The idea that diverse diets place stress on the immune system is new to me.
“Eat a varied diet” is a fairly standard piece of advice. But some research suggests that less variety might lead to healthier diets. What’s the truth?
Nutrition Diva is hosted by Monica Reinagel. A transcript is available at Simplecast.
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Hello. I’m Monica Reinagel and you’re listening to the Nutrition Diva podcast, a show where I take a closer look at nutrition trends and headlines, explain what the latest research means for you, and answer your nutrition questions. Welcome!
Nutrition Diva listener Sandy writes:
“I‘m reading Dan Beutner’s Blue Zones Kitchen and ran across a couple of dietary recommendations that contradict conventional medical/nutritional wisdom. I try to avoid crank theories and junk science where nutrition is concerned. Dan Buettner, along with the scientists and medical professionals he developed these recommendations with, don’t seem to fall into that camp. So, I’d be very interested to hear your thoughts.”
For those who aren’t familiar, the Blue Zones project was an attempt to codify the dietary and lifestyle habits of the planet’s longest-living populations, with the hopes that we might learn how to live longer ourselves. They focused on five communities, each of which was relatively isolated or insulated, either by geography or culture, so that both the genetics and the cultural practices were somewhat homogenous: the Japanese island of Okinawa, the Greek island of Ikaria, the Sardinian region of Italy, the Nikoyan peninsula of Costa Rica, and the Seventh Day Adventist community of Loma Linda, California.
And, as Sandy suggests, this was a very serious and rigorous scientific exploration.
But Beutner and the Blue Zones collaborators were not the first to tackle this question. Other researchers have analyzed the traditional diets and lifestyles of Cretans, Icelanders, Cameroons, and Pima Indians in an attempt to pinpoint the secret to a long and healthy life.
At a glance, it might seem that health and longevity are about the only things that these cultures have in common. The robust Tarahumara Indians, for example, eat a diet of 80% carbohydrates (mostly in the form of starchy vegetables like corn and potatoes!), while the indestructible Cretans get almost 50% of their calories from fat. Some long-lived cultures eat almost no meat, while the hale and hearty Icelanders eschew vegetables as “animal feed” and eat large quantities of lamb and fish.
But one thing that all of these indigenous diets have in common is that they’re all composed of a relatively small list of foods. In most cases, about two dozen foods provide 95% of the calories—in some cases, fewer than a dozen! Compare this with the tens of thousands of food products we are confronted with at the grocery store. Hundreds of kinds of produce from every climate in the world. Dozens of kinds of grains. Scores of protein sources. We think nothing of eating Indian food on Monday, Chinese on Tuesday, sushi on Wednesday, Latin-American on Thursday, and Greek on Friday.
I’ve also talked in the past about the benefits of eating a varied diet. Eating a variety of fruits, vegetables, whole grains, nuts, and protein sources helps to ensure that you’re covering your bases, nutritionally. Green peppers, for example, are a terrific source of vitamin C but don’t offer that much in the way of vitamin A. Carrots are the other way around. The different food groups tend to feature different nutrient profiles as well. Fruits and vegetables contain lots of antioxidants, nuts and seeds supply fat-soluble nutrients, grains and legumes supply minerals, and so on. Rather than trying to memorize which foods contain which nutrients, it’s easier to just aim for a reasonable amount of variety.
Secondly, variety is an easy way to avoid over-exposure to a potential hazard. Some otherwise healthful foods contain compounds that may pose a concern if consumed in large quantities. Albacore tuna, for example, contains small amounts of mercury. It’s unlikely to be a problem if you’re eating tuna once or twice a week. But if you’re popping open a can of tuna every single day for lunch, it could be a problem.
Raw spinach contains natural compounds called oxalates. Oxalates aren’t harmful or toxic but they do have a bad habit of binding to calcium, making it more difficult for your body to absorb this important nutrient. Eating a spinach salad a few times a week isn’t likely to pose a big problem, but eating spinach every single day for lunch could potentially start to impinge on your calcium status—especially if you’re counting on the milk in your lunchtime latte as one of your primary source of calcium.
I don’t want to make too big a deal out of this. I’ve yet to see a case of spinach-induced calcium deficiency. But mixing it up a bit takes that concern completely off the table. If you enjoy sautéed spinach, try sautéed Swiss chard or beet greens once in a while. If you like spinach in salads or wraps, try substituting arugula or even fresh basil leaves every once in a while.
There are even a few situations in which you can get too much of a good thing. Brazil nuts are uniquely high in the mineral selenium, for example. Eat a handful of Brazil nuts every day and you could easily end up with a selenium overload. Eat a handful of mixed nuts every day, and there’s no such risk.
But one of the key dietary recommendations included in the Blue Zones kitchen is to use fewer ingredients. According to Gianni Pes, the Sardinian gerontologist whose research basically launched the Blue Zones project, “Dietary monotony may be an important component of longevity.”
He goes on to explain it this way:
“When you eat too many types of food, you’re asking your immune system to work harder and undertake stress. When our immune systems are confronted with foreign invaders—whether it’s a bacteria, virus, or new molecule—they turn on genes to mount a defense. We only have so many genes to ‘turn on’ before our immune systems begin to wear out. Eating the same food every day may preserve them.”
I’m certainly not an immunologist or a geneticist. But this idea that diverse diets place stress on the immune system is new to me.
I checked in with Tamara Freuman, whose name will be familiar to listeners—she’s my go-to expert for all things digestion related. She had this to say:
“The immune system can tolerate a variety of foods without being taxed, it does not have a finite capacity. More diverse diets (especially with respect to plant foods) are associated with more diversity in gut microbiomes. And this characteristic is associated with reduced inflammation, greater resilience or resistance to pathogens, and reduced frailty in old age.”
But even if you don’t buy Pes’ hypothesis about dietary variety stressing the immune system, there may be another reason that reduced variety leads to better outcomes.
A 2018 study published in the American Heart Association journal Circulation found that
“greater dietary diversity is associated with… higher intakes of processed foods, refined grains, and sugar-sweetened beverages.” In other words, it’s not the variety that’s the problem. It’s the type of foods that are contributing to that variety.
There’s also the fact that we do tend to eat more when confronted with a large variety of foods than we do when we eat just one or two things at a meal. (Just think of your behavior at buffets, or at Thanksgiving.) This is a phenomenon known as sensory-specific satiety and you’ve probably experienced this countless times. After eating a bowl of chili, we might feel no desire to continue eating… until a piece of cheesecake appears. Suddenly, we have a little more room.
But I think there’s a way we can use this effect to our advantage—and bring these two seemingly conflicting pieces of advice into alignment: limit the variety of snacks and sweets that we keep around but increase the variety of fresh vegetables, fruits, and other whole foods.
Just for fun, why not take an inventory of what’s in your house right now? How many different types of crackers, chips, or other snack foods are on hand? How many different kinds of cookies, cereal, muffins, granola bars, ice cream, chocolate, or other sweet treats? How many types of bread, rolls, tortillas, and other starchy foods?
Now open up that crisper drawer. How many different kinds of vegetables and fruits are in there, ready to eat?
And finally, consider how the variety (or lack thereof) of the various categories of food correlates to your consumption patterns.
If you want to cut down on snacking, try keeping fewer snack foods around. If you want to eat more fruits and vegetables, surround yourself with more different kinds of produce. And if you’d like to add little extra accountability around these habits, check out my free Nutrition GPA app.
Elsewhere in the Blue Zones Kitchen, Dr. Pes advances another novel theory: that eating cruciferous vegetables enhances longevity by suppressing thyroid function. This also got Sandy’s attention. “If low thyroid function enhances life expectancy,” she wondered, “why is my doctor prescribing medication to boost my thyroid function?”
That’s another excellent question, Sandy, and one that we will tackle in NEXT week’s episode!
This is Monica Reinagel (the Nutrition Diva). Thank you for listening today. If you have a question you'd like me to answer, you can email me at nutrition@quickanddirtytips.com You can also leave me a message at 443-961-6206
I’d also like to invite you to check out my other podcast. It’s called the Change Academy, where we explore the art and science of creating positive behavior change, both in our own lives, as well in our workplaces and communities. You can find it on all the major podcast platforms. Just search for Change Academy.