Fructose is essentially fruit sugar. It’s a monosaccharide like glucose that occurs naturally in plants, mostly fruits, but in other plants as well like beets, carrots, sugar cane and so on. The trouble is, poorly informed opinion is treating fructose like some sort of poison.
Metabolism of fructose
The body metabolises fructose a little differently to glucose. The liver (and muscles) can’t store fructose like glucose, so it has to do something with it. One pathway is to metabolise it to glucose, which then gets handled like any other source of glucose, and the other main pathway is conversion to fat, which seems to have everyone up in arms about the dangers of fructose. It’s amazing how the mention of the conversion of carbohydrates to fat gets the internet nutrition mob in a lather. (Hey, what about direct consumption of fat?) In any case, the dreaded high-fructose corn syrup (HFCS) and other sources of fructose have come under increasing focus. Table sugar, or sucrose, is 50% fructose and 50% glucose, and so is HFCS, more or less.
Okay, fructose has some other things about it that may not be great, like being a possible cause of insulin resistance and leptin inhibition. Leptin is the hormone that tells us when we’re full and signals us not to eat. Inhibiting leptin might increase our ability to overeat. Even so, as Paracelsus warned us: “the dose makes the poison.” In this case, not only does the dose of fructose makes the poison, but the dose of everything you eat makes the poison. I’ll explain.
Fructose can turn to fat, but so what?
A recent book about sugar (I won’t be giving it any publicity) made a statement to the effect that the fructose in one mouthful of apple juice will be turning to fat before you can finish your glass of juice. That’s a very naive statement, and anyway, so what? The pathway of fructose to fat is really only going to be lipogenic (fat creating) when the liver is replete in glycogen or stored glucose. In the reverse, that is, when the liver is low in glycogen, fructose will follow the metabolic pathway to glucose and be stored in liver, and even muscle, blood, brain and all the other places that glucose ends up, in the right circumstances. Fructose is not destined for fat storage as a matter of course.
And what if the liver is full and some fructose gets stored as fat? Fat storage is dynamic and this fat will get used for fuel just as any other fat you eat. Fat that results from the lipogenesis of carbohydrates is not magically labeled “never to be used again” for fuel, destined to forever glug over into your buttocks and belly. Mind you, the fat is going to be a saturated fat just like much of the animal fat you eat, so it’s not completely benign.
How much fructose consumption is healthy?
That depends on how you eat, exercise and your general weight management. People who are fat or obese and who eat too much of everything may have a worse response to fructose because of their poor existing metabolic circumstances. On the other hand, athletes or heavy exercisers can get away with much more fructose in the diet because it will be metabolised mostly to glucose, or, the balance of energy input and output will result in any fructose to fat conversion being inconsequential.
As a general rule, a diet of less than 10% fructose (energy equivalent) is probably going to be benign in most people. Most of the studies showing excessive lipogenesis and other adverse effects are in the range 15 to 20% of dietary calories from fructose. For example, for a 2000 kcal (8400 kjoule)/day diet, eating 500 grams of fruit (about 3 large pieces) at an average content of 5% fructose would mean 25 grams (100 kcals) of fructose and 5% dietary fructose per day. A few teaspoons of sugar in coffee or on cereal or other sweet things in moderation might see you reach 10% dietary fructose per day at the most — probably not ideal if you’re in poor shape and a little overweight, but not a disaster.
Soft drinks and juice the main sources of fructose
Where it gets serious is when you start taking in fluid sources of fructose and glucose. A can of soft drink or large glass of juice at 375 mls (13 fl oz) has about 10 teaspoons of sugar equivalent (or more) — even in some fruit juices, like grape, with ‘no added sugar’ on the label. In fruit juice, most of the sweetness will be from fructose. The sucrose in soft drinks is half glucose and half fructose. If you’re in the habit of knocking down two cans of cola a day, that’s about 20 teaspoons of sugar and 10 teaspoons or 40 grams of fructose equivalent. It’s easy to see how this, and other enticements for the sweet tooth, could get you up around the 15-20% fructose per day. Don’t do it. Soft drink consumption is increasingly associated with the obesity epidemic.
Athletes have more room to move
Allowing for the fact that some people do have a rare fructose intolerance, athletes and heavy exercisers can reasonably take in more fructose in the form of fruit and drinks and sugars in general. Many doing heavy training and competing in endurance sports have no working alternative to meet energy requirements. As long as the diet has a good balance of healthy starches and sugars, this is not a problem. Elite sportspeople, marathoners, triathletes and other heavy exercisers can take in quite a bit of fructose without getting fat. It’s simple ‘first law of thermodynamics’ stuff — inputs and outputs.
What about a fruit diet?
Here’s something to consider for the zealots who reckon fructose is some evil nutrient that makes us fat. What happens to fruitarians, who eat only fruit? A few people do eat like this. Their diet is probably going to be up to 50% fructose by energy intake. Do they get fat? What about animals (some primates) that eat fruit at 65% or more of the diet? Do they get fat? It’s going to be almost impossible to get fat on a fruit diet — a diet very high in fructose.
It’s another beat-up
Like many things in the health and fitness roundabout, something with a grain of truth to it has been extrapolated well beyond what is reasonable, to imply impending doom or to sell something. I’m no patsy for the sugar and soft drink industries, but if you balance fructose intake and attain normal weight and fitness, fructose will not be a problem, especially fruit consumption.