Probably unwise for me to challenge you as this is your field and definitely not my field.
Almost all fat in humans comes from diet. De novo lipogenesis is possible but is rare in humans
http://www.ncbi.nlm.nih.gov/entrez/...ve&db=PubMed&list_uids=10365981&dopt=Abstract
I post this abstract in its entirety because (1) it is short; (2) I want to ask about section I made bold; and (3)most reading here will not go to the link:
"
The enzymatic pathway* for converting dietary carbohydrate (CHO) into fat, or de novo lipogenesis (DNL), is present in humans, whereas the capacity to convert fats into CHO does not exist. Here, the quantitative importance of DNL in humans is reviewed, focusing on the response to increased intake of dietary CHO. Eucaloric replacement of dietary fat by CHO does not induce hepatic DNL to any substantial degree. Similarly, addition of CHO to a mixed diet does not increase hepatic DNL to quantitatively important levels,
as long as CHO energy intake remains less than total energy expenditure** (TEE). Instead, dietary CHO replaces fat in the whole-body fuel mixture, even in the post-absorptive state.
Body fat is thereby accrued, but the pathway of DNL is not traversed;*** instead, a coordinated set of metabolic adaptations, including resistance of hepatic glucose production to suppression by insulin, occurs that allows CHO oxidation to increase and match CHO intake. Only
when CHO energy intake exceeds TEE does DNL# in liver or adipose tissue contribute significantly to the whole-body energy economy. It is concluded that DNL is not the pathway of
first resort #for added dietary CHO, in humans.
Under most dietary conditions, the two major macronutrient energy sources (CHO and fat) are therefore not interconvertible currencies;## CHO and fat have independent, though interacting, economies and independent regulation. The metabolic mechanisms and physiologic implications of the functional block between CHO and fat in humans are discussed, but require further investigation."
It seems to me, that this alone does not support your claim that humans rarely make fat, DNL, because it is speaking ONLY about the conversion of CHO to fat.
BTW, someone, perhaps BM, was critical of your tendency to quote from a reference instead of give your POV. I, in contrast, praise you for this. When I doubt what you are stating, as was the case this time, since I know cows eat grass and make a lot of fat, even in their milk they yield, I went to the original (your reference). - Much better than just giving an unsupported POV.
Are humans so different from cows that they can make fat from grass and we can not make it from our much more varied range of foods? Is not the cholesterol, that I know humans make, as well as get from food they eat, not a fat? After reading your reference, I accept that eating boiled potatoes (my idea of nearly pure CHOs) will not make me accumulate directly fat if I do not exceed myTEE (but may by displacing some of my energy requirements, might do so indirectly if my TEE is exceeded.) but I am not ready yet to accept the idea, that by no pathway do humans make significant fat when their caloric intake far exceeds their energy requirements, even if there is little fat in their diet. e.g. high volume of sugar in diet.
In part I am biased to this POV by what I understand to be evolutions reason for bodies making fat. - I.e. for most of human history, there were some lucky days with abundant food, followed by several with very little - making and storing fat being evolution's discovery as to how to efficiently cope with this variation. For example, Are you saying that when a primative man got lucky and found a hollow tree the wind had blown over was full of honey and ate all he could hold for two days until it was all gone that it did him no good during the next four days when he had nothing to eat?
---------------------------------(notes/comments on the bold I added to the abstract)
*are there other “pathways”?
**seldom does for big eaters, who get fat.
***clearly some other “pathway” is.
#(two parts taken together) seem to state that if you eat a lot more than you need for energy requirements, then you will do DNL (make fat) as a “second resort” (no fat people in concentration camps is all this is explaining?)
## does this, together with the foregoing, not clearly define that by “under most dietary conditions” the authors mean "caloric intake less than TEE"? - I understand that to be their meaning. See (2) below.
SUMMARY I think:
(1)the articles “under most dietary conditions” is referring to a diet in which the caloric intake is not greatly in excess of TEE, but that is not the typical diet of someone who is significantly or grossly over weight (a “fat person”).
(2)You have over generalized what is being said with the claim “humans rarely make fat” perhaps as you had a different POV as to what article is assuming as “under most dietary conditions” than I do.
Could you comment a little more (again with reference links, in case I still do not take your word for it)?
