Any movie worth watching is worth analyzing, so a brief scene in Captain America: Civil War involving some plums has triggered a lot of discussion plums’ potential health benefits.
So, if your memory has been damaged by years of brainwashing can you actually improve it by eating plums?
Fortunately, two Australian scientists have systematically reviewed* the research of the health effects of plums and their findings were published just two months ago.
They did identify several studies where eating plum extract or powdered plums improved memory or cognitive skills.
So is that a yes?
Well, maybe. Every single one of those studies was done in rats or mice. They didn’t find any studies showing that plums improved cognition in humans.
We do studies in animals because they can tell us a lot more about what will happen in people than a dish full of human cells or a computer model, but they aren’t perfect. When something improves cognition in a rat or a mouse it suggests that it might do the same in a human, but it doesn’t mean that it will for sure. Studies about diet are especially difficult to generalize from, because lab rats eat precisely controlled diets with carefully measured components, while human diets are complicated and variable.
There’s a few more reasons not to get too excited about these studies. First off, none of them looked at actual plums, they used plum powder or plum extract, which is less variable in content than fresh plums, and easier to add to rat chow.
Incidentally, the researchers noted that the human studies they did find (which looked at different health effects of plums), tended to use either dried plums, or plum juice. There are very few studies on fresh plums. Dried, extracted, or juiced plums have slightly different nutrient contents than fresh ones. Some chemicals become more concentrated, and others tend to be lost during processing.
Secondly, the amounts of plums in the rat diets were comically enormous. In a lot of them, the plum component was 2-5% of their entire diet. Imagine putting all the food you eat in a day on a scale, then replacing 5% of that with dried plums and you’ll get some idea of how much plum these rats were eating. Way more plums than you would ever want to eat. Any effect that eating a normal number of plums would have would be much smaller.
Why would you do such an unrealistic study?
These studies aren’t really trying to mimic the effects of a human diet, even a very plum rich human diet at all. They’re simply asking the question can plums affect the brain conditions they were studying, at all, in any way? And when you just want to find that out its best to get the biggest effect you can. A small dose of plums might produce an effect so small that it couldn’t be detected, and that’s much less likely to happen with a giant dose. It also means that you can do a good study with fewer experimental animals, which is a good thing. This is due to a statistical concept called statistical power, in essence, the smaller an effect is the bigger the sample you need to detect it. If plums produce only a small change in brain function, you’d need to study a large group of rats to find it, but if the effect on the brain is large, you can find it with only a small number of rats.
And thirdly, these positive studies, even if they do translate to humans, are not suggesting that plums are magic brain-food. Each of the studies looked at a different condition, one found that plums extract could protect against the effects of a (very) high cholesterol diet. Another found plums improved cognition in elderly rats, and another in rats with diabetes.
What you can take from this variety of studies is that plums appear to protect the brain from various conditions that can impair it (called neuroprotection), not that they necessarily improve cognition or memory in and of themselves. And none of these studies suggest, or even look at, if they can reverse damage that has already occurred, which is really totally different.
But what about brainwashing? Could plums protect you from brainwashing?
Well, some of the sources of damage that happen when your brain is physically injured are the same as when your brain is damaged by aging or illness, so its not impossible that the same sources of neuroprotection might work too, but that would still only apply to damage that was still occurring, or had just occurred, not things that happened months or years ago.
Generalizing research from one illness to another is a little bit like generalizing between species. A finding in one case suggests that another study would be useful, but the only way to prove it, is to do the research and prove it.
*A review is a paper that compares and summarizes earlier studies to draw conclusions based on the totality of many studies, instead of just one. A systematic review is a review where papers are collected in a fixed way so the researchers won’t be biased in which papers they choose to include.
Igwe, E. O., & Charlton, K. E. (2016). A Systematic Review on the Health Effects of Plums ( Prunus domestica and Prunus salicina ). Phytotherapy Research, 30, 701–731.