Thanks so much for your very detailed response to my question. It is incredibly helpful, and I'm sharing it with pretty much everyone I know. One very brief follow-up question though, if you wouldn't mind indulging me:
Excess carbs obviously lead directly to storage. But is it not possible for the body to convert and store fat and/or protein as well? In other words, a normal/reasonable amount of fat, protein, and cholesterol won't lead to fat conversion and storage when that same amount of carbs would. However, is this still the case if one consumes a serious excess of fat protein and cholesterol? (as might happen, say, when I spend a few hours at my favorite Brazilian steakhouse, Fogo de Chao).
Much thanks once again!
No problem and I like your follow-up questions too; I'm going to think this through as I type and explain kinda like an impromptu lecture, so enjoy! :-)
It is possible for the body to store excess fat and protein consumption as fat. Here's how; we'll look at fats first;
When we consume fats we consume them usually in the form of triglycerides. A triglyceride is composed of 3 (three) fatty acid side chains (hence the tri) attached to a glycerol backbone. While beta-oxidation of the acyl side chains will release acetyl groups that will shuttle into the Kreb's, this will also release the glycerol backbone. The glycerol backbone can indeed be transformed to glucose via gluconeogenesis.
It needs to be re-emphasized that gluconeogenesis is not the reversal of glycolysis; that gluconeogenesis occurs mainly in the liver and to a lesser extent in the kidneys (these are organs which are essential to glucose homeostasis); and that gluconeogenesis ends with glucose-6-phosphate and free glucose is not usually generated, but it can be, say after sleeping 8 hours (nighttime fast) or prolonged starvation. And finally we need to remember that the usual fate of glucose-6-phosphate is conversion into glycogen. Any glucose released will be used by the brain (but remember, the brain prefers ketone bodies for function if given a choice), or any other organ that needs glucose; there generally will not be enough glucose released to create a long chain fatty acid to be stored in the adipocyte, nor will the physiological signals be present to support this.
But we need to remember that the question pertains to the over consumption of fat, so we certainly aren't fasting in this scenario. Over consumption of fat is very difficult to do despite what everyone thinks. Now, I am not talking about eating a double cheeseburger with the bun along with a regular coke and fries; unfortunately a common meal out there these days. Which, by the way, also has an overabundance of carbohydrates. I'm referring to eating a bunch of bacon, sausage, butter, rib eye steaks, non lean burgers with cheese (with no bun or regular coke), or say four eggs or so...man, I'm beginning to drool...
Remember the scenario above with the eventual release of the glycerol backbone which could be converted into glucose (this, of course, is occurring intracellularly, not extracellularly). Now here's where we run into some trouble. Assuming one is a glutton and eats a mega amount of bacon, what is really going to happen. Yes, alot of fat was consumed; yes, there could be a large release of the glycerol backbone which can be converted to glucose-6-phosphate; will there be a release of glucose extracellularly or does there even need to be a release? Why can't the glucose-6-phosphate created just start shuttling down the glycolytic pathway to pyruvate, then to acetyl CoA and we all know the rest.
The answer lies in the fact that the signals for the creation of fatty acids will not be there. Remember, we ate no carbs and all fat (of course protein too which we'll analyze later). What are the signals needed? Overabundance of glucose (there is none), relative increase in insulin release (there was none), relative decrease in glucagon concentrations (the exact opposite occurred as more glucagon is secreted); so with an over consumption of fat it is still very difficult physiologically to get fat creation from fat consumption, so long as not too many carbs were consumed with the fat, as is the case in our scenario.
Another fact about eating alot of fat; fat consumption is a natural appetite suppressant. We think it has to do with the release of cholecystokinin, but the fact remains; when we eat alot of fat, we are fuller longer; satiety prevails. When we are fuller longer we dont eat as much..so we are giving the body nothing to store. Another problem our fat glutton runs into is either malabsorption of the fat or ensuing emesis due to their gluttonous behaviour, both of which will not allow any absorption of any of the fat at all.
So I guess my final answer is over consumption of fat could in theory lead to fat storage but it is extremely difficult to do since the biochemical signals favoring a glycolytic run into fat will not be favored when we over consume fat. (Can anyone say 'Run-on sentence' LOL)
Now let's look at protein over consumption;
Proteins are broken down into amino acids. These amino acids can be either glucogenic, ketogenic or have qualities of both glucogenic & ketogenic. Glucogenic amino acids will be converted into glucose and ketogenic amino acids can be converted into ketones or fatty acids. So right away we can see that over consumption of proteins containing predominantly ketogenic amino acids (only leucine and lysine are strictly ketogenic) can give rise to fatty acids. An amino acid is referred to as ketogenic if degradation gives rise to Acetyl CoA or Acetoacetyl CoA. These two species can be transformed to fatty acids as we discussed in an earlier post. Amino acids giving rise to pyruvate, oxaloacetate, fumarate, succinyl CoA or alphaketoglutarate are termed glucogenic as they can be used to create glucose. Fans of the Kreb's Cycle will immediately recognize oxaloacetate, fumarate, succinyl CoA and alphaketoglutarate as intermediaries in the Kreb's, a simple reminder that these amino acids are glucogenic.
I can certainly go through the degradation pathways of gluco/ketogenic amino acids, but I feel it is beyond the scope of this post :-) Of course I find it very exciting and anyone who wants to review the degradative pathways of amino acids further will be referred to pp. 690-696 of Stryer's (now really Berg, Tymoczko & Stryer) 7th Edition of Biochemistry, my personal favorite source for biochemical facts.
To summarize, we can see that it is quite difficult to create fat from the consumption of fat. Indeed this flies in the face of what most self-purported nutritional experts proselytize, that eating fat makes us fat; we see from a simple review that this is biochemically difficult to do. But over consumption of protein, in the form of the mixed or ketogenic amino acids will lead to the creation of fatty acids. But then this question is immediately raised by myself; since we over consumed protein to get to this stage; there will be low glucose, low insulin, increased glucagon, hence all the signals to favor B-oxidation of free fatty acids, not creation and storage of fats, so now we are right back to where we started...gosh I love this stuff :-)
Also, just as we will either vomit or poop out (mainly poop thank goodness) some of what we ate, a portion of our protein gluttony once again will not even have a chance for absorption. So while the question asked what happens when we over consume protein and/or fat and can this lead to the storage of fat, the short answer is that all depends and it depends mainly on us over consuming carbs with the protein/fat indulgence. Over consumption of fat/cholesterol without carbs can cause a fat buildup but is extremely unlikely due to the signals favoring fatty acid degradation and not storage.
I hope I didn't confuse anyone too much. Oh yeah, anyone know a good Brazilian Steak House on Long Island, NY. :-)
Have a great rest of what remains to our weekend everyone and as always thanks for reading and keep those biochemical questions coming!