Burning Ketones for energy versus Glucose from Glycogen or Gluconeogensis Question

Factual Questions
1

I’m going to start this in FQ because I think there is a factual answer and I’m not asking for medical advice. Mods feel free to move it if it really belongs somewhere else. After scouring the Internet I’m having trouble finding a straight answer to what I think is a fairly straight question. Let’s see if anyone here can explain to me how it should all work.

I am currently on 16/8 Time Restricted Eating, which means in a 24 hour period I have an 8-hour eating window and I fast for the remaining 16 hours. My eating window is from 9 a.m. to 5 p.m., which means I am fasting from 5 p.m. to 9 a.m the following day. I’m on a low-carb (<50 g carbs per day) but not a strict ketogenic diet.

After my last meal of the day, my body digests what I just ate and extracts energy/glucose from any carbohydrates I consumed over the next 3-4 hours. By the time I go to bed at 10 p.m., five hours after my last meal ended, my blood glucose has returned to it’s normal unfed level of ~100 mg/dL. The question is what does my body do now to get the energy needed to keep me alive for the next 11 hours?

As I understand it, my body has three options: 1) use glucose produced by my liver via gluconeogenesis, 2) use glucose stored in my liver’s glycogen, or 3) use ketones from the breakdown of body fat. You can assume I am metabolically flexible since I was on a strict Ketogenic diet for a number of months and lost a lot of weight in the process.

So what does my body do? There isn’t a lot of glucose around but there is glucose available in my glycogen and my liver can make glucose from amino acids. I would prefer it to burn ketones, but what is it most likely to happen given those three options, and why?

2

Glycogen goes first. Blood sugar will be maintained.

Proteins always have turnover so amino acids are floating about (and some from that early dinner are being absorbed over multiple hours later). Fats can be mobilized only so quickly (although I believe that occasional fasting state exercise can lead to more efficient fat mobilization).

Strong stimulus to use available amino acids for synthesis will encourage a net balance in. So exercise, especially incorporating some strength training, will decrease the use of amino acids for energy.

Without that stimulus more of the amino acids floating about from turnover and intake will be used for energy and more muscle mass will be lost.

I am sure the exact balance ends up as a consequence of many factors, including various stress hormone levels and individual efficiency at utilizing fat for energy (a goal of those participating in endurance sport training is to become more efficient at using stored fat for energy).

So bottom line is that there may not be an actual factual answer across the board.

3

Thanks for your reply. I realize that as far as any specific person goes, there is no way to predict what their body will do. I was looking for general principles which should apply to most people, which is what you articulated.

I now understand that your body will first and foremost use liver glycogen stores to maintain your blood sugar levels, without the need to burn fat, and that makes sense. So throughout most of my fasting period, including when I am exercising and sleeping, my liver glycogen is slowly being depleted to provide glucose for my entire body, except for my muscles, which apparently have their own glycogen stores. Unless I were to somehow completely deplete my glycogen, there’s really no reason for my body to burn body fat.

According to one source, The human liver can produce about 125–150 mg glucose per min or 180–220 g per 24 h. About 100 g glycogen are stored in the liver and this store can, therefore, last about 12 hours.

This tells me that my 16-hour daily fast probably isn’t long enough to deplete my liver glycogen, so that’s why I’m not losing weight, even though I am burning an additional 600 calories by exercising daily. I lost a lot of weight on my strict Keto diet because I was consuming almost zero carbs, and I must have been routinely depleting my glycogen, which meant my body had no choice but to switch to fat burning mode. Does that sound right?

4

Nah. And honestly you may be overthinking it.

The obvious must be said: you probably don’t really want to lose weight so much as lose fat mass, while preserving or even gaining fat free mass, especially muscle mass. And of course gaining fitness.

Especially on lower carb diets weight loss can be very misguiding. Initial weight loss associated with relatively depleting carbohydrate stores is to large degree the water obligatorily stored with the glycogen.

There is also no magic to fasting for weight or fat loss (nor glycogen depletion) other than that it is sometimes associated with fewer calories in. It is for some people an effective way to end up with fewer calories in, so great for them. You highly probably over time ate less while on ketogenic. That is why it worked. And time restricted fasting with low ish carb doesn’t decrease your calorie intake as much.

There are lots of possible reasons why an hour of intense exercise each day (a rough guess of what you must be doing to be getting your calculator to tell you you are burning 600 calories doing it) is not being associated with weight loss.

One. Some people do more compensatory intake than they think, than they are consciously aware of. Even during a limited time each day to eat.

Two. Many have less other incidental daily movement that burn calories, non-exercise activity thermogenesis (NEAT) as their bodies compensate without any conscious recognition.

In both of those cases benefits are still being gained.

Three. That much daily exercise without recovery days may be increasing stress hormones and preventing fat loss. Really it takes a good long intense exercise session for an average man to burn 600 calories over resting state. Most adults, including those marathon training, need recovery days. Even without stress hormones per se there are a variety of other responses that see the body resist weight loss.

Four. The best possibility. Some may be losing fat mass but simultaneously gaining modest amounts of muscle mass. For them the scale changes little but the belt still is a notch tighter. That happens most for those keeping up with a balanced program of cardio and strength training both and a modest calorie deficit that of course is light on the more highly processed foods and solid on the real foods high in fiber and with adequate protein. If timed restricted fasting helps achieve that for a particular person, fantastic.

There really is no fat burning mode per se. The body just can’t mobilize fat all the quickly. Again endurance training and occasional fasted state exercise can help a body adapt to doing it better, but the end result of loss of fat by end of week will still be most due to the actual (not just calculated but all after all of the subtle adjustments the body makes, including to behaviors) calories in vs calories out, coupled with how much lean body mass is preserved, gained, or lost.

5

It’s a lot more complicated than I thought, but I think you hit the nail on the head when you said my scale may not show it but my body composition is changing. I’m definitely decreasing my belt size and I feel that my moderate daily cardio and moderate daily upper body resistance work means more muscle mass and less fat mass, which will inevitably lead to weight gain, not loss.

I’m 70 years old and don’t do extreme exercise. I power-walk during the day and do upper body weight training first thing in the morning and last thing before bed. Today, according to my iPhone, I power-walked 9.41 miles, ~23k steps, and “moved” 728 Calories. I ate three healthy meals today day, no snacks, no seed oils, and no added sugar. My overall stress levels are low, and I get at least 8 hours of good sleep at night.

It seems that while my weight is staying the same, I am actually building muscle while I am losing body fat. When I lost 40 lbs. on my keto diet two years ago I got used to seeing my weight go down. Now it just sits there, which is something I should probably be happy with. Case closed.

6

Very impressive!!!

7

Two small points to add, which add little to DSeid’s excellent summary.

  1. FWIW I’ve read the liver stores about 700 calories worth of glucose. I’m sure there is considerable variation.
  2. Lifters in particular are often worried about losing muscle mass by using protein for energy. In the short term, your body has plenty of other protein sources that need to be recycled apart from muscle. Something like 75% of proteins are expressed in the bowels, so over a short fast the fear of losing muscle can be overblown. Your body does prioritize holding onto muscle when possible. Lifting and doing cardio on the same day is not that big a deal either.
  3. That is an impressive amount of weight loss. Still, weight is hard to change (likely resetting points requires many weeks), and replacing fat with muscle is often helpful even if the weight changes little.
8

Noted, thanks. I never intended to lose 40 lbs., but when I was only eating one meal a day, which was highly calorie restrictive, and I was on a strict Keto diet, which was highly carbohydrate restrictive, and I was power-walking 12 miles a day, which burned a lot of calories, the weight came off quickly. However, as my dietician told me when I was struggling to transition into Maintenance, I lost too much too fast, and I never gave my body a chance to adjust to my new weight. As soon as I went off my diet the weight came back with a vengeance. It finally settled at about 30 lbs. below where I started, and I decided that was good enough. My BMI was down to 25, and I was no longer at the high end of the unhealthy range. I now know that BMI is bunk, but at the time I thought it meant something being in the 20 to 25 range, so that became my goal. It’s been two years since I lost the weight and I feel better now and my mind is clearer than it was 30 years ago. I just hope I never have to go through a diet like that again in my lifetime.