Can dietary antioxidants slow the progression of parkinsonian symptoms?

A recent study finds that increased dietary intake of vitamins E and C can reduce the risk of parkinsonian symptoms in older adults.

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Parkinson’s disease is one of the most common neurodegenerative conditions in the world, with approximately 10 million people suffering from the disease. Characterised by a progressive decline in motor ability, tremors and impaired balance, Parkinson’s disease is a hugely disabling condition in elderly people. However, these symptoms aren’t exclusive to Parkinson’s disease per se. It is estimated that over half of over 85-year-olds have mild parkinsonian symptoms, such as tremors and mobility issues, resulting in an increased risk of disability, mortality, and other adverse health outcomes. Despite the huge number of people who suffer from parkinsonian symptoms, there are currently no treatments that will cure the disease, and even medications to improves symptoms only work temporarily.

It has recently been suggested that increasing your intake of antioxidants could be one way to reduce the risk of parkinsonian symptoms. Antioxidants are molecules that protect biological systems from oxidative damage caused by free radicals produced during normal cellular respiration, being found in high concentrations in vegetables, citrus fruits, oils, and nuts. Dietary antioxidants, including carotenoids (for example, alpha- and beta-carotene), vitamin C, and vitamin E, play key roles in the cell to reduce the potentially damaging effects of free radicals. Parkinson’s disease, and the related mild symptoms, are caused by the degeneration of dopamine cells in a part of the brain called the substantia nigra. Dopamine released from these cells plays a critical role in the control of movement, in particular the inhibition of unwanted movement, meaning it is necessary for them to have very high cellular respiration rates to maintain their constant firing. This produces a lot of free radicals, which are thought to contribute to the selective degeneration of dopaminergic cells in Parkinson’s disease. Therefore, it was hypothesised that if antioxidant intake is increased, there is more capacity to remove these free radicals, and hence reduce the risk of the resulting parkinsonian symptoms.

And there is some evidence that this intervention works. In a study published in 2018, patients eating a Mediterranean diet — that is, one high in vegetables, whole grains, and healthy fats — had a reduced risk of parkinsonian symptoms and hence lower levels of disability [1]. Patients with Parkinson’s disease also have lower levels of antioxidants in their blood [2], and a clinical trial recently found that increasing vitamin E intake by giving Parkinson’s disease patients flaxseed oil can improve their movement symptoms [3]. However, these studies were done in patients who had already been diagnosed with Parkinson’s disease — it would be really interesting to know is whether increasing dietary antioxidants can play a role in reducing the risk of parkinsonian symptoms developing in otherwise healthy individuals.

This is exactly what was investigated in a published just last month in the journal Nutritional Neuroscience. Here, Agarwal and colleagues undertook the first longitudinal study to assess whether the antioxidant content of the diets of older adults was associated with their risk of developing parkinsonian symptoms. They recruited over 1,800 participants from retirement communities from around Chicago, Illinois, for a staggering 20-year longitudinal study. Throughout their time in the study (which was on average just under 6 years), participants were assessed on a modified test to detect Parkinson’s disease, identifying symptoms such as rigidity, tremor, and the characteristic parkinsonian gait. Participants also completed surveys on how often they consumed 137 different food items and dietary supplements, from which the researchers could calculate their average intake of carotenoids, vitamins C and vitamin E. Other important covariants were also noted, such as their age, sex, BMI and whether they had a family history of Parkinson’s disease, as well as lifestyle factors including smoking, physical activity, and educational attainment.

The authors found that there was in fact an association between the total intake of carotenoids and slower progression of parkinsonian symptoms, even when they took into account the physical activity, BMI, or family history of the participants. Vitamins E and C in food, as opposed to coming from dietary supplements, also decreased the rate of symptom progression. Interestingly, when food and supplement vitamin intake were combined, there was no overall association with symptom progression, suggesting a specific benefit of dietary antioxidant intake as opposed to supplementation. Other interesting findings were also only seen in some subgroups of the study population — for example, higher beta-carotenoid intake from food sources slowed progression in participants under the age of 80, but not in those over 80, and vitamin C only slowed progression in men and not women.

Overall, the authors conclude that dietary antioxidants can slow the progression of parkinsonian symptoms. However, it’s important to also consider the caveats of research such as this before fully believing its claims. For example, the patient sample they studied were all from the same geographical area and hence are likely to have similar backgrounds and experiences of the healthcare system. Moreover, the authors report that 75% of the participants were female, primarily white (93%) and with an average BMI of 27, were considered overweight. It is therefore only accurate to claim that in this population, dietary antioxidant intake was associated with slowed progression of parkinsonian symptoms, although due to the underlying population bias, these conclusions cannot simply be generalised to the entire population, especially those of other races and different socioeconomic backgrounds. Interestingly, a Boston study in 2015 also found that vitamin C could reduce the risk of Parkinson’s disease, but only in males [4], whereas a study in Sweden found the exact opposite — the benefits were only seen in female participants [5], the opposite finding to the current study. These varying results highlight the importance of the underlying population in these findings, as well as casting doubt over the reproducibility of the current study.

Critically, this study only shows a correlation between dietary antioxidants and symptoms — it does not provide causal evidence that increasing antioxidant intake will reduce the risk of parkinsonian symptoms. Because the study is not sufficiently controlled so that dietary intake of antioxidants can be the only explanation for the reduced risk observed, there may well be other factors influencing the findings that the authors do not address. For example, although in calculating each participant's dietary intake of antioxidants it is adjusted to take into account the number of calories that were also consumed, there is no attempt to consider the other dietary constituents, for example, the fat or sugar content, or other key nutrients and vitamins. It could be that participants that consumed more antioxidants generally had healthy diets compared to other participants, and this could be influencing the risk of developing parkinsonian symptoms rather than the antioxidants per se. Another huge caveat is that diet was self-reported by participants based on an estimate of their intake over the preceding year. I’m not sure about you, but I’d be hard-pushed to tell you the frequency at which I ate over one hundred different foods in the last year, and this will uncertainly be more difficult in older adults with potentially deteriorating memory! People will also be biased in what they report, potentially not accounting for their entire diet. Hence, all intakes will have been estimated rather than directly measured, which adds another layer of uncertainty to the conclusions.

So, can dietary antioxidants actually reduce the risk of older adults developing parkinsonian symptoms? Well, this is certainly what the authors of this article suggest. But as discussed, there are a number of other factors that have to be considered before increased antioxidant intake can be proven to slow the progression of such symptoms. Moreover, the underlying population used for the study is also influential in the outcome of the study. Future research is therefore needed into whether targeted dietary interventions can improve symptom progression in a case-control trial. It will also be important to assess whether dietary intake across a lifetime, and not just during old age as studied here, affects your risk of parkinsonian symptoms emerging, or indeed developing Parkinson’s disease itself. Nonetheless, while increasing antioxidant intake involves eating an overall healthy diet, there’s certainly no harm in increasing our intakes — it may well help us stay healthy when we’re older!

References

The article discussed: Puja Agarwal, Yamin Wang, Aron S. Buchman, Thomas M. Holland, David A. Bennett & Martha C. Morris (2020). Dietary antioxidants associated with slower progression of parkinsonian signs in older adults, Nutritional Neuroscience, DOI:10.1080/1028415X.2020.1769411

[1] Agarwal P, Wang Y, Buchman AS, Holland TM, Bennett, DA, Morris MC. (2018) MIND diet associated with reduced incidence and delayed progression of Parkinsonism in Old Age. J Nutr Health Aging, 22:1211–5.

[2] Yuan Y, Tong Q, Zhang L, Jiang S, Zhou H, Zhang R, et al. (2016) Plasma antioxidant status and motor features in de novo Chinese Parkinson’s disease patients. Int J Neurosci., 126:641–6.

[3] Taghizadeh M, Tamtaji OR, Dadgostar E, Daneshvar, Kakhaki R, Bahmani F, Abolhassani J, et al. (2017) The effects of omega-3 fatty acids and vitamin E co-supplementation on clinical and metabolic status in patients with Parkinson’s disease: A randomized, double-blind, placebo-controlled trial. Neurochem Int., 108:183–9.

[4] Bartali B, Curto T, Maserejian NN, Araujo AB. (2015) Intake of antioxidants and subsequent decline in physical function in a racially/ethnically diverse population. J Nutr Health Aging, 19:542–7.

[5] Bovier ER, Renzi LM, Hammond BR. (2014) A double-blind, placebo-controlled study on the effects of lutein and zeaxanthin on neural processing speed and efficiency. PloS One, 9:e108178.

DPhil Student in Neuroscience at the University of Oxford 🔬 Science 🧠 Neuroscience 🎓 University Life

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