Nutrition fads come and go. Certain foods and diets get demonized or exalted, often without appropriate scientific or practical evidence or justification. Promoters of such things are often very vocal and get plenty of publicity in the mainstream and scientific media. Fructose is one of those demonized foods.
In certain sections of the health, fitness, and weight training communities, fructose has become a "bad guy" -- along with carbohydrates in general. Fructose is blamed for causing obesity, liver damage, diabetes, and metabolic syndrome. Here's what you need to know about fructose.
What is Fructose?
Fructose is fruit sugar. It’s a monosaccharide (single carbohydrate molecule) like glucose, and occurs naturally in plants -- mostly in fruits, but also in other plants such as beets, carrots, corn, and of course, sugar cane and honey. The trouble is, poorly informed opinion says that fructose is a poison that requires regulation, just like alcohol.
Metabolism of Fructose
The body metabolizes fructose a little differently than glucose. Glucose is another simple carbohydrate in fruit and in starchy carbohydrates as you digest them. The liver (and muscles) can’t store fructose like they can glucose; the liver has to do something with it. One pathway is to metabolize it to glucose, which the body then handles like any other source of glucose -- mostly storage -- and the other main pathway is conversion to fat. Any mention of the conversion of carbohydrates to fat seems to get the fad dieters in a lather. (What about direct consumption of fat?) In any case, the much feared high-fructose corn syrup (HFCS) and other sources of fructose have come under increasing scrutiny. Table sugar, or sucrose, is 50% fructose and 50% glucose, and HFCS has a few percent more fructose and less glucose.
Effects of Fructose
Fructose has some other qualities that may not be healthful in excess, such as 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 propensity to overeat. Even so, as Paracelsus, the father of toxicology, warned us: "the dose makes the poison." Here's why.
Fructose Can Turn to Fat, But So What?
A recent book about sugar 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 an incorrect and misleading statement, and ultimately, so what? The pathway of fructose to fat is really only going to be lipogenic (fat-creating) when the liver is replete in glycogen (stored glucose). However, when the liver is low in glycogen, fructose will follow the metabolic pathway to glucose as a priority and be stored in your liver, and eventually in your blood, muscle, brain and all the other places that glucose ends up in a normal metabolism. 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 like any other fat you eat. Fat that results from the lipogenesis of carbohydrates is not magically labeled "never to be burnt again" for fuel, destined to forever glug over into your buttocks and belly. Ultimately, it's how much you eat of everything that makes you fat. Even so, this 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 overweight 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 metabolized mostly to glucose and fuel, or the balance of energy input and output will result in any fructose to fat conversion being inconsequential.
As a general rule, a diet that includes less than 10% fructose is probably going to be benign for most people, especially when fruit is the main source. The American Heart Association recommends that people consume only five percent of calories as added sugar, meaning sugar from the bowl or added to cakes, sweets and drinks. Consumption of fructose in fruit and juices could bring you up to 10%. Most of the studies showing excessive lipogenesis (fat creation) and other adverse effects are in the range of 15 to 25% of dietary calories from fructose. For example, for a 2,000 calorie per day diet, eating 500 grams of fruit (about 3 large pieces) at an average content of 5% fructose would mean 25 grams (100 calories) of fructose and 5% dietary fructose per day. A few more 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.
Soft Drinks, Pop, and Juice the Main Sources of Fructose Excess
Where it starts to get serious is when you start taking in fluid sources of sugar and fructose. A 13 fl. oz. glass of soft drink (about a regular can) or juice has about 10 teaspoons of sugar equivalent -— and so do some fruit juices, like grape, even if they say "no added sugar" on the label. In fruit juice, most of the sweetness will be from natural fructose. The sugar in soft drinks is sucrose -- half glucose and half fructose. If you’re in the habit of knocking down two cans of pop 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 Choice
Even 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 sugary drinks and sugars in general. Many athletes in heavy training and competing in endurance sports have few working alternatives to meet energy requirements. As long as the diet has a good balance of healthy starches and sugars, this is not a problem.
For bodybuilders, body shapers, weight loss dieters, and others with much less energy output in physical activity, added sugars and sugary foods and drinks should be the first to go, with the exception of fruit. The fructose in fruit is bound up in fiber to some extent and is released more slowly, which provides the liver with time to process it more evenly compared to soft drinks or juice for example. In addition, the antioxidants and phytonutrients in fruit are important nutritional constituents.
Frugivores and Fruitarians
Here’s something to consider. What happens to frugivores (fruitarians), who eat only fruit or predominantly fruit? A few people do eat like this and so do many animals, including some monkeys and primates. Do they get fat or ill? Do they get metabolic syndrome? Their diet is going to be up to 50% fructose by energy intake in peak season. 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. Fructose in fruit has been an important part of the nutritional evolutionary history of Homo sapiens. If you balance fructose intake and attain and maintain normal weight and fitness, fructose will not be a problem, especially fruit consumption.
Sources:
Evidence-based review on the effect of normal dietary consumption of fructose on blood lipids and body weight of overweight and obese individuals. Dolan LC, Potter SM, Burdock GA. Crit Rev Food Sci Nutr. 2010 Nov;50(10):889-918.
Consuming fructose-sweetened, not glucose-sweetened, beverages increases visceral adiposity and lipids and decreases insulin sensitivity in overweight/obese humans. Stanhope KL, Schwarz JM, Keim NL, et al. J Clin Invest. 2009 May;119(5):1322-34.
