The title pretty much sums up my question. What are the metabolic differences, if any, between fasting and eating 100% fat?
I'm not clear that there are any, so I am very curious to hear what others think. No carbs/protein means gluconeogenesis and ketosis will be in full force and blood glucose will be low. Fat mobilization is still going to be in effect as far as I can tell, the dietary fat would just be topping up adipose tissue.
Anything I'm missing?
Update Dec 10: no one has pointed out a metabolic difference yet, but Eva pointed out that the digestion of fat could itself involve various signaling pathways. Does anyone know details about digestion of a 100% fat meal? Have any studies been done on this?
asked byWill (911)
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on November 10, 2010
at 08:45 PM
If I can find the post or comments Peter Dobromylskyj has made at his blog, Hyperlipid, about high fat mimicking fasting, I will post them, with the links. Also, there might be something in the Stefansson, Pennington, Kekwick, Donaldson, MacKarness realm on high fat diets being similar to fasting. If I can find references, I will post those, too.
Meanwhile, I hope this is of some use:
Dr. Kurt Harris, in this post, the last paragraph, emphasis added:
As far as raising your insulin levels, glucose has the biggest effect, and fructose is bad because it induces insulin resistance, which requires you to produce higher levels of insulin to handle the same amount of glucose. Protein requires insulin but less than glucose and there is some insulin response to dietary protein. There is no insulin response to dietary fat. All three require some basal insulin for normal metabolism.
Here is something Stan Bleszynski posted, related to this subject, at his blog:
Snacking and glucose/ketogenic cycling
This is a theory, very speculative. I came to think about it, inspired after reading the latest articles by Peter (Hyperlipid blog, see also his Kitava articles). I am thinking about the effect of frequent snacking in between the meals on a high carb diet, may be more damaging than we thought. Under a healthy metabolism, even on a high carb diet, one's body goes back to ketogenic state some time after every meal, when most body tissues revert back to using lipids and ketone bodies as fuel. This should happen in between the meals providing that the body is given enough time to burn glucose off and to switch. On the other hand, any time one eats something sugary or starchy in between the meals, insulin inrush causes the body to switch back to glucose processing state, turning off production of ketone bodies and turns off the release of adipose fat into bloodstream. It takes about 1-2 hours to work out all that insulin peak. Probably longer for older people.
For some reason, our bodies probably require this alternating between glucose-burning/ketogenic cycles to be done throughout the day to work properly. This is based upon the assumption that ketogenic mode is essential for the body and must take place at least some of the time throughout the day, every day. I suspect that frequent snacking on a high carbohydrate diet derails that cycling and may be one of main triggering factors behind the development of the metabolic syndrome.
From my personal observation, one can notice that people who are used to snack frequently are often obese and often have metabolic syndrome. Perhaps there is a causual connection? Perhaps it is not just what and how much one eats but how often and when?
Diets based on natural whole food like our vegetarian friends recommend, usually discourage sugary or starchy snacks, or discourage consumption of the processed food that forms the typical fast food snacks. It makes snacking more difficult, especially that preparation of meals out of natural unprocessed whole food is time consuming and labor intensive. Perhaps that may be one of the reason why such whole food natural diets are often helpful?
Of course, the high fat low carb diets make that issue irrelevent since a high fat snack would not cause the glucose+insulin spike thus would not break the ketogenic mode. This could be another reason why the high fat low carb diets work so well!
on November 10, 2010
at 11:26 PM
Fasting isn't just a matter of keeping insulin low. It is giving the body a break from digesting, period - stomach, pancreas, intestines. This allows autophagy to occur.
"Art DeVany on autophagy - Art Devany discusses how fasting - being hungry - can promote autophagy (literally self-eating) a process in which your cells consume and recycle damaged internal material.
The process seems to be triggered when the energy content of the cell declines so that the cell literally consumes itself. It goes after the damaged materials first, so there is a strong link between repair of damaged tissues and fasting or low energy state in the cell. So, it you are over-fed you down regulate cellular repair. You want to go hungy episodically to turn on cellular autophagy and repair those damaged tissues."
If you are going to fast, fast; if eating, eat. Why muck around with 100% fat which would be unsatisfying for more than a snack.
on November 11, 2010
at 05:56 AM
An interesting story: Long ago, doctors noticed that some epileptics did not have seizures during times of starvation. Thus starvation was considered a short term treatment technique for those suffering seizures. This observation goes back so far that it was even written about in the Bible. But the obvious drawback is you can't stay on such a treatment long term and still be alive.
In the 20s, smart doctors reasoned that a high fat diet might accomplish similar effects to starvation, ie ketosis. And it worked. Ketosis can eliminate seizures in 50% or more of siezure patients, similar to fasting/starvation. Metabolically, they are very similar.
But one obvious difference is you can maintain weight on the fat eating diet. Energy comes from the fat you eat instead of the fat on your butt! ;-) Other obvious diffs, when eating, you will need to funnel a lot of energy to digestion. Whereas in fasting, you may trigger some kinds of alertness and food gathering instincts and hormones. You may also have to deal with any side effects of fat leaving the fat cells includng (if such is really what happens) potential release of stored toxins.
on November 10, 2010
at 09:46 PM
can't imagine what one can be on 100% fat for - but that's perhaps due to lack of imagination
on a side note, as far as i can tell from reading the blog "carbsanity.blogspot.com", you can't "run on" dietary fat directly - at first, all consumed lipids get incorporated into adipose / muscle tissue, and released (as 'your own fat') only in an "on-demand" mode during time of negative energy balance, thus being an ideal "fat storage" fuel: carbs or protein require at least some energy to be converted into adiposity - that shattered my personal long-standing fat-centric paradigm! - before i knew the "truth" (=eat fat, a lot of fat, mostly fat) but now i'm in the "darkness"
on November 10, 2010
at 08:05 PM
You ask a very good question. To make sure I understand, what do you mean by metabolic differences? For this response, I will assume you mean blood lipids and glucose.
Before addressing your question, you should note that in a metabolically normal person (although this definition may be circular), blood sugar will stay between 80-120 mg/dl despite fasting. Your question anticipates this because increased gluconeogenesis partially compensates for decreased dietary intake (increases glucose supply) and ketosis lessens the body's demand for glucose (decreases glucose demand).
Although I hypothesize that eating more increases insulin secretion, I cannot find supporting data. The studies I found through PubMed and Google Scholar all require one to extrapolate beyond the scope of the study. For example say decreasing carbs from 30% to 10% causes a decrease in fasting insulin levels. Given human physiology's complexity (nonlinear responses, counter-regulatory mechanisms to maintain homeostasis), this observation does not tell us anything about what cutting carbs from 10% to 0% would do.
However, Peter at Hyperlipid details how different types of fat have unique effects on plasma lipids (triglycerides, chlyomicrons, etc).
Studying how the body reacts to a pure fat diet may unearth metabolic processes we didn't know about. However, since that is an extreme situation I am unsure how to reason to it or from it.
on November 10, 2010
at 09:10 PM
Nope. When the body senses starvation it goes to the easiest source of energy ... protein aka lean body mass which includes the heart muscle. Protein is easily converted BY THE BODY OVER to sugars for energy. This is why most anorexics are clinically obese because all the lean tissue has been scavenged. Hope I am remembering this correctly from my PT days. HTH, ~S