HFCS Follow-up: What the Rats at Princeton Can and Can’t Tell Us

Ed called my attention to last week’s press release about the study at Princeton currently getting some mass media attention. The press release claims:

Rats with access to high-fructose corn syrup gained significantly more weight than those with access to table sugar, even when their overall caloric intake was the same. 

i know it's a squirrel, not a rat. apparently no one's gotten a rat to do this and then circulated it with the right keywords to match my google search. this image likely not original to: http://ybfat101.com/notyourfault.shtmlThat’s pretty surprising, given that other studies have suggested that there is no difference between HFCS and sucrose. The Princeton study doesn’t offer a definitive explanation for the difference they found, but they suggest that it may have something to do with the slightly greater proportion of fructose in the HFCS.

As I noted in the first post on high-fructose corn syrup, HFCS-55, which is the kind used in soft drinks and the Princeton study, has roughly the same proportions of fructose and glucose as table sugar. Table sugar, or sucrose, is composed of fructose bonded to glucose so it’s a perfect 50-50 split. HFCS-55 contains 55% fructose, 42% glucose, and 3% larger sugar molecules. There’s a lot of evidence that fructose is metabolized differently than glucose, and may promote the accumulation of fat, especially in the liver and abdomen. Indeed, that’s why I believe that agave nectar is probably nutritionally worse than table sugar. Still, I’d be pretty shocked if a 5% increase in fructose could produce a statistically significant difference in weight gain, unless the rats were eating nothing but sugar-water. And they weren’t—in both of the experiments reported in the original study, the rats had access to unlimited “standard rat chow,”

Experiment 1: Rats Who Binge?

In the first experiment, 40 male rats were divided into four groups of ten. All of them had 24-hour access to rat chow and water. Group 1 was the control, so they just had chow and water. Group 2 had 24-access to an 8% solution of HFCS (.24 kcal/mL), which the press release claims is “half as concentrated as most sodas”. Group 3 had 12-hr access to the same HFCS solution. And Group 4 had 12-hr access to a 10% solution of sugar dissolved in water (.4 kcal/mL), which the press release claims is “the same as is found in some commercial soft drinks.” The two things of note so far are that none of the rats had 24-hr access to sucrose-sweetened water, and that the concentration of the sucrose was nearly 2x that of the HFCS syrup.*

Why the 24 hr vs 12 hr groups? According to the study:

We selected these schedules to allow comparison of intermittent and continuous access, as our previous publications show limited (12 h) access to sucrose precipitates binge-eating behavior (Avena et al., 2006).

In other words, they fed the sucrose group on a schedule that they already knew would cause binging. And they didn’t include a 24-hr sucrose group to control for that.

That helps to explain the results: the rats that had 24-hr access to HFCS-water gained less weight than either the rats who had 12-hr access to sucrose-water or the rats that had 12-hr access to HFCS-water. So according to the experiment, it’s better to consume some HFCS than it is to binge on sugar (not, obviously, how they chose to frame it in either the formal write-up or the press release).

Princeton rats

The only difference between the four groups in the first experiment that was statistically significant at a p<0.05 was between the rats who got chow only and the rats who got 12-hr HFCS. There was no statistically significant difference between the rats who had 12-hr access to sucrose-water and the rats who had 12-hr access to HFCS-water. There wasn’t even a significant difference between the rats who had 24-hr access to HFCS-water and the chow-only rats. So the only basis for the claim in the press release that HFCS is worse than sucrose is the fact that the rats with 12-hr HFCS got a “significant” amount fatter while the 12-hr sucrose rats didn’t. Even though the 24-hr HFCS rats didn’t either.

I am not the only one who’s picked up on this—both Marion Nestle (a vocal critic of the food industry) and Karen Kaplan (not, as far as I can tell, a shill for the Corn Refiners Association) also dispute the claim that this research demonstrates anything conclusive about HFCS vs. sucrose. The lead researcher replied to Nestle’s post, and rather than addressing the discrepancy between the 12-hr and 24-hr HFCS groups, he merely corrects her assumption that the 24-hr rats should be fatter:

There have been several studies showing that when rats are offered a palatable food on a limited basis, they consume as much or more of it than rats offered the same diet ad libitum, and in some cases this can produce an increase in body weight. So, it is incorrect to expect that just because the rats have a food available ad libitum, they should gain more weight than rats with food available on a limited basis. –Bart Hoebel

Which just makes it all the more baffling why they didn’t include a 24-hr sucrose group. Additionally, according to their results, binging or “consuming more” doesn’t explain the results, because:

There was no overall difference in total caloric intake (sugar plus chow) among the sucrose group and two HFCS groups. Further, no difference was found in HFCS intake and total overall caloric intake in the groups given 12-h access versus 24-h access. Both groups consumed the same amount of HFCS on average (21.3±2.0 kcal HFCS in 12-h versus 20.1±1.6 kcal HFCS in 24 h), even though only the 12-h group showed a significant difference in body weight when compared with the control groups.

The only explanation they offer for these results is the slight difference in the amount of fructose the rats in the HFCS and sucrose groups consumed. But even that relies on the idea that the HFCS rats did not feel as satisfied by their sugar water and compensated by eating more:

…fructose intake might not result in the degree of satiety that would normally ensue with a meal of glucose or sucrose, and this could contribute to increased body weight.

Unless satisfaction itself makes rats thinner.

Experiment 2 (Males): Wait, Where’s the Sucrose?

In the first part of the second experiment, 24 male rats were divided into three groups of eight. Again, all three had unlimited chow and water. Group 1 had 24-hr access to the HFCS-solution, Group 2 had 12-hr access to the HFCS-solution, and Group 3 was the chow-only control. Sucrose, you’ll note, drops out entirely. According to the study:

Since we did not see effects of sucrose on body weight in Experiment 1 with males, we did not include sucrose groups in this long-term analysis in males.

But there were no effects of HFCS on body weight on the 24-hr schedule! The omission of sucrose from this experiment makes as much sense as the omission of a 24-hr sucrose group in the first one. The lead researcher’s reply to Marion Nestle’s criticisms offered no further clarification about this choice. 

We explain in the article that we purposefully did not compare HFCS to sucrose in Experiment 2 in males, because we did not see an effect of sucrose on body weight in males in Experiment 1.

This study went on for 6 months instead of 2 months and, as the table above shows, the groups with both 24-hr and 12-hr access to HFCS-water gained a significantly greater amount of weight than the chow-only rats. This time, the 24-hr HFCS rats gained more weight than the 12-hr HFCS rats.

Experiment 2 (Females): Sucrose is back (still only 12-hr)! But chow is limited.

In order to “determine if the findings applied to both sexes,” they also ran a slightly different version of the second experiment on some female rats (n unknown). The control group, as usual, got unlimited chow and food. Group 1 got 24-hr access to HFCS-water. The remaining two groups got 12-hr access to chow (“to determine if limited access to chow, in the presence of HFCS or sucrose, could affect body weight”) and either 12-hr access to HFCS-water or 12-hr access to sucrose-water. Yeesh. How about testing one thing at a time, guys?**

So this time, only the rats with 24-hr access to HFCS gained a significantly greater amount of weight than the chow-only rats, which flies in the face of the claim that rats with limited access to a palatable food eat more. And the 12-hr sucrose rats actually gained slightly more weight (though not a statistically significant amount) than the 12-hr HFCS rats.

In other words, the findings in the three studies were completely inconsistent. For male rats in the short term, 12-hr access to HFCS induces significant weight gain but 24-hr access to HFCS does not. For male rats in the long term, both 12-hr or 24-hr access to HFCS induces significant weight gain, but they didn’t test sucrose. For female rats in the long term, only 24-hr access to HFCS with unlimited chow induces significant weight gain and limited chow, HFCS, and sucrose do not. And yet, based on this, they claim:

In Experiment 2 (long-term study, 6–7 months), HFCS caused an increase in body weight greater than that of sucrose in both male and female rats. This increase in body weight was accompanied by an increase in fat accrual and circulating levels of TG, shows that this increase in body weight is reflective of obesity.

Despite the fact that Experiment 2 didn’t even test the long-term effects of sucrose consumption on male rats, and 12-hr HFCS (albeit with limited chow) didn’t cause significant weight gain in female rats.

As Usual: Needs More Research

Based on the results of all three experiments, they conclude:

Rats maintained on a diet rich in HFCS for 6 or 7 months show abnormal weight gain, increased circulating TG and augmented fat deposition. All of these factors indicate obesity. Thus, over-consumption of HFCS could very well be a major factor in the
“obesity epidemic,” which correlates with the upsurge in the use of HFCS.

Despite the fact that obesity has also increased in many countries where HFCS is virtually never used, like Australia. According to a 2008 USDA paper:

Australia and the United States have a high and rising prevalence of obesity. They have opposite sugar policies: virtually no distortions affect Australia’s use of sugar, whereas sugar policy in the United States taxes sugar use. Sugar consumption per capita in Australia has been flat from 1980 to 2001, after which it increased by 10%-15%. Sugar is the major sweetener consumed in Australia.

The fact that the experiment doesn’t seem to show that HFCS is necessarily worse than sucrose doesn’t mean the findings aren’t intriguing. I really do want to know, for example, why rats with 12-hr access to HFCS gain more weight in the short term than rats with 24-hr access to HFCS, but the 24-hr HFCS rats gain more in the long term. And if, as they claim, the rats in all the groups consumed the same number of calories—which Nestle doubts because, "measuring the caloric intake of lab rats is notoriously difficult to do (they are messy)”—why were there any differences at all at the end of the trials? If none of the rats are eating more (and indeed, it seems that in some cases the HFCS rats were eating slightly less), what is the mechanism causing them to gain more weight, at least on some feeding schedules?

Does the concentration of the sugar have anything to do with it? In his reply to Nestle, Hoebel says:

Eating sucrose does not necessarily increase body weight in rats, although it has been shown to do so in some studies, usually employing high concentrations of sucrose, such as 32%. Our previously published work, has found no effect of 10% sucrose on mean body weight. At this concentration, rats seem to compensate for the sucrose calories by eating less chow.

I want to know if that’s true for HFCS as well. And did the difference in the concentrations of the HFCS and sucrose drinks have anything to do with the difference in the rats’ weight in this study?

Or does it maybe have something to do with sucrase, the enzyme that splits the fructose and glucose in table sugar? From what I’ve read, sucrase is present in the human digestive tract in sufficient amounts that it doesn’t rate-limit the absorption of those sugars in sucrose compared to the consumption of free fructose and glucose. But is it somehow involved in metabolism or appetite-regulation?

So rather than answering any questions about HFCS vs. table sugar, this really just raises a lot of new ones.

*It’s also not clear why they gave them different concentrations of sweetener. You’d think they would make them both soda-strength, or at least calorically equivalent.

**The failure to control for multiple variables does, in fact, complicate their ability to make any conclusions about gender difference:

In the present study, male rats maintained on 12-h access to HFCS also gained significantly more weight than chow-fed controls, while female rats maintained on 12-h access did not. It is possible that this can be accounted for by the fact that these males had ad libitum chow, while the females had 12-h access to chow. It is possible that the lack of chow for 12 h daily suppressed weight gain and TG levels
that might have otherwise been elevated in the female 12-h HFCS access group. This would indicate an effect of diet rather than a gender difference.