A new study suggests that mothers pass on a craving for junk food to their babies. The study was conducted on pregnant and lactating laboratory rats who, given the choice of eating crap (including marshmallows, doughnuts, and chocolate bars) or traditional rat chow, preferred the crap. And their baby rats, once weaned, also showed a preference for crap.

But, interestingly, the offspring of the female rats who weren't given junk food during pregnancy and lactation also showed a preference for junk food, once they were weaned and given the choice.

The big difference between the two groups of rat spawn: The ones whose mothers ate junk food ate a lot more of it. Thus, there's a suggestion that eating junk food during pregnancy and while nursing someone predisposes kids to overeat.

That's scary enough, but the same group of researchers found something even scarier in a study published two years ago. In that one, rats whose mothers had overfed during pregnancy and lactation were born with more fat cells and fewer muscle fibers.

Check out this final line from the study's abstract:

And, as long as I'm rolling with the rat studies, I'll mention this one, from a research team in Mexico, which showed that a low-protein diet during pregnancy and/or lactation did more metabolic damage to male offspring than to the females.

Do these findings apply to humans? I can't possibly say. But it wouldn't surprise me a bit if they did. I mean, how could you argue with the idea that people given the choice between traditional food and junk instinctively prefer the junk? The other findings are more speculative, but if they do apply to humans could explain a lot about the rise in obesity rates, with overfed moms giving birth to kids whose metabolisms are malfunctioning right out of the gate.

That said, the obvious weakness in the hypothesis is that humans have free will, along with our unimpeded access to fast food. You have to think that very few pregnant women are so oblivious to the health implications of their diets that they'll gorge on junk food on a daily basis. (Yes, we all someone who did. But I doubt if any of us knows a lot of people who went crazy for fried Snickers bars while in the family way.)

Meanwhile, we also know from epidemiological studies that fast food can do its nasty work without in-vitro exposure. Immigrants and their children tend to gain weight and develop metabolic disorders at any age, regardless of when they were first introduced to Whoppers and Big Macs.

But now I want to shift gears, and mention another new study, this one from researchers at Penn State. It shows that children use a higher percentage of fat calories for energy, compared to adults. (Female children and adults also burn a higher ratio of fat to carbohydrate than men of any age.)

The researchers say this means that the fat in children's diets shouldn't be restricted for health reasons. (Comically, news reports like this one suggest that adults should eat low-fat diets, which is a bizarre assertion at this late stage of the Diet Wars.)

That seems like a fine, responsible admonition. But I'm still worried. Someone who comes across all these headlines may have no idea that they aren't in any way contradictory, and think the Penn State study shows that it's okay to give kids a lot of fast food.

The rat studies looked specifically at junk food, much of which was high in sugar or starch, rather than fat. I mean, a marshmallow is not high in fat. The Penn State study looked at the relative amounts of fat and carbohydrate that human bodies use for energy. We store fat in the form of triglycerides, and those triglycerides can be made from anything -- fats, carbohydrates, or even protein. So that study isn't really providing evidence for or against any type of diet.

All that said, I think almost everyone agrees about which foods are healthier for children and adults. It's interesting that kids use more fat for energy, as a percentage of their total energy expenditure, but that single finding doesn't mean it's okay for kids to eat high-fat fast food because they'll burn more of it off, or that adults should eat low-fat diets because they don't burn fat for energy as easily as children do.

For most healthy people, of any age, a moderate-fat diet seems better than the low-fat alternative. Does anyone seriously dispute that?

(Thanks to Anthony McInnis for the heads-up on the junk-food story.)

Thursday blog meat

• Confirmed: People my age have no business climbing Mt. Everest. Good to know I can scratch that one off my to-do list.
• Just in case you suspect this business over Chinese manufacturers using lead paint on children's toys is overblown, click here for scary facts about lead poisoning in children. (As every parent knows, babies and toddlers put everything in their mouths.)
• Here's a counterintuitive idea: Overdependence on trainers, coaches, and structured exercise classes can be detrimental to one's fitness over the long term.