TLDR: If resisting insulin is rendered impossible due to very, very high PUFA diets then mitochondria resort to uncoupling.
reiterated the well known essentiality of FFAs for the activation of UCP-1 and added in the significant role of reactive alkenyl species. In more common parlance that would be lipid peroxides, primarily those derived from linoleic acid. I thought about it in this post.
This is the next step, which I specifically went looking for because this is how I think life should be organised. It is a perfect example of me having a huge bias, knowing what I want, and going looking for it. You have been warned.
It is utterly clear that UCPs in general suppress ROS generation and that some members of the family are not being used for thermogenesis at all, more likely for prevention of the generation of supra-physiological levels of ROS.
We have an interesting situation where PUFAs, the prime pathology of which is their failure to generate adequate ROS, are also extremely effective at facilitating uncoupling, which needs significant ROS generation to happen. Where does this superoxide come from?
This is the core question.
Warning: I think step f) is the one I have least evidence for. Life is like that sometimes.
Okay. The story so far:
Caloric ingress in to a cell is controlled to an appropriate level while calories are freely available by limiting insulin signalling.
Insulin signalling is curtailed by driving electrons in reverse through complex I to generate superoxide and hydrogen peroxide which disable/limit said insulin signalling.
This reverse electron transport rises in direct proportion of FADH2 supplied compared to the amount of NADH.
The presence of double bonds in fatty acids limits FADH2 generation, limited FADH2 generation limits the control of insulin facilitated caloric ingress and continued insulin signalling diverts calories in to storage.
I have to accept that not all excess calories will be stored, some will continue to enter the electron transport chain as both NADH and FADH2. If electrons enter the electron transport chain when there is a low (relative) demand for ATP then
a) mitochondria membrane potential will rise, a direct consequence of the inability to limit electrons entering the ETC through complex I and multiple FADH2 entrance points. Protons are pumped but not used
b) the cytochrome C pool will become progressively more reduced, limiting it's ability to accept electrons from complex III
c) electrons will be transported through the cytochrome b centre of complex III back on to the CoQ couple
d) these electrons will replace those absent as a result of double bond containing fatty acid oxidation and will restore superoxide production
e) superoxide production, under these circumstances, is being facilitated only at the cost of high membrane potential secondary to low ATP synthase activity in parallel to excess calorie storage
f) if the double bond content of the lipids providing input to the CoQ couple from beta oxidation is very, very high then the need for negative feedback through complex III will be very, very high as well and will be associated with an excessively high membrane potential
g) the answer to excess membrane potential is uncoupling
h) superoxide positively regulates the activity of UCP-1 as do superoxide derivatives of LA
i) uncoupling allows the flow of electrons down the electron transport chain and lowers membrane potential, it immediately stops reverse electron transport/superoxide generation and converts the excess calories entering the cell in to heat, water and carbon dioxide
Summary:
Normally regulated calorie ingress to a cell occurs when linoleic acid is low.
Excess ingress from LA is dealt with by diversion to storage, leading to obesity.
Marked excess is dealt with by uncoupling which does not "require" obesity.
There are no hard boundaries between the above conditions, there simply appears to be an inverse "U" shaped curve in response to the content of linoleic acid in the diet. Such curves are common in nature.
A few last thoughts:
Simply looking at the experimental data the situation appears to be that including linoleic acid at levels between somewhere just under 10% up to somewhere in the high 20%s will facilitate excess insulin action and excess lipid storage. As we increase the supply of linoleic acid through the 30%s and in to the 40% region of calories then uncoupling becomes the primary molecular solution to a progressively more intractable problem.
It is worth pointing out that in a given mitochondrion there are lots of CoQ/CoQH2 molecules, lots of beta oxidation sites and, while the calculation of FADH2:NADH is useful for insight as to how RET works for a single fatty acid molecule, the mitochondria are integrating lots of FADH2 production sources, lots of NADH sources and are continuously monitoring the membrane potential (which is an integration of energy input/need) as well as the redox state of the CoQ couple.
What happens on a whole organism basis is also an integration of all of these effects. The mechanism for weight loss in high PUFA diets is both comprehensible and plausible.
The studies are real, the data are real.
Personally I still wouldn't go there, but understanding them is key.
Peter




































