscle? - I hope this isn’t the case, but I don’t know what other sources of energy are left. If one is fasting and lifting weights, Dr Jason Fung says that HGH is secreted and it’s muscle sparing. But I would suspect that would only apply to the parts of the body that where exercised during the fast leaving other parts ‘fair game’ since they where no being used.


The reason why I’m asking is that I performed 2 DEXA scans over 4 months apart (different offices / machines / software, etc). Between the two scans it showed that I lost 2 lbs of lean tissue in my legs even though I lift weights.

I did an extended 7 day fast between scans, and a couple of 48 hour ones with some OMAD sprinkled around everywhere. During the 7 day fast I still weight lifted but didn’t do lower body during that period.

One theory is that in the week that I did the 7 day fast and had a large caloric deficit, that my body used up my leg muscles to fuel my workouts for my upper body. I also happen to have gained 2 pounds of fat and some bone.

The other theory is that since it was different DEXA machines and I was slightly dehydrated the 2nd time, that the scan was less than optimal:

(Give me bacon, or give me death.) #3

You raise an interesting question, one I haven’t ever really seen addressed. My thoughts are that either (a) the calculated limit is erroneous, or (b) the metabolism adapts to the available energy. Since feasting/fasting is the pattern we apparently evolved to follow, obviously the body can handle exercise during fasting, and at the moment, I am leaning toward answer (b).

There are a number of processes that are not essential to immediate survival, among them being hair and nail growth, and the reproductive system. They could easily be cut back during times of strenuous activity during fasting. Also as muscle tissue becomes more metabolically efficient, the same tasks can be accomplished with less energy.

The amount and quality of the food during the feasting phase is surely important, as well. I know as the owner of pet rats that in the wild, rat fertility is regulated by the available food supply. Wild litters are never as large as the litters of domesticated females, because the pets have a stable abundant food supply. I suspect that the human body has a similar regulatory system.

I don’t know how accurate these speculations are, but I’ll be interested to follow the conversation that develops around this question.

(Doug) #4

“A final source of error is the same error that all 2-compartment models also have: the hydration of fat-free mass.”

–This is often a problem. Fasting usually has us losing water weight, and depleting muscle glycogen will mean more of that, i.e. carbohydrates are stored in the body with water (always, as far as I know).

(Michael - When reality fails to meet expectations, the problem is not reality.) #5

This caught my attention. Apparently, after fat adaptation, glycogen stores return to approximately the same levels they were prior to keto. If you think about it; makes sense from an evolutionary perspective. Our ancestors spent a lot of time in ketosis, probably the majority of their lives. If they had not also retained the ability to mount a sudden burst of energy to avoid a predator we wouldn’t be here now. The process involves gluconeogenesis and is somewhat complex:

This is interesting, too:


I appreciate the thoughtful response Paul.

This had me thinking so I looked at my workout log. About 70 hours into a fast I was able to lift more then I did the previous week. According to Dr Fung, metabolism increases during the first 3 days, but I am not sure what happens after that.

I had a workout a 2-3 days later and wrote in my notes that I struggled and couldn’t do as much as I did the previous week. So I take it that my metabolism slowed down.

Next time I’ll try and notice changes to hair (do I have to shave as much?) , nails, etc.


Thanks for the good reading.

I just read read the FASTER study which shows how keto adapted athletes where able to replenish their glycogen stores:

I falsely believed that most glycogen would be depleted when starting a keto WOE and not come back until someone would start to eat carbs again.

This might explain why there’s sometimes an uptick in weight while in keto even if keeping below 20 net carbs per day. It could possibly be glycogen replenishment, not necessarily muscle gain.

Metabolic Characteristics of Keto-Adapted Ultra-Endurance Runners
(Michael - When reality fails to meet expectations, the problem is not reality.) #8

Thanks for the link! I had trouble finding something that just states this right out there.


Compared to highly trained ultra-endurance athletes consuming an HC diet, long-term keto-adaptation results in extraordinarily high rates of fat oxidation, whereas muscle glycogen utilization and repletion patterns during and after a 3 hour run are similar.

(Alec) #9

My hunch (no evidence) is that if you are truly fat adapted, the amount of energy you can get from fat stores is much higher than the quoted figure.

If you are not fat adapted, I think the delta is initially dealt with by the body dropping expenditure, but as there is a limit to this, I think the body then starts to burn the furniture.

So, what’s the lesson? Be fat adapted if you want to fast.


@Mavro Yes, this ^. The calculator is based on folks on a calorie-restricted high-carbohydrate diet. It’s a completely different metabolic situation from fasting, especially for someone who’s fat adapated.
When you’re fasting you’re getting energy from fat stores. In my personal experience, any weakness has to do with electrolytes - very hard to get those right during a fast - rather than from the fast itself.


Excellent points about the calculation being based on people that are not fat adapted. I would imagine that we would be better suited to metabolize fat for energy then the folks that partook in the Starvation Study. How much more than 32 Calories, who knows?

Thinking back again, I wasn’t supplementing electrolytes until after the poor exercise day when I was going through the EF. I felt cramping in my legs overnight which was a good indicator.

I now consume a form of ketoaid every morning in place of breakfast. Helped me a ton! Also having some Himalayan pink salt crystals every so often throughout the day.

(Michael - When reality fails to meet expectations, the problem is not reality.) #12


Lessons from Low Carbohydrate Athletes

Perhaps the situation perceived as most challenging for someone on a ketogenic diet is the ability to maintain glucose/glycogen reserves with prolonged, high intensity exercise. For a greater part of the last century, the accepted paradigm has been that one’s initial muscle glycogen is positively correlated with the ability to sustain endurance performance during moderate-to-high intensity exercise¹²,¹³. However, given that even with ‘optimized’ muscle glycogen obtained by using a carbohydrate-loading diet strategy, an endurance athlete has a peak total body glycogen content of only about 2000 kcal. To attempt to simultaneously train the muscles to use more fat and reduce one’s dependency upon glycogen in order to extend performance creates somewhat of a metabolic oxymoron. This is because very high insulin levels induced by carbohydrate loading actually suppress adipose fatty acid release and oxidation.


Awesome find! Thanks for sharing.

Blockquote They reported the maximum rate of fat oxidation for the best individual fat burner in this group (which included a number of highly trained athletes) to be 0.99 grams of fat per minute. However long before that, one of us reported that bicycle racers who were keto-adapted for just 4 weeks were able to burn fat at 1.5 grams of fat per minute¹⁰.

Blockquote But indirect calorimetry testing (measuring 02 consumption and CO2 production) indicated that almost 90% of ketogenic runner’s net energy use was from fat. This result is a clear indication that glycogen mobilization does not equate with carbohydrate oxidation in the keto-adapted state. Rather glycogen stores can be optimized and available for anaerobic (aka glycolytic) muscle function and then quantitatively recycled back to glucose by the liver.

(Doug) #14

Yes, Alec - there is an enormous world of difference at work here. I think that in the end it would be quite complicated, with potentially many influences to think about, rather than any necessary single limit being in place.

Keys’ subjects were depressed, food-obsessed, etc. Hormones/neurotransmitters affect metabolism and the usage of fat. Catecholamines definitely do. They include dopamine - known to increase heat and energy usage in brown fat - and adrenaline (wide range of effects).

They definitely affect fat usage:

The most striking and possibly the most important effect of the catecholamines on lipid metabolism lies in their ability to accelerate acutely the mobilization of FFA from adipose tissue.

Rather than an arbitrary cap on how much fat can come out and be used, there seem to be a mix of mechanistic inducements. No idea at the present time how they all come together, but those ‘poor buggers’ in Keys’ study are a long way from massively-trained exceptionally fit subjects in the ‘FASTER’ study, for example.

A good many things to think about… Even among non-keto athletes, there is more storage of triglycerides in muscle tissue (close to the equivalent amount of energy in our stored glycogen) and somewhat more usage, per unit of time when exercising, than in non-trained people (about 1/5 of energy, versus glycogen, over the first couple hours of strenuous exercise). In a 24 hour cycle, would the replenishment of this mean more total ‘fat loss’ beyond what we picture being taken out of storage and directly ‘burned’?

Rates of fat metabolism differ, depending on where the fat is - visceral, under the skin near the abdomen, hips, butt.

In the ‘FASTER’ study, they ran for 3 hours on a treadmill. This is similar to running a marathon (per the pace/percent of maximum oxygen uptake), and even with that thought in mind, the stored glycogen would provide the majority of the energy required. Stored fat in the muscles would contribute a little more. In the end, not much is coming out of fat tissue while the exercise was going on.

Oddly enough, the author of A limit on the energy transfer rate from the human fat store in hypophagia - Seymour Alpert - apparently later revised his estimated amount downward. While it doesn’t really make sense to me, here is what he was quoted as saying:

"The value used of 31 kcal/d lb is correct only if there is no energy loss to activity. In the original paper which you cited. I took a value of activity losses from some of my previous work, but I have recently developed a much better way to estimate activity losses which changes the maximum practical value for the idealized value of maximum loss from fat alone to the actual loss to only 22 kcal/d lb. This mean that it is very hard to lose fat alone. That is life."