Not all sugar is created equal. (Photo by anyjazz65)
Compared to glucose, which sounds like something extracted and processed in some dull laboratory, fructose has a nice ring to it: You instantly think of bundles of fresh fruit or a refreshing glass of fruit juice. Those couldn't possible be anything but healthy, could they? It's the glucose that is the bad sugar, right?
Well, not exactly. Excess sugar in general is bad for you, but there are differences between fructose and glucose, and not all of them are in favour of the former. Spesifically, it is the effect of fructose on triglycerides that is of most concern. In this post, I review some of the research on sugars, cholesterol and triglycerides.
Studies on rats
This rat study from 2000 compared the effects of different carbohydrate sources on blood triglycerides, total cholesterol and HDL cholesterol. The rats were either given a free or 60% restricted access to one of the sources (cornstarch, sucrose, glucose, fructose or a fructose-glucose mix), so the study effectively compared both a calorie-restricted version and an ad-lib version of all five diets. The authors conclude that the source of dietary carbohydrate did not have a significant effect on body composition or cholesterol but did increase serum triglyceride in the fructose group - both in the CR and the ad-lib fed rats.
Unsurprisingly, the calorie-restricted rats lived longer than their peers. What is interesting is that while the source of carbohydrate did not affect the lifespans in the ad-lib groups, the calorie-restricted fructose group actually lived longer than other CR groups.
In a newer study from 2008, rats were fed a 63% glucose or fructose diet for two weeks and then killed either after feeding or after a 24-hour fast. The authors conclude that fructose feeding induced a broader range of genes than previously identified and also increased triglycerides in the liver when rats were killed after feeding. In rats killed after the fast liver triglycerides were lower, however, but plasma triglycerides were higher.
Studies on humans
The more interesting part are obviously the human studies, of which there thankfully are quite many. The sugar-sweetened beverage and 100% fruit juice consumption of children and adolescents in the United States today is about 270 kcal per day, up from 242 kcal/day in 1988-1994. An increase of less than 30 kcal is not as great as one might have guessed, but the total amount is still 10-15% of total calorie intake, which is quite a lot, and the percentage has increased in the past few decades.
In this study from 2000, a subject group of 12 men and 12 women was given either a diet providing 17% of energy as fructose or a diet using glucose instead. Both diets lasted for six weeks. In men, the fructose diet produced significantly higher (as high as 32%) triglyceride levels than the glucose diet. In women, there was no difference. No effect was observed for cholesterol levels in either men or women.
In this recent study from 2008, 4 men and 2 women consumed either glucose, a 50:50 mixture of glucose and fructose, or a 25:75 mix of glucose and fructose in the morning. Higher fructose consumption stimulated lipogenesis, which is the process of simple sugars being converted into fatty acids, eventually forming triglycerides. Unfortunately, the sample size is quite small and the experiments were apparently done only once.
Another study from 2008 compared the effects of fructose in healthy males and females. The subjects were fed fructose (the abstract doesn't say how much) for 6 days, which increased serum triglycerides by 71% in men and 16% in women. Fasting insulin increased by 14% in men, but did not significantly increase in women.
Authors of this study from 2008 came to a different conclusion: It found no increase in triglyceride levels after a diet with fructose accounting for either 6% or 24% of total energy intake. Again, the sample size was small (six persons) and the diets lasted only a week.
Though not all researchers agree, it seems likely that fructose has a negative effect on triglyceride levels, at least in men. Why the effect is not seen or is smaller in women is not clear. Cholesterol, on the other, is unaffected in both men and women.
Fresh fruits certainly have their benefits - not to mention they taste good - but they have their downsides as well. Fruit juices especially are high in fructose, so just because something says "no added sugar" or "100% fruit juice" does not necessarily mean it should be drunk in gallons. Personally, I still consume fruit juices every now and then, but try to limit the amount and choose juices made from fruits that are lower in fructose (such as grapefruit).
For more information on cholesterol and triglycerides, see this post:
Coconut Lowers LDL, VLDL and Triglycerides, Raises HDL