July 30, 2019
Contrary to some previous findings that it may enhance cognitive function, a high intake of hot chili peppers is associated with an increased risk of cognitive decline, new research shows.
A large longitudinal study found a high chili intake was positively associated with lower cognitive scores and doubling of the risk of self-reported memory loss.
The main "take home" message from this new study is that people "should eat moderately" and follow a diet that includes "a variety of foods," study investigator Ming Li, PhD, senior research fellow in chronic disease epidemiology, School of Health Sciences, University of South Australia in Adelaide, told Medscape Medical News.
The study was published online May 27 in the journal Nutrients.
Chili is one of the most commonly used spices worldwide, and is especially popular in Asia. In certain regions of China, for example, almost one third of adults consume spicy food, including chili, every day.
The active component of chili is capsaicin, which is the source of its pungent taste. Some research suggests that capsaicin may have beneficial effects, including increasing metabolism, improvement of vascular disorders, and pain relief. Li noted that capsaicin is sometimes included in topical analgesic creams.
Evidence on the association between capsaicin and cognition from animal studies is inconsistent, with some showing it is beneficial for cognitive function and others suggesting it is neurotoxic, said Li. In addition, she noted that there has been no longitudinal research into the potential relationship between chili consumption and cognition.
For the study, investigators used data from the China Health and Nutrition Survey (CHNS), an open, prospective household-based cohort study conducted in nine provinces of China between 1989 and 2011.
Investigators carried out cognitive screen tests among those over age 55 in 1997, 2000, 2004, and 2006.
From these in-person screenings, investigators calculated a global cognitive score using composite scores of memory, counting back, and subtraction scores. The total global cognitive score ranged from 0 to 27.
Study subjects were also asked to rate their memory and whether they had experienced memory changes in the past year.
Dietary intake was assessed in each survey between 1991 and 2006. Researchers used records from 3-day dietary intake surveys and calculated chili intake as the total amount of fresh and dry chilli consumed.
The analysis did not include intake of sweet capsicum or black pepper, as these do not have high levels of capsaicin.
Two Tablespoons per Day
Researchers categorized cumulative mean chili intake into four groups: nonconsumers, 1-20 g/day, 20-50 g/day, and >50 g/day. An amount of 50 grams is equal to about a couple of tablespoons of chili.
Researchers used the cumulative average chili consumption between 1991 and 2006 for the analysis.
Investigators constructed two dietary patterns to control for potential dietary confounding. These were "traditional southern," which is characterized by high intake of rice, pork and vegetables, and low intake of wheat; and "modern," which is characterized by high intake of fruit, soy milk, eggs, milk, deep fried products, and beer.
The study included 4852 participants with a mean age of 63 years.
A greater percentage of those who ate the largest amount of chili had a low income, low education level, and body mass index (BMI), and were more physically active, compared with nonconsumers.
The investigators found that compared with nonconsumers, those whose cumulative average chili intake was above 50 g/day had a regression coefficient for global cognitive function of -1.13 (95% confidence interval [CI], -1.71 - 0.54; P = .001).
These results were based on a model adjusted for age, gender, energy intake, fat intake, smoking, alcohol intake, income, urbanicity, education, physical activity, dietary pattern, body mass index (BMI), and hypertension.
Overall, compared with nonconsumption, a chili intake higher than 50 g/day was associated with more than twice the risk of having self-reported poor memory (odds ratio [OR], 2.12; 95% CI, 1.63 - 2.77), and a 56% increased risk of having self-reported memory decline (OR, 1.56; 95% CI, 1.23 - 1.97).
The effect of chili intake on cognitive function appeared to be stronger among those with normal body weight than among those with a high BMI.
"Lean people with a high chili intake may be more sensitive to chili," said Li. "Other dietary components may confound the link between chili and cognition among those with a high BMI."
Beneficial for Weight, Blood Pressure
Earlier studies by these same investigators showed chili intake was beneficial for body weight and blood pressure.
Although chili intake has been shown to be inversely associated with the risk of hypertension, this beneficial effect has not translated to a better cognitive function, say the authors.
There was a significant difference in chili intake among people with different education levels. "High education was positively associated with cognition in the study," said Li.
The new results are in contrast to some earlier research. The authors cited a cross-sectional study of Chinese adults aged at least 40 years in the city of Chongqing, which found that a capsaicin-rich diet was positively associated with cognitive function and inversely associated with blood amyloid-β levels.
"The study in Chongqing was a cross-sectional one, so reverse causation is possible," said Li.
The new study does not shed any light on possible mechanism linking high hot chili intake with cognitive decline. There's some evidence from animal studies that capsaicin may be neurotoxic and that it impacts levels of brain-derived neurotrophic factor (BDNF), which is important for neuronal survival and growth, and plays an important role in learning memory.
Also, high chili intake may be an indicator of a plant-based diet that could be related to other factors such as heavy metal contamination and high iron intake, said Li.
The researchers are looking at the impact on cognition of various other dietary factors. So far, they have also reported on the association with iron intake, nut intake, and a plant-based, iron-related dietary pattern.
Interesting, Not Conclusive
Commenting on the study for Medscape Medical News, Amy Loughman, PhD, postdoctoral research fellow, Food & Mood Centre, Deakin University, Melbourne, Australia, said that the although the observed association between high chili intake and cognition is "interesting," it is not conclusive, as the authors indicated.
"It isn't possible to conclude that chilli is causal to cognitive decline, that reducing chilli might help to prevent it, or that chilli has a biological effect on the brain," said Loughman, who was not associated with the study.
Even though the interactions between nutrients and foods within diets are "incredibly complex," research in the field does not always account for the effects of the overall dietary pattern, said Loughman.
But this new study did adjust for diet, and did so using a "gold standard method" of dietary intake assessment involving factors corresponding to "traditional" and "modern" diet types, which were derived from 3-day diet recalls, she said.
Overall, the approach used in the study to adjust for potential confounders "is relatively strong," said Loughman.
"Most of the important differences between the highest and lowest chili consumers were included as covariates in adjusted models, and in the most stringent statistical model, important lifestyle, demographic, and medical factors are accounted for," she said. "Also, participants for whom cognitive function was only available at a single time point were excluded."
However, she added, there may have been "important differences" between the high/low chilli consumers that could "more plausibly" explain the findings.
A notable study limitation was the lack of apparent adjustment for the number of previous assessments, said Loughman.
"This is relevant because in a test administered as little as 2 years apart, practice effects are entirely possible, which would mask cognitive decline among participants who participated more often than others in this open cohort study."
As the high chilli intake group had "extraordinarily high" levels of consumption, results may not be generalizable to other consumers, said Loughman.
The secondary findings for self-reported cognitive decline "are considerably weaker" than those of the primary outcome, and "should be interpreted very cautiously," she noted.
"It often bears only modest association with objective measurement of cognition, and often reflects depressive symptoms, which were not controlled for in this study," Loughman added. "True, clinically significant loss of memory function is not always associated with a high degree of insight into it."
The study authors and Loughman have disclosed no relevant financial relationships.
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SOURCE: Medscape, July 30, 2019. Nutrients. Published online May 27, 2019.