Fructose and lipolysis

You have to be very, very careful with fructose feeding papers. It is very easy to slant your methods to give strange and conflicting results. Some really weird stuff happens when you give a sugar which fails to trigger insulin secretion and itself rapidly turns in to fat. The combination of low insulin secretion and high fat production can end up looking very much like a genuine high fat diet! There are papers out there where this can be pushed to the point where fructose fed rats are slim, exquisitely insulin sensitive and apparently very health. Increasing starch/glucose alongside the fructose seems, generally, to produce more obesity. PUFA add a whole new dimension. So be careful...

Onwards.

Another confirmationally biasing paper:

Adipose tissue remodeling in rats exhibiting fructose-induced obesity

Here are the diets

Not too bad. Some changes between sucrose and starch but most of the other variables are pretty well held constant. The study ran for eight weeks. Here are the body compositions at the end:

The fructose fed rats carried 18g of extra fat, just over 12g of which were in mesentery and the epididymal fat pads. Visceral fat. The fructose fed subcutaneous adipocytes had an average volume of 25,200μm3 vs 40,950μm3 in the controls. The situation is reversed in the visceral adipocytes, fructose fed are 28,540μm3 vs 19,870μm3 in the controls.

So, are FFAs being released from adipocytes under the influence of fructose, being picked up by the liver, repackaged in VLDLs and stored in visceral adipocytes long term? Well, as far as I can find, no one has done the tracer studies to check this. We do have these measurements in this paper relating to lipids:

Those elevated FFAs along with elevated fasting triglycerides are both suggesting routes in to and out from the liver respectively. I also rather like the elevated lipid peroxidation, this is not happening to palmitate!

So it's all very suggestive that fructose might be working on subcutaneous adipocytes much the way that alcohol does. I suppose it could be acting on all adipocytes, subcutaneous and visceral, just the repackaged FFAs are targeted to visceral adipocytes, hence the overall shift in size differential. Just as neat vodka makes you thin so a very high fructose diet should do the same. Adding in more starch and/or glucose should go more towards the beer belly look.

Of course you could just argue that fructose or ethanol simply generated lipid in the liver which was shipped out destined for visceral adipocytes. Until you look at the alcohol tracer study and realise that it is certainly not that simple for ethanol. I just have to wish that someone had done a similar tracer study on fructose feeding. Can't have everything I guess.

Peter