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Anyone prone to depression may do well to steer clear of sugar-laden puddings, sweets, and treats over the holiday season. Clinical psychologists at the University of Kansas (KU) analyzed a wide range of research on the physiological and psychological effects of eating foods with added sugar. Their results support the hypothesis that consuming foods containing a lot of added sugars—which are hard to avoid during this time of year—can trigger metabolic, inflammatory, and neurobiological processes tied to depressive illness. Their study results are published in the journal Medical Hypotheses.

“When we consume sweets, they act like a drug,” said Stephen Ilardi, PhD, KU associate professor of clinical psychology. “They have an immediate mood-elevating effect, but in high doses they can also have a paradoxical, pernicious longer-term consequence of making mood worse, reducing well-being, elevating inflammation, and causing weight gain.” Ilardi and colleagues reported the findings from their study in a paper titled “The depressogenic potential of added dietary sugars.”

Sugar has become a dietary staple throughout the developed world, the authors wrote. People in the United States currently derive an estimated 14% of all their calories from added sugars that are typically introduced into foods and beverages during processing, and this amounts to “the equivalent of 18 teaspoons’ worth each day,” they added. “These sweeteners are even found in an estimated 75% of all packaged foods.” A high-sugar diet is believed to represent a risk factor for disorders including cardiovascular disease, type 2 diabetes, as well as obesity. “As a result, U.S. dietary guidelines now advise limiting the consumption of added sugars, both to promote better overall health and to help reduce the burgeoning toll of obesity,” the researches further commented.

It’s also been found that when people improve their diets to boost nutrition or lose weight, it can also reduce the symptoms of depression, they commented. “Moreover, healthy and unhealthy diets, respectively, each appear to exert independent effects on mental health, which suggests that depressogenic processes can be affected both by the relative absence of key nutrients and by the excessive presence of harmful food.”

Couple diet with dwindling light in wintertime and corresponding changes in sleep patterns, and it’s possible that high sugar consumption could result in a “perfect storm” that adversely affects mental health, the researchers reasoned. “For many people, reduced sunlight exposure during the winter will throw off circadian rhythms, disrupting healthy sleep and pushing 5% to 10% of the population into a full-blown episode of clinical depression,” said Ilardi, who co-authored the study with KU graduate students Daniel Reis, Michael Namekata, Erik Wing, and Carina Fowler. And these symptoms of what’s known as winter-onset depression could then further prompt people to consume more sweets.

“One common characteristic of winter-onset depression is craving sugar,” Ilardi said. “So, we’ve got up to 30% of the population suffering from at least some symptoms of winter-onset depression, causing them to crave carbs—and now they’re constantly confronted with holiday sweets.” Kicking the extra dietary sugar can be hard, Ilardi suggested, because of the quickfire mood boost that sugar offers, and this leads some people with depressive illness to seek its temporary emotional lift. “Therefore, given the established link between excessive sugar intake and adverse health outcomes, as well as the broader association between dietary quality and mental health, we hypothesize that added dietary sugars constitute a risk factor for the onset of depression, particularly at high levels of consumption.”

To investigate this hypothesis further, the authors analyzed a wide range of research on the physiological and psychological effects of consuming added sugar, including the Women’s Health Initiative Observational Study, the NIH-AARP Diet and Health Study, a study of Spanish university graduates, and studies of Australian and Chinese soda-drinkers. The researchers found inflammation to be the most important physiological effect of dietary sugar related to mental health and depressive disorder. “Elevated systemic inflammation is recognized as a potent physiological trigger of depression,” they commented. The results of their analyses indicated that “added sugars have a profound effect on inflammatory processes within the body and brain, and inflammation may serve as a key mediator of sugar-induced depression onset.”

Ilardi further noted, “A large subset of people with depression have high levels of systemic inflammation. When we think about inflammatory disease we think about things like diabetes and rheumatoid arthritis—diseases with a high level of systemic inflammation. We don’t normally think about depression being in that category, but it turns out that it really is—not for everyone who’s depressed, but for about half. We also know that inflammatory hormones can directly push the brain into a state of severe depression. So, an inflamed brain is typically a depressed brain. And added sugars have a pro-inflammatory effect on the body and brain.”

The studies also identified four additional depressive physiological pathways affected by sugar intake. “ … disruption of the gut-brain axis, oxidative stress, insulin resistance, and the production of toxic advanced glycation end products (AGEs)—are also associated with increased inflammation.” Ilardi further commented on the impact of sugar on the microbiome as a potential contributor to depression: “Our bodies host over 10 trillion microbes and many of them know how to hack into the brain,” he said. “The symbiotic microbial species, the beneficial microbes, basically hack the brain to enhance our well-being. They want us to thrive so they can thrive. But there are also some opportunistic species that can be thought of as more purely parasitic—they don’t have our best interest in mind at all. Many of those parasitic microbes thrive on added sugars, and they can produce chemicals that push the brain in a state of anxiety and stress and depression. They’re also highly inflammatory.”

Additional studies evaluated by the team indicated that sugar impacts on the release of the neurotransmitter dopamine (DA), which plays a key role in brain circuitry that regulates reward-based behavior. “Interestingly, acute sugar consumption tends to stimulate the DA system: even a sip of sugary beverage immediately activates rewards areas within the brain,” they noted. Excessive sugar intake has also been linked with alterations to the structure and function of DA pathways, they commented. Similarly, they reported evidence indicating that added sugar consumption can induce oxidative stress, which contributes to “the dysregulation of various physiological pathways associated with depressive processes.” The study, in addition, reported that AGEs have been proposed as markers of lifestyle-related diseases, including depression, and commented on evidence of a positive association between AGE levels and the risk of depression, and severity of depressive symptoms. “Taken together, these findings provide substantial support for the hypothesis that AGEs exert depressogenic effects,” they stated.

The authors acknowledged that while the hypothesized link between depression and added dietary sugar is neither definitive nor conclusive, as more “rigorous experimental manipulations of added sugar intake,” will be needed in large samples of people, the evidence to date is still “persuasive.” They suggested that if the link can be demonstrated, then the ideal would be to develop low-sugar diets that are easy to stick to, and so can help a wide range of people. … depression-focused diets could aim specifically to reduce the intake of added sugars while simultaneously ignoring other types of carbohydrates, thereby eliminating the need for restrictive (or expensive) dietary changes that act as a barrier to adherence,” they suggested. One simple change that could have a major effect on added sugar intake is the reduction of sugar-sweetened beverages, they said.

Ilardi cautioned that it might be appropriate to view added sugar, at high enough levels, as physically and psychologically harmful, akin to drinking a little too much alcohol. “We have pretty good evidence that one alcoholic drink a day is safe, and it might have a beneficial effect for some people,” he said. “Alcohol is basically pure calories, pure energy, non-nutritive, and super toxic at high doses. Sugars are very similar. We’re learning when it comes to depression, people who optimize their diet should provide all the nutrients the brain needs and mostly avoid these potential toxins.”

Ilardi recommended a minimally processed diet rich in plant-based foods and omega-3 fatty acids for optimal psychological benefit. As for sugar, the KU researcher recommended caution—not just during the holidays, but year-round. “There’s no one-size-fits-all approach to predicting exactly how any person’s body will react to any given food at any given dose,” Ilardi stated. “As a conservative guideline, based on our current state of knowledge, there could be some risk associated with high-dose sugar intake—probably anything above the American Heart Association guideline, which is 25 g of added sugars per day.”

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