Much has been written about fructose in the past few years as it has been postulated that it may be linked to the rise of obesity, type 2 diabetes, and the metabolic syndrome. Observational studies have linked high-fructose corn syrup with the obesity epidemic in the U.S. over the past 30 years. In addition, animal studies have shown that a diet high in fructose can induce obesity, insulin resistance, high triglycerides, high blood pressure, and high uric acid. So should we start a campaign to eradicate fructose from the face of the earth? Not just yet.
Published this month in the Annals of Internal Medicine is an extensive review and meta-analysis of all the published trials looking at the effects of fructose on body weight. They started with 1,984 trials and narrowed it down to 41 trials that were reviewed in detail. They concluded that fructose does not seem to cause weight gain when it is substituted for other carbohydrates. Weight gain is only seen when it leads to an overall increase in calories. So it seems that it’s more an issue of overconsumption than it is carbohydrate selection. A limitation of this review is that most of the studies out there are of a very short duration so it’s hard to say what would happen if you maintained a high fructose diet for say, a year. Another big limitation is that in an attempt to isolate the effect of fructose itself they excluded studies that used high-fructose corn syrup or sucrose (table sugar).
Anyway, I thought we might mention a few facts about the simple sugars.
The simple sugars are called monosaccharides or disaccharides. The main monosaccharides are:
The main disaccharides are:
- sucrose: one molecule of glucose bound to one molecule of fructose. This is table sugar extracted from sugar beets or sugar cane.
- lactose: one molecule of glucose bound to one molecule of galactose. This is the sugar found in milk.
- maltose: one glucose bound to another glucose. This is malt sugar, you know like a malt shake; yummy! 3 glucose together make maltotriose. More than 3 glucoses together are called maltodextrin. A bunch of maltodextrin bound together forms starch.
As you see from the chart below, fructose is the sweetest sugar of them all.
Fructose (fruit sugar) is the main sugar in fruit. It also occurs naturally in vegetables, honey, and maple syrup. All sweeteners are a mixture of glucose and fructose either bound in the form of sucrose (table sugar) or in free form. So at the end of the day, it all turns into glucose and fructose. This includes, honey, maple syrup, corn syrup, high-fructose corn syrup, table sugar, brown sugar, evaporated cane juice, molasses, and agave nectar.
High-fructose corn syrup (HFCS) has received a lot of attention because since being introduced in the U.S. in the early 1970’s, it’s use has sky-rocketed. See the chart below.
HFCS has become the most common added sweetener in processed foods and sodas mainly because it’s cheap to acquire and manufacture as compared to sucrose. It’s also 1.5 times sweeter than table sugar so you can get away with using less and it keeps food moist.
As the name implies, it contains a higher amount of fructose. However, if you look at the percentages, it’s actually just a little higher than honey (55% vs. 50%) and actually agave nectar has even more fructose! (56-92%). Ouch!
Here is the problem with fructose as compared to glucose: As opposed to glucose (and galactose) which are under strict metabolic control, fructose is not. It gets immediately broken down in the liver where it is either converted into glucose or fatty acids (triglycerides). There is no storage form of fructose, so a lot of it gets turned into fat. The proponents of fructose will tell you that fructose has a low glycemic index (which normally that’s a good thing) and this makes sense; it doesn’t raise blood glucose much when you eat it. What happens instead is that it turns into fat. So I suppose if we had a “fat index” it may turn out to be a chart topper!
The chart above shows how the use of HFCS has increased over the years and it parallels almost exactly how the waist line of America has expanded as well. However, take a look at total sugar consumption, it has also increased! The studies implicating fructose and specifically HFCS are mainly observational studies which are quite limited and also small studies done in rats. This large meta-analysis I reviewed suggests that this possible connection between obesity and HFCS may be more due to the fact that our total sugar consumption has increased as opposed to just fructose consumption.
In conclusion, I think the lesson here is that we need to pay attention to our intake of carbohydrates and especially the simple sugars. When your body has enough glucose it doesn’t just get rid of the excess, it stores it in the form of fat as any organism trying to survive would. My thoughts on fructose are that it is an unregulated substrate in the body and I think we were not meant to handle big gobs of it.