We've been hearing a lot about sugar lately. Every few years, a new bugbear of diet comes to haunt us. First it was fats. Then it was carbs. Now it's sugars. Sometimes it's gluten or cholesterol. And it's pretty scary to see all the studies come out. I worried about my sugar intake, now I'm worried about my artificial sweeteners, and now I should be worried about sugar again?!
Possibly I should be. A new study out yesterday showed that "safe" levels of sugar are still harmful to mice.
But being a good little science nerd, I'm not just believing Nature News (though in general I like them very much). I'm getting my hands on the study itself. And I'm sharing it with you.
Ruff et al. "Human-relevant levels of added sugar consumption increase female mortality and lower male fitness in mice" Nature Communications, 2013.
(Admit it, your mouth started watering. With GUILT. Source)
The authors were interested in whether or not added sugar in the diet had negative effects on mice. They went for a diet that was 25% sugar. Believe it or not, up to 25% of Americans eat a diet that is 25% sugar. The authors define it at 3 cans of soda per day and a sugar free diet otherwise. With a statistic like that...I'm surprised we're not ALL above the recommended daily sugar allowance. I mean, I don't drink soda, but my chocolate intake has to make up for it.
What kind of sugar? This was probably a tough thing to call. Americans have increased sugar consumption in general over the last 50 years. But the biggest bugbear is high fructose corn syrup (HFCS). HFCS is not as "high" in fructose as some people would have you believe. It's a combination of the monosaccrahides (single molecule sugars) glucose and fructose. In sucrose, table sugar, the ratio of glucose to fructose is 50/50 (roughly). In HFCS, it's more like 55/41 fructose/glucose (NOT the pure fructose that some people claim. Seriously, this site says that HFCS was "purged" of glucose. No. Believe me, no one wants to eat straight fructose. Anything super high in fructose gives you...the runs. Badly). So the authors of this study went for 55:41 ratio of fructose/glucose.
The authors trapped wild mice and bred them in the lab for 10-11 generations. They exposed the resulting animals to the high sugar diet (or to control) after weaning until the mice were 26 weeks Old. Then they released them into a large environment, a mousey paradise, where they had nest boxes, food and water, bedding, little runs and fences and things. The sort of place where a whole colony of mice could set up shop and perform all their normal behaviors. They then tracked the mice in the environment, looking at mortality rates, how many pups (babies) they had, testing their insulin responses, etc.
They found that the high sugar diet produced several different effects. First, female mice died more.
The above are survival curves for female (top) and male (bottom) mice on the control (dark line) and high sugar (dashed line) diets. The females overall didn't die as much as males, but females on the high sugar diet were more likely to die than those on the control diet.
Males did not escape unscathed, however. The males on the high sugar diets ended up defending less territory and having fewer pups than males on the control diet.
As you might expect, the authors also checked for insulin resistance, something you definitely want to look for in a high sugar diet. They found that females on the high sugar diet had insulin resistance problems...but only in a home cage, NOT in the big environment. The males had insulin resistance problems in the home cage REGARDLESS of diet. There was no effect on body weight at all.
What can we conclude from this? Well, it looks like this level of sugar may have negative effects in mice, limited to higher risk of mortality in females and fewer pups in males, though the real specifics of this are not very clear. But this didn't stop the authors spreading the worry a little:
But Potts and Ruff think that their results are enough to indicate that there is a problem, and that the recommended safe level of dietary sugar should be lowered. “If I show that something hurts mice, do you really want it in your body before we’ve determined whether it’s a mouse-only problem?” asks Potts.
(Way to ruin my evening chocolate attack, guys. Source)
Perhaps they are right...but perhaps they are not.
Here's the thing: while I think this study supports high sugar increasing risk of death in female mice and decreased reproductive fitness in males...I'm not sure how much it has to do with humans. Humans and mice process sugar deep down in the same way, breaking down bonds, insulin being released to get the sugar into the cells, etc. But generally, mice don't have as much sugar in their diets as we are used to getting. It's also interesting to note that the fasting measures of things like glucose and insulin were unchanged even though the diet was so high in sugar. I don't know that the same can be said for humans. The overall metabolism of mice is different. So while the high sugar may have effects in humans, I don't know if they'd be the SAME effects. They might well be worse. Or better. Mice make GREAT models of many diseases, all the way up to psychiatric disorders. But it's important to keep in mind that there are differences.
The study also raises a lot of questions. What were the female mice dying OF? Was it a result of insulin resistance? Something else? Did females have fewer offspring? What were the offspring like? Why were the males defending less territory? Reduced testosterone? Are they less good in fights? Why were they having fewer offspring? Reduced territory? Reduced sperm count? Changed mating behavior? How does the diet and its effects compare to, say, a high fat diet? What about high fat AND sugar? I also noticed that when the mice entered the behavioral environment, they ALL consumed the high sugar diet. This would mean that even the controls were exposed to the diet for a pretty long time. I understand that that is really difficult to control, but I also wonder if it had any effects. The high sugar diet began at weaning, when the mice were still very young. Do the effects differ if the diet starts as adults? The mice didn't show differences in weight, but did they have different fat levels? Other suggest suggest that they might, but it wasn't looked at here.
Obviously they couldn't address it in a single paper. Hopefully they are already working on those questions. In the meantime, while it's probably always a good idea to reduce our sugary soda intake, I don't know that this paper should make us panic. The mice, though? They might want to cut the candy, stat.
NOTE: I'd be really interested to hear from people in the field! How much does metabolism differ in mice vs humans? Is this diet realistic and relevant to the human condition? Anything I'm missing? Please speak up in the comments and let me know!
PS: I'm also very interested into why they felt that had to lab breed wild mice for this because lab mice won't 'exhibit the right behaviors'. Really? Lab mice defend territory all the time (you couldn't do social defeat without this trait), and even if you worried about inbred strains, there are lots of outbred strains you can use. Why did they need to catch mice in a bakery?