As I implied in my comment above, this appears to be overkill to me. Fructose is not poison, no matter what terms are thrown around. Eating too much fructose is obviously bad for many reasons (liver health, AGEs, absorption, etc). But the dose makes the poison, and fructose is not an alien substance that the body can not fathom. For example, we have set metabolic pathways for it...people have eaten it since humans have been alive...we are not morbidly obese people or gravely ill people who need to dissect grams of each nutrient.

This is falling into the same trap as zero-carbers and fruitarians fall into (sorry for that last comparison!) 4 grams of fructose a day is tiny, and worrying about fructose in onions is a slippery slope. Our bodies are quite robust, especially if they haven't been irrepairabily metabolically deranged. If you're looking for thresholds for fructose, my guess is it shouldn't be in the 4 gram range, but more like the "couple pieces of fruit" range, unless you have some illness or feel acutely bad eating it.

It is my belief that fructose is the most lipogenic naturally-occurring substance that we encounter. There's a whole host of problems ranging from adiposity to dyslipidemia to the creation of AGEs. These are obviously much worse with large amounts of unbound fructose, as occurs in products containing HFCS. Honey also technically has a large amount of free fructose, but there are other factors that supposedly mitigate that fact. If I were working for a pharmaceutical company and was tasked with creating obesity in a pill at low cost to the company, I would just make a pill that was pure fructose. That's one sugar pill that wouldn't be a placebo.

I'm of the opinion that we should count fructose as being more of a dense carbohydrate inasmuch as lipogenesis is concerned. I'm not sure how to fully quantify the effect, but I count grams of fructose as 3-4X that of starch. While it's true that insulin will store extra glucose (that which can't be stored in muscle or the liver as glycogen) as fat, there is just more going on with fructose.

I think this is an excellent piece about how we've evolved alongside fructose: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2917125/

I get only what fructose is in spinach and potato currently because I feel that the best "bang for your buck" when it comes to fat loss comes from first decreasing fructose as low as possible, then decreasing starch to where it is supplying only enough glucose to replete glycogen stores.

There is no minimum daily requirement for fructose, and really, any amount of it will be more toxic than the equivalent amount of glucose.

Edit: I suppose my primary goal in all of this is longevity, and as such, I'd like to stack the deck in my favor to the greatest extent. I have mechanisms whereby the DNA damage from ionizing radiation produced by diagnostic x-rays can be repaired. Does this mean that I should be cavalier about x-rays and radiation in general simply because my body is robust enough to handle the occasional x-ray? No, I'd rather avoid them unless I break a bone or something so I have that much more chance of not developing cancer. This is the same motivation that leads me to cook meat at low temperatures. It's not going to kill me today or tomorrow if I don't, but who knows what that effect is over decades.

20g of fructose a day doesn't sound like a lot, but that 15g difference over 80 years is 438,000 grams of fructose. That's a lot of AGEs. I rather think that makes a difference.

Those saying "you'll have to pry the fructose from my cold, dead hands" may be creating something of a self-fulfilling prophecy.

50g/day; not by coincidence (since both are metabolized by the liver), this is the same amount of alcohol that is associated with increased risk of nearly all types of cancer

For many of the newer paleohacks here, Dr Lustig at UCSF School of Medicine, Pediatric Obesity specialist, has a good lecture which covers the evils of fructose that is a couple of years old. http://www.youtube.com/watch?v=dBnniua6-oM

There is a lot of chemistry that is way over my head...so I just accept the fact that table sugar is half fructose and half glucose. I avoid all the plain sugar and all the sugar words ending in "ose".

Consumption of fructose brings on the following over a lifetime:

An increase in visceral fat, the kind that embeds itself between tissues in organs

Less sensitivity to insulin, one of the first signs of diabetes

Increased fat production in the liver

Elevated LDL (bad) cholesterol

Increased levels of triglycerides http://www.sixwise.com/Newsletters/2009/April/29/Glucose-Fructose-Sucrose-Whats-the-Difference.htm

And fruit has some naturally occuring fructose so I also avoid. It is way too sweet for marketing purposes due to being hybridized and our ancestors never could eat fruit year around...except for the few folks in the tropic regions and Africa.

Visceral Fat and non alcoholic fatty liver disease are nothing I want to have anything to do with. I have been working on my VF for 3 years since going paleo and I am winning....almost. The project is not yet complete, though.

My general philosophy about such experiments is to start with complete elimination. If you have some sort of intolerance to a substance itself or some common dietary item you eat that carries it, you might not feel a big difference unless it is totally gone. I can't remember where I read this idea recently, but toxins in the body may have a superlinear effect with dosage. The bonus with this method is that your body can get really clear of many substances, and when you reintroduce them you might see a sharp difference you wouldn't necessarily see with just a reduction.

I'm also influenced by my own experience of a vast difference in my own health between a very low carb diet and a zero carb one.

Anyway, however you decide, best of luck. Let us know how it goes.

P.S. What kind of magnesium do you take? A few months ago I changed from oxide to citrate, and it was amazing. I actually felt almost euphoric for a period of a few weeks, after which the same dose became too much and I settled down at a lower dose.

I think a good chunk of it depends on individual body types and tolerances. I have some friends who eat tons of fructose laden foods, who don't get cravings after eating them and are still easily able to lose weight. For myself, if I have too much fructose I end up going into a carb craving and its also much harder for me to lose weight. I am fairly active, I work out 5 times a week for 45 minutes (crossfit and kettlebells), but I find that I still need to be careful about my fructose intake.

The bad news in all of this is fructokinase. Read The Fat Switch by cardiologist/renal specialist, Richard J. Johnson, M.D. It explains that excessive amounts of fructose (and sucrose--table sugar--is 50% fructose) in our diets is the problem. There are relatively small amounts of fructose in fruits (a small Valencia orange has a little over 2 grams of fructose) but a 12-oz. can of orange soda has 26 grams of fructose in a total sugar load of 44 grams. THAT is the problem. No one pigs out on 10 oranges in one sitting (or even in a day) but it is easy to quaff a can of orange soda in one sitting (that is, if you like fake orange flavoring--yuk). Fructokinase is the enzyme that is used by the liver to assimilate fructose and most of the destructive metabolic problems come from the action of this enzyme in raising uric acid levels. Because humans lack the ability to lower their own uric acid levels---we don't have uricase--high uric acid creates a host of problems in the body. Any substance taken in excess can be potentially lethal (remember the deaths that occurred from drinking too much water?) A couple of servings of fruit have health benefits that make taking the fructose hit worth it. Fruit can have a powerful healing effect on the body, just as vegetables do. I personally eat a lot more vegetables than I do fruit---I listen to my body and it tells me to eat a lot of vegetables and not so many fruits.

I use to consume tons of tomatoes and onions because I was under the dumb belief that they contained no fructose, they were like candy to me. I would down about 2 whole onions steamed and seasoned with sea salt, cayenne pepper and lemon. Now I know better.

So, what CAN I eat if I can't have carbs and apparently I now have to stay away from fructose-containing fruits and veggies...

There's no way I can get enough calories for the day from protein and greens alone...

if you're super sensitive to fructose, maybe its worth cutting them all out. I'm mongolian and our ancestral diet consists of nothing but meat (lamb, beef) and dairy (lactose, which is glucose + galactose) with the occasional wild vegetable (fibrous, little fructose) and seasonal tart berries (again only seasonally). i used to be morbidly obese and i lost the weight getting rid of all fructose sources meticulously. i still eat white rice and milk as carb sources, but little vegs and no sweet foods