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Why would fructose, a sugar found naturally in fruit, be so poorly tolerated by modern humans? It doesn’t make intuitive sense, until we consider the scarcity and seasonality of fruit up until a few short decades ago.

Like the casino of a Las Vegas resort, our modern food complex has lost any sense of time, place, and season. Within a single generation we have gained unprecedented access to sweet fruit. A pineapple from the tropics, berries grown in Mexico, and Medjool dates from Morocco are now flown to our towns and cities so that they may line our supermarket shelves all year long. These fruits are bred to be larger, and contain more sugar, than ever before in history.

We are frequently told that it’s OK — beneficial, even — to consume “unlimited” fruit, but looked at through an evolutionary lens, fruit (and particularly today’s cultivated, high-sugar versions) may be uniquely adept at tricking our bodies’ metabolisms. This is theorized to be an adaptive, temporary quality that helped us pack away fat so that we might survive the winter.

In fact, it is thought that our ancestors developed red-green color vision for the sole purpose of distinguishing a ripe, red fruit from a green background — an evolutionary testament to the lifesaving value of fruit for a hungry forager. Today, 365 days of high-sugar fruit consumption is readying our bodies for a winter that never seems to come.

Fructose in particular, whether from processed sugary foods or excessive fruit sugar, is problematic because it impairs its own absorption when consumed in large amounts. While this may sound vaguely positive, the excess fructose left to linger in the gut can create many unpleasant symptoms, from bloating and cramps to diarrhea and symptoms of irritable bowel syndrome (IBS).

The gut lining is the precious matrix across which we absorb nutrients from food. It also helps keep intestinal bacteria in the gut where it belongs. The last thing you’d want to do is poke holes in your gut lining, but this is what concentrated fructose seems to be able to do. The technical term for this is “increased intestinal permeability,” which is when the gut lining allows the leakage of inflammatory bacterial components from the gut into circulation. The seepage of these bacterial components into your blood is a major driver of systemic inflammation, and it can induce symptoms of depression and anxiety as it switches the immune system of your brain and body into high alert.

What other consequences of gorging on grapes and other sweet fruit could there possibly be for our brains? A few large studies have helped to shed some light. In one, higher fruit intake in older, cognitively healthy adults was linked with less volume in the hippocampus. This finding was unusual, since people who eat more fruit usually display the benefits associated with a healthy diet. In this study, however, the researchers isolated various components of the subjects’ diets and found that fruit didn’t seem to be doing their memory centers any favors.

Another study from the Mayo Clinic saw a similar inverse relationship between fruit intake and volume of the cortex, the large outer layer of the brain. Researchers in the latter study noted that excessive consumption of high-sugar fruit (such as figs, dates, mango, banana, and pineapple) may induce metabolic and cognitive derangements on par with processed carbs.

Fruits, however, do contain various important nutrients. Luckily, low-sugar fruits are among the most concentrated sources of these nutrients. Some examples include coconut, avocado, olives, and cacao (no, this does not mean that chocolate is a fruit — but dark chocolate does have myriad brain benefits and is one of our Genius Foods).

Berries are also great because not only are they low in fructose but they are particularly high in certain antioxidants shown to have a memory-boosting and anti-aging effect. The Nurses’ Health Study, a long-running dietary survey of 120,000 female nurses, found that those who ate the most berries had brains that looked 2.5 years younger on scans. In fact, while a recent analysis of the literature found no association between overall fruit intake and reduced dementia risk, berry consumption was the sole exception. Berry nice!

This piece was excerpted from the new book GENIUS FOODS: Become Smarter, Happier, and More Productive While Protecting Your Brain for Life by Max Lugavere. Copyright © 2018 by Max Lugavere. Published by Harper Wave, an imprint of HarperCollins Publishers. Reprinted by permission.

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