When readers first linked to the “paleo diet makes you fat!” study in comments, I replied that I generally dismiss mouse and rat studies as irrelevant to humans, except in certain circumstances. I should probably talk about those circumstances.
But first, I’ll explain which studies I dismiss outright: pretty much all diet studies that involve rodents. We’re not rodents. The foods that have negative effects on rodents may have positive effects on humans and vice versa.
The big cholesterol scare started more than 100 years ago when scientists fed cholesterol to rabbits, who rapidly developed heart disease as a result. Oh my gosh, cholesterol must cause heart disease in humans!
Stupid conclusion. Rabbits are herbivores. They don’t eat cholesterol. There’s no reason they should have the biological machinery to deal with cholesterol. So – duh! – it builds up in their systems and causes problems. I’m pretty sure if we fed lions an all-vegetarian rabbit diet, they’d become quite ill. But that doesn’t mean carrots are bad for rabbits or humans. It simply means lions are obligate carnivores.
So even if researchers fed rats and mice a true paleo diet of meats and vegetables, I still wouldn’t give a rat’s ass (pardon the pun) about the results — positive or negative – because there’s simply no reason to assume those results translate to humans. We’re not rats. The diet that’s perfect for them is very unlike the diet that’s perfect for us. The researchers in the dumbass “paleo diet makes you fat!” study mentioned that standard rat chow is 3% fat. Has there ever been a group of paleo humans who lived on a 3% fat diet? I sincerely doubt it. Rats are probably biologically geared to thrive on an extremely low-fat diet. We’re not.
But of course, researchers in these studies rarely feed mice and rats anything like the human diet they’re supposedly testing. The “paleo” diet in the dumbass study consisted largely of isolated casein, sugar and canola oil. It was nothing like a paleo diet. The study has absolutely zero relevance to humans eating an actual paleo diet.
This little sleight-of-hand seems to be a habit among some researchers. More than once, I’ve dug into a mouse or rat study of the “Atkins” (ahem-ahem) diet and found that the primary fats were Crisco or corn oil, and the sole source of protein was casein … you know, just like Dr. Atkins recommended. Then when the rats or mice became fat or sick, dingbats in the media dutifully reported that New Study Show Atkins Diet Causes (insert scary result here)!!
So when should we pay attention to rodent studies? Well, I’ll least give them a look if they test the result of drugs or hormones. In the animal kingdom, hormones are the chemical messengers that trigger the code written into our biological software. Hormones have been around since before humans existed. I think we can safely assume hormones produce similar effects in a man and a mouse.
So if researchers pump male mice full of testosterone and those mice become leaner, stronger, and start throwing punches in bars at the slightest provocation, I’d expect to see similar effects in male humans. If researchers inject rats with high doses of insulin and the rats start eating like crazy and getting fat, I’d expect a similar result in humans. But I’d still take those studies with a grain of salt.
Here’s the type of rodent study I don’t take with a grain of salt: those that disprove a supposed Immutable Law of The Universe. Back in 2011, I wrote about a study in which researchers calculated how much food mice were eating ad libitum. Then they took one group of mice and cut their daily calories by just 5%. Here are some quotes from my post:
Now, according to Jillian Michaels and the other leading experts in thermodynamics, there are only a couple of possible outcomes for these experiments:
- The calorie-restricted mice, who were prevented from making little pig-mice of themselves, ended up weighing less and were leaner.
- If the calorie-restricted mice somehow ended up fatter, it could only be because they were far less active than the mice who ate freely.
Yup … if you get fat, by gosh, it means you’re either eating more or moving less. Now let’s look at the actual results:
At the end of the second experiment (three weeks), the average weight for both groups was virtually identical — it was also virtually identical to their baseline weights. But the calorie-restricted mice had 43.6% more fat mass and 6.4% less lean mass than the free-eating control mice.
Ah, well then, the mice who gained fat mass must’ve been less active, right?
Nope. According to the study data, there was no difference in locomotor activity levels between the two groups.
The calorie-restricted mice ate less, they moved around just as much, but they ended up weighing the same as the mice allowed to eat freely, and also ended up with more fat and less muscle. Oh, dear me … did these mice find a way to violate the laws of thermodynamics?
I paid attention to that study because the calorie freaks insist that according to the laws of physics, if you eat less and move around just as much YOU MUST BURN FAT FOR FUEL AND LOSE WEIGHT. IT’S AN IMMUTABLE LAW OF THE UNIVERSE. But these mice ate less, moved around just as much, and gained fat mass while losing muscle.
Yeah, it’s just a mouse study, but the laws of physics are the laws of physics, period. They don’t apply to humans and then go on vacation when mice saunter into the room. So if the laws of physics say eating less while remaining active must always lead to fat loss, that would apply to both large and small furry creatures.
No, those mice didn’t violate the laws of physics. And no, the experiment didn’t disprove any laws of physics. But it did disprove the calorie-freak argument that cutting calories while remaining just as active MUST ALWAYS LEAD TO BURNING AWAY BODY FAT BECAUSE THE LAWS OF PHYSICS SAY SO.
The laws of physics say no such thing. They merely say that if you lose weight, you burned more calories than you consumed. These mice – despite remaining just as active – slowed down their metabolisms and burned muscle tissue for fuel in order to get fatter. It was probably a programmed reaction to what their little mouse bodies interpreted as a risk of starvation. No laws of physics were harmed in the process.
So here’s the one part of the “paleo diet makes you fat!” study I found relevant:
After 3 weeks, mice fed the LCHFD began to diverge from the chow-fed group, and at 5 weeks the difference in body weight was statistically significant. At the end of the study, white adipose tissue mass was also significantly increased. The LCHFD has a higher energy density than the chow diet (24 vs 13.5 MJ kg−1); however, the increased body weight of mice fed the LCHFD was not associated with a higher energy intake.
Yup, the mice fed the full-of-crap “paleo” diet (which tripled their sugar intake) gained more weight and more body fat. We can’t blame it on palatability, because they didn’t say, “Oooh, this is yummy!” and eat more. We can’t blame it on consuming too many calories, because they didn’t consume more calories. So if the researchers kept accurate records on food consumption (and it appears they did), we have a situation where mice eating a crap diet got fatter than their control-group cousins, despite not eating more.
That’s a relevant result, even though it’s a mouse study. It disproves the dearly-held belief among the calorie freaks that getting fatter is always and forever the result of eating too many calories BECAUSE THE LAWS OF PHYSICS SAY SO. And it lends credence to the belief that food quality affects how calories are partitioned, burned and stored. Different foods send different commands to the biological software. That’s the only useful lesson from an otherwise garbage study.
And once again, no laws of physics were harmed in the process.
Of mice and men. Again.
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