Journal of Preventive Medicine

Introduction
Telomere length is known as a biological marker of the aging process. The role of telomeres in aging and aging-related chronic disease has been addressed in several studies. According to studies, lifestyle and environmental factors are related to telomere length. Recent studies have provided useful scientific evidence to confirm the role of diets rich in fruits, vegetables, fish, and low-fat dairy products in reducing the incidence of chronic diseases and increasing life expectancy. Longer telomeres are associated with healthy diets. Diet has been shown to affect telomere length by affecting inflammation, oxidative stress, DNA integrity, and DNA methylation.
The Mediterranean Diet (MD) is considered one of the healthiest eating plans in the world. Increasing evidence shows that adherence to the Mediterranean diet is associated with longer life. According to recent studies, healthy eating patterns protect cells from oxidative stress, aging, apoptosis, and telomere shortening. However, the effect of healthy diets (MD, traditional diet, etc.) on telomere length is still debated. Therefore, the present study aims to investigate the effect of different healthy eating patterns on telomere length and telomerase enzyme activity.

Methods
This is a systematic review study. Related articles were searched in PubMed, Embase, Scopus, ISI and Web of Science databases. The observational studies on adults or older people that evaluated the effect of one of the types of healthy eating patterns on telomere length were included in the review. Also, these studies should report telomere length as a primary or secondary outcome. Studies that were conducted on pregnant or lactating women or people with diseases (such as cardiovascular diseases and stroke) and clinical trial studies were excluded from the review.
The title and abstract of the articles were checked separately by two researchers. If there was a disagreement between them, a discussion would resolve the discrepancies. Information about the published study, including the authors’ name, year of publication, study area, number of participants, telomere length in each dietary pattern, and duration of follow-up were extracted and recorded. The quality of articles was evaluated using the Cochrane Library guide.

Results
Five studies were eligible to be included in the review. In a cross-sectional study that aimed to determine the relationship between dietary patterns and telomere length in elderly and middle-aged people, it was shown that diets rich in whole grains, seafood, legumes, vegetables, and seeds have a positive and significant relationship with telomere length. A study by Gu et al. [20] who evaluated the relationship between adherence to the MD and telomere length, showed that the MD score was not related to telomere length in the elderly. However, a statistically significant relationship between telomere length and MD score was observed in white Hispanics based on subgroup analysis. Nettleton et al. [12] used two dietary patterns for analysis. The first pattern included the intake of high-fat foods and processed meat, and the second pattern included the intake of whole grains, fruits, and vegetables. None of the patterns were associated with telomere length. In the study by Bethancou et al. [22] no correlation was found between telomere length and dietary patterns. Karimi et al. [23] reported a negative relationship between the consumption of nuts and seeds and telomere length in healthy cyclists.

Discussion
The significant relationship of dietary patterns with telomere length was reported in a few studies (2 out of 5). In Gu et al.’s [20] study, the relationship between vegetable intake and telomere length was observed only in women. Based on the findings of some studies, dietary components such as grains, sugar-sweetened beverages, and processed meats may be associated with shortened telomere length. The studies included in this systematic review were cross-sectional studies; therefore, genetic differences in telomere length were not investigated. Inter-individual differences in telomere length may be a limiting factor for determining the relationship between telomere length and dietary pattern. In other words, studies in different countries have been conducted on different races and ethnicities. In addition, inflammation and oxidative stress status may vary in different population groups. The contradiction in the findings of the studies can be related to the difference in the cooking methods and the type and amount of food consumed by the study participants. 

Ethical Considerations
Compliance with ethical guidelines
This is a review study. No experiments were performed on human or animal samples. Therefore, there was need for considering ethical principles.

Funding
This study was funded by Larestan University of Medical Sciences.

Authors' contributions
Searching and screening articles: Somaye Yosaee, Sepideh Soltani and Shamayel Madadi; Writing: Somaye Yosaee, Sepideh Soltani, Mahkameh Ashoorpour and Shamayel Madadi; Data extraction: Sepideh Soltani, Mahkameh Ashoorpour and Mohamadreza Zare; Editing & review: Mohamadreza Zare and Shamayel Madadi.

Conflicts of interest
The authors declared no conflict of interest.

Acknowledgements
The authors would like to thank Larestan University of Medical Sciences.


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