[INTRO ♪].

Junk food: you know it’s not healthy, but it’s so good. And sometimes, it’s just too hard to resist.

And scientists have been trying to figure out why junk foods have so much power over us for a while. According to a study published yesterday in the journal Cell Metabolism, it probably has a lot to do with the fact that they’re usually packed with both fat and carbs. Pff, my favorite!

The researchers found that the reward centers of our brains are more active when those nutrients are combined in a snack— above and beyond what you would expect if you just added the fat and the carbs together. Which is why milkshakes, and french fries, and dipping your french fries into milkshakes. And that means that it’s not just you—junk food basically hacks our brains.

Now, there have been hints to this phenomenon for some time. Mice, for example, can stay trim if they’re given either carbs or fat to eat, but they pack on the pounds if they’re given a mix of the two. But it was less clear how that plays out in the brain, or how it applies to people.

So neurologists and physiologists in Germany and the US set up a kind of food auction where participants bid for snacks while the activity in their brains was measured with fMRI. The calories in the various snacks were either mostly from carbs, mostly from fat, or from a mix of both. And the items were sized so that each category contained the same number of calories, to make them equal from an energetic perspective.

The team also made sure that all the foods were similarly well known and liked in a previous experiment. That way, the group couldn’t prize any set of foods over the others. But the participants consistently put higher bids on the combo foods— things like chocolate chip cookies and candy bars— rather than things like nuts, cheese, or crackers.

This willingness to shell out more for the foods that had both fat and carbs was associated with more of a response in brain regions associated with reward, like the top part of the striatum. The researchers suspect this may be because we have separate reward pathways for fats and carbs, both of which are simultaneously turned on by the combo foods. And they think this, like, simultaneous reward circuit firing is something our brains just don’t know how to handle.

That actually makes sense if you look at our ancestors. For eons, people mostly ate one food group at a time, like when it became available— fatty meat one day, sugary honey or berries the next. They didn’t really have the option to do anything else because very few foods are naturally rich in carbs and fat.

Agriculture made it easier to mix nutrient groups in meals, but it still wasn’t until the last 150 years or so that we started actually making single food items that contain a dozen or more calories of each nutrient type. So for the vast majority of our evolutionary history, our brains simply haven’t had to try to estimate the nutritional value of fatty, carb-filled foods. The researchers also discovered that we’re pretty terrible at making such judgements.

When they asked the participants to guess the calories in the snacks they were bidding on, they kinda failed when it came to the carb-y and combo foods. The researchers even identified a part of the brain called the fusiform gyrus— a long strip that's at the base of our brains—that’s important for making these kinds of estimations. So, if we can't estimate calories very well when carbs are involved, and we're getting a bigger reward from those fat/carb combinations, that goes a long way to explaining why it's so hard for me to not eat more donuts when somebody brings donuts in to work.

I’m not saying I resent you for bringing the donuts in, but it's just hard, okay? They're so good! Even when they’re not that good I keep eating them.

Eventually, scientists may be able to use this information to better understand overeating and obesity. And hopefully, that means they can come up with ways for people to make better food choices, even when they're surrounded by lots of junk, as we so often are. Our next topic, weirdly enough, is about one of those rare natural foods that’s rich in the carb and the fat department.

African baobab trees produce fruits that have a starchy pulp and a fatty seed. So in a way, that makes them, like, kind of a proto-junk food, except that they're also packed with fiber and protein, so they’re pretty healthy. The problem is, a new report out this week in the journal Nature Plants found that many of the biggest and oldest baobabs— some of which have been feeding people for thousands of years— are inexplicably dying out.

The international research team surveyed 60 trees across the globe, checking on their health, measuring their size, and taking wood samples to estimate their ages. While you might think you could just, like, count the rings like other trees, baobabs can grow more than one trunk throughout their lives. These sometimes fuse together, creating what looks like one big trunk, but inside there are open spaces.

And this complexity makes it really hard to date them— the baobab’s structure is so wacky that rings don’t tell you very much. So, the scientists relied on special type of radiocarbon dating instead. In this method, you use a very small sample from multiple parts of the tree, and count the number of all the different types of carbon atoms, which can then give you a date estimate.

The team found that many trees were more than 1,000 years old, including one that was nearly 2,500 years old, making it the oldest flowering tree on the planet. But, in a horrible and unexpected twist, they also discovered that 9 of the 13 oldest trees and 5 of the 6 biggest trees were dead or had at least one trunk that was dead. And those deaths all happened within the last 12 years.

Scientists aren’t sure why this is happening, although they suspect climate change might be to blame. With warmer temperatures and more drought, the trees might be struggling to get enough water to support their large frames. But further research is needed to confirm that suspicion.

So now, the race is on to figure out exactly what’s going on before we lose these iconic trees for good. Thanks for watching this episode of SciShow! If you want to keep learning about this wacky world we live in, including staying up to date with all the new, exciting scientific research, you might want to head on over to YouTube.com/SciShow to watch a bunch of episodes and also click on that subscribe button.

Thanks! [OUTRO ♪].