by Marion Nestle
Nov 30 2012

Do artificial sweeteners make rats fat?

Artificial sweeteners are a terrific example of why correlation does not necessarily mean causation (see previous post).

Their use has increased in parallel—is highly correlated—with rising rates of obesity.  But could artificial sweeteners cause obesity?

Unlikely as that idea may seem, Brazilian researchers thought  it was worth careful investigation.

They did a preliminary study of sweeteners and weight gain in rats.

They report: although total calorie intake was similar in all rats, the rats fed artificial sweeteners gained more weight than those fed sucrose (table sugar).

The investigators fed the rats yogurt containing either sucrose, aspartame, or saccharin along with unlimited amounts of rat chow.

The rats must not have liked the taste of the artificial sweeteners because they ate more of the sucrose-containing yogurt than the kind with artificial sweeteners. They compensated for less yogurt by eating more rat chow.

Although saccharin and aspartame promoted relatively fewer calories from yogurt intake when compared to sucrose, increases in calories from chow intake effectively compensated for decreases in calories from yogurt, in such a way that there was a similar total caloric intake among all groups after the 12-week period of the experiment.

As they put it, “Possible explanations for weight-gain in saccharin and aspartame groups without increasing energy intake are still widely speculative.”  They suggest that artificial sweeteners might induce:

  • Reduced energy expenditure.
  • Excessive insulin secretion.
  • Increased fluid intake and retention.

What are we to make of this?

This is a small, preliminary study using only 10 rats in each of the three groups.

The differences in calorie intake were small and small calorie differences are difficult to measure.

Bottom line: this is an interesting result that needs to be repeated with greater numbers of rats and even more careful calorie measurements.

Reference: Fernanda de Matos Feijó et al.  Saccharin and aspartame, compared with sucrose, induce greater weight gain in adult Wistar rats, at similar total caloric intake levels.  Appetite 2012; 60:203-207. doi: 10.1016/j.appet.2012.10.009




  • Jill

    There was a similar study (rats who ate artificially sweetened yogurt gaining more weight than those who ate glucose-sweetened yogurt) in 2008 by Purdue University researchers. Their take was that sweet signals not followed by calories weakens the body’s ability to prepare for calories coming in. In other words, messes up a finely tuned metabolic signal.

  • yadayadayada

    “Excessive insulin secretion.”

    There are many diabetic and pre-diabetic people who will tell you that these fake sweeteners mess up their blood sugar. Lots of info about this on the net, with people drinking Pepsi Max and then measuring their blood sugar. These sweeteners can cause a weird response in the insulin resistant, which is just about everyone nowadays.

  • Cathy Richards

    Another explanation could be that yogurt helps increase a rat’s metabolism or some other mechanism of weight control due to yogurt. Maybe it has nothing to do with the sweetener.

    I’d be more impressed with the study if they had added sucrose or sweetener to a portion of the rat chow or a portion of the water. That would eliminate the variable of yogurt intake.

    Did they actually measure insulin levels pre/post-pandrial? Did they do body mass/fat/lean/water proportions? Otherwise they are just hypothesising. What about defecation? Maybe yogurt causes changes in their digestion and fatty acid production by gut flora, which maybe changes calorie absorption or metabolism?

    Note: in this study, artificial sweeteners were given WITH food (ie. yogurt), therefore the sweet signal WAS accompanied by calories.

  • My first thought is that this is terrible news for rats trying diet by substituting artificial sweeteners for sugar.

  • As you noted, the study size is too small to extrapolate anything useful.

    I don’t use artificial sweeteners. I feel that a treat is a treat, and I don’t try to cheat by using fake sweeteners.

    If we want to lose weight, we need to eat less. It just doesn’t get more complicated than that.

  • The question of whether artificial sweeteners affect insulin resistance somehow will need to be looked at on the molecular level, in which case they should be examined in both the presence and absence of digestible sugars (the yogurt has those), to see if perhaps they have an adjuvant effect.

    Meanwhile, the practical question for us is, can artificial sweeteners do what they’re being sold (very expensively) to do, which is lower the carb count of treat foods significantly? I don’t think they really deliver in practice–well, maybe for chewing gum. Nothing carbier than that, though.

    My daughter is a Type I diabetic and still a kid, so I was very wary of everyone’s “helpful” suggestions about using Splenda, Stevia and other artificial sweeteners when she was first diagnosed. What I noticed in the diabetes and weight-loss mags (which take advertising from these and similar products), and even more glaringly in commercial products like artificially sweetened ice cream, is that desserts made with artificial sweeteners are only a gram or so less of total carb per serving than a normal standard recipe made with sugar. They’re also usually a lot sweeter and more cloying than normal versions, as if to make up for the supposed loss.

    Total carb is what counts most for diabetics, and probably for everyone–we all have the same underlying digestive machinery. Although more free sugars make a food likelier to spike (raise blood sugar quickly), and certainly you don’t want a huge amount of free sugar per meal, within an hour or two the body has broken down and processed most of the starches to sugars too. It all requires insulin.

  • Paul

    Given that artificial sweetners are already on the market, why not cut to the chase and conduct similar research on humans?

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