In the current study, we found no relationship of PDI, hPDI, and uhPDI with NAFLD in adults. Moreover, individuals with high adhering to PDI consumed higher amount of vitamin B6, B9, calcium, and zinc.
There are limited evidence examining the association between PDI and NAFLD in adults. Similar to our findings, a study carried out by Choi et al investigated that vegetarian diet, an antioxidant rich source, was not associated with odds of NAFLD in Buddhist priests (15). In contrast, Chiu et al observed that people having vegetarian diet were protected from fatty liver than non-vegetarians (26). In contrast to our results, a research conducted on elderly population examined three priori dietary patterns including Mediterranean Diet Score (MDS), the Dutch Dietary Guidelines (DDG), and the World Health Organization (WHO). Finally it was investigated that WHO diet as plant-based, high-fiber, and low-fat diet was linked to regression of NAFLD (27). Moreover, another study in which liver function was examined using alanine aminotransferase (ALT), aspartate aminotransferase (AST) and fatty liver index (FLI) reported that adults adhering to hPDI had lower risk of NAFLD and more optimal liver function (18). Overall, there are contradictory results related to PDI and vegetarian diet; however, it seems to have more likely a protective role in odds of NAFLD.
A review came to the conclusion that PDI can alleviate fatty liver disease (28). There are different mechanisms related to the impact of this specific diet on NAFLD. Since this crucial liver disease accompanied by inflammation and oxidative stress, the influence of some food as anti-inflammatory and antioxidant sources could help (28, 29). It is noteworthy to point out that fruits such as grapes having resveratrol as antioxidant source can inhibit oxidative stress and upregulating signaling pathway in HepG2 cells (30). Vegetables such as tomato inhibits autophagy in the NAFLD cell model by upregulating the peroxisome proliferator-activated receptor alpha (PPAR-α) signaling pathway (31). A hypothesis is that the prevalence of NAFLD in our population was not great enough to show this inter-relationship. Another hypothesis is related to the difference between raw and cooked vegetables stated that cooked vegetables cannot influence on NAFLD than raw ones which we did not consider this contrast in the current study (32). Furthermore, PDI had minimum level in our participant needing to be achieved for health benefits. Considering the use of sample-based scores, the diets of the population may be less healthy (or less plant-rich) than those of professional recommendations.
Our study had some strength needing to mention. We considered individuals with and without NAFLD. Also, we analyzed three type of PDI as PDI, hPDI, and uhPDI. However, this research has some limitations. First, it was case control study that did not allow us to interpret the causal relationships. Second, we did not consider the prevalence of NAFLD in the participants that it might reduce the power to detect the association.
In conclusion, there was not any association of PDI, hPDI, and uhPDI with NAFLD in adults. More research needs to examine whether this specific diet could impact and improve NAFLD in cohort study.