From what I've read, fructose stimulates hunger (causing you to want to eat more of it) and is processed by the liver directly into stored fat. That is not what my question is about, because those both make sense to me from an evolutionary standpoint: the plant wants its fruits to be eaten and seeds spread, and the body wants to store extra energy during times of plenty (when fruits are in season) for later when times are scarce.
Why, exactly, is fructose hepatotoxic? Is it because we are consuming it in such great quantities, greater than we ever would have encountered in natural, wild fruit? Or is there some sort of evolutionary benefit to either us or the plant? Is it a chemical defense, like capsaicin? Just curious.
If you are liver glycogen replete and continue to eat fructose in excess, your liver will attempt to convert it into fat to pass around the body. If you overeat fructose chronically, excess fat will begin to accumulate in your liver, causing non-alcoholic fatty liver disease.
This happens when you chug a lot of soda (or some other pure sugar "food") chronically, something which is entirely outside everything's evolutionary experience. If you are exercising constantly fructose can help you quickly replenish liver glycogen (a lot of those carb replenishment goos for endurance athletes are half or a third fructose for this reason), but most people aren't working at that level.
Incidentally, it's incorrect to say that "fructose is bad for you". Like all things, fructose is context sensitive and ranges from beneficial to benign to bad. If you eat it in excess, as you may if you are eating SAD without thinking about it, bad things can happen, but if you're incidentally consuming it as part of a healthy diet then it's probably not an issue.
Read this story:
Hominids about 15M years ago developed a mutation in the way they metabolized fructose, which is why it goes straight to fat in humans today. At the time, it proferred an evolutionary advantage, perhaps because our ancestors were leaving the jungle and moving to the savanna, where fruit would be seasonal.
In any case, other mammals do not metabolize fruit the way we do. We pack it away into fat for a rainy day. Other mammals use it for immediate energy. Our strategy made perfect sense when fruit was only available at a few select times per year. But now, it's a recipe for disaster. Compound that with our relatively recent discovery of cane sugar and HFCS, and we are loading up on orders of magnitude more fructose than we were evolved to handle.
HFCS "high fructose corn syrup" commonly called "fructose" the processed sugar contained in most sodas is really bad for your health for a number of reasons including the empty calories associated with soda consumption. Fructose in whole fruits is not bad for your health, unless you still need to lose weight or have metabolic syndrome. Fruits have been a main staple of primates and humans for millions of years and fruit is an essential component in almost any hunter-gatherer diet!
Though technically a monosaccharide, practically speaking, we may as well think of fructose as a lipogenesis hormone. Talking about fructose as any other "carb" is far too simplistic.
HFCS is comparable in fructose:glucose ratio to sucrose and honey, both "natural" sweeteners considered healthier than HFCS. It is also comparable in Glycemic Index. If there is a problem with HFCS, it may be toxic processing, not the fructose.
I just want to make sure that everyone knows that a good portion of one of Dr. Harris's most recent posts gives his position on fructose. It's here. I would suggest reading the whole thing, not just the section marked "fructose," in order to get the context. Sorry if someone's mentioned this on another thread, recently I haven't been reading paleohacks as obsessively as usual.
Are you asking about the biochemistry of why it's bad? If you want to know the nitty gritty, watch this lecture, it's amazing: http://www.youtube.com/watch?v=dBnniua6-oM
But in brief, it's a toxin--by definition--because the liver is the only organ that can deal with fructose (in contrast to glucose, which can be used by any cell in the body).
Fructose in fruit is not actually good for you. The only reason that it's okay to eat fruit is that fruit contains a healthy dose of fiber along with the fructose, which prevents some of the fructose from being absorbed during digestion. Fructose in moderation won't kill you, but it's not, strictly speaking, "healthy."
Also, just FYI, you can order powdered dextrose (which is pure glucose--I recommend http://www.barryfarm.com/) to use as an alternative sweetener. It's still sugar, but it won't fry your liver.