Plant-based diets, genetic predisposition and risk of non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) has emerged as the most common chronic liver disorder globally [1]. Currently, nearly 32.4% of adults worldwide have NAFLD [2], while around 1 in 3 people has an early stage of the disease in the UK [3]. Diet is an essential modifiable risk factor for NAFLD, and dietary patterns, in which nutrients and foods are consumed in combination, reflect real-world dietary practice [4].

Plant-based diet patterns nowadays are gaining attention as their environmental sustainability benefits [5], and diet patterns characterized by high consumption of healthy plant-based foods were associated with lower NAFLD prevalence and liver fat content [6, 7]. However, not all plant-based foods were beneficial to NAFLD, as less nutrient-dense plant foods, including refined grains, fruit juices, and sugar-sweetened beverages, are associated with higher NAFLD risk [8‐10]. To distinguish the plant-based diets with different quality, previous studies have developed three plant-based diet indices (PDIs), an overall plant-based diet index (PDI), which emphasizes the intake of all plant foods; a healthful plant-based diet index (hPDI), which emphasizes higher consumption of healthful plant-based foods such as whole grains, vegetables, nuts, legumes, coffee, and tea; and an unhealthful plant-based index (uPDI), which highlights the consumption of less healthful plant-based foods associated with increased risks of several chronic diseases [11]. The associations of lower overall PDI and hPDI but higher uPDI with higher liver fat content and the prevalence of fatty liver have been reported [12, 13], while insignificant associations were shown in other studies [14, 15]. Given the limited sample size and inconsistent findings of existing studies, evidence from large population-based studies with a prospective design is warranted.

The development of NAFLD is a consequence of an interaction between environmental and genetic factors [16]. To date, several NAFLD-associated loci have been identified in genome-wide association studies [17]. However, no studies have examined the interaction between diet patterns and genetic predisposition on NAFLD primary prevention, and only one study showed that improved adherence to the Mediterranean diet pattern or the Alternative Healthy Eating Index (AHEI) was associated with reduced liver fat content, particularly among individuals with high genetic risk [18]. Therefore, our study aimed to longitudinally investigate the association between PDIs and NAFLD risk and to explore whether such associations would be modified by the genetic risk of NAFLD.

The baseline characteristics of 159,222 participants are shown in Table 1. The mean age was 58.0 ± 8.0, and 55.7% were female. The mean (SD) times of 24-h dietary assessment were 2.2 (1.2). The overall PDI ranged from 25 to 74, hPDI ranged from 27 to 82, and uPDI ranged from 27 to 78. Participants with higher overall PDI and hPDI but lower uPDI tended to be female, well-educated, non-current smokers, and with lower BMI. Total energy intake was higher among participants with higher overall PDI but lower in those with higher hPDI and uPDI. The baseline characteristics and PDIs were generally consistent among total participants and those with MRI-PDFF data (n = 20,692, Additional file 1: Table S4). The baseline characteristics by NAFLD status are shown in Additional file 1: Table S5.

During a median follow-up of 9.5 years, 1541 NAFLD cases were documented. We did not observe significant departures from linearity when the non-linearity of overall PDI, hPDI, and uPDI with NAFLD risk was tested using RCS (Fig. 1, p-nonlinearity > 0.05 for all PDIs). The cumulative risks of NAFLD by PDIs quintiles are shown in Additional file 1: Figure S3. Compared to the lowest quintile, multivariable-adjusted HRs of NAFLD in the highest quintile were 0.78 (95% CI, 0.66–0.93, p-trend = 0.02), 0.74 (95% CI, 0.62–0.87, p-trend < 0.0001), and 1.24 (95% CI, 1.05–1.46, p-trend = 0.02) for PDI, hPDI, and uPDI, respectively. These associations were stronger when not adjusting for BMI (Table 2). Additionally, per 10-point increment of PDIs was associated with an 11% lower, 20% lower, and 14% higher risk of NAFLD (with HRs 0.89 [95% CI, 0.81–0.97], 0.80 [95% CI, 0.73–0.88], and 1.14 [95% CI, 1.05–1.24] for overall PDI, hPDI, and uPDI, respectively (Table 2). When the liver fat content was indicated by MRI-PDFF and further adjusted for age at MRI in the final model, higher overall PDI and hPDI were associated with lower intrahepatic fat content (β [95% CI] per 10-point increment were − 0.34 [− 0.44, − 0.25] and − 0.45 [− 0.54, − 0.36], respectively), while higher uPDI was associated with higher liver fat content (β [95% CI], 0.41 [0.32, 0.49], Fig. 2, Additional file 1: Table S6).

When assessing the joint association of PDIs and PRS with NAFLD risk, compared with the highest risk combinations, most other groups for overall PDI and hPDI had significantly lower NAFLD risk, and NAFLD risk was lowest in participants with low genetic risk and highest PDI/hPDI scores. On the contrary, compared to those with the lowest PRS and lowest uPDI, the NAFLD risk was increased with higher PRS and uPDI (Additional file 1: Figure S4). In joint associations of MRI-PDFF, the β-coefficients were gradually increased with increased PDIs and PRS (Additional file 1: Figure S5). In stratified analyses by genetic risk, the associations of higher overall PDI and lower uPDI with lower NAFLD risk were not modified by genetic susceptibility to NAFLD, but the association of hPDI with NAFLD risk was significantly modified by genetic risk (P-interaction > 0.05 for overall PDI and uPDI, P-interaction = 0.03 for hPDI, Additional file 1: Tables S7-S9).

Associations of PDIs with NAFLD risk and liver fat content persisted when using sex-specific quintiles of PDIs, and no significant interaction between sex and PDIs was observed (Additional file 1: Tables S10-S11, Figure S6). In addition, we observed no significant modifications by other major risk factors (age, obesity, energy intake, alcohol consumption, or physical activity) of NAFLD risk or MRI-PDFF, except the association between uPDI and NAFLD risk was modified by obesity (p for interaction = 0.0009 < 0.003 (0.05/(3 exposures * 5 groups)), Additional file 1: Tables S12-S13). Associations between PDIs and NAFLD remained largely unchanged when we further adjusted for chronic disorders and liver functions (Additional file 1: Table S14, sensitivity analysis 1&2); when we further adjusted for metabolic indicators and waist circumference (Additional file 1: Table S14, sensitivity analysis 3&4); when we excluded participants with less than twice diet assessments (Additional file 1: Table S14, sensitivity analysis 5); and when we excluded participants with less than 2 years of follow-up (Additional file 1: Table S14, sensitivity analysis 6). In addition, we observed significant mediating effects of BMI on PDIs-NAFLD risk associations, which were 51.8%, 47.0%, and 46.5% for PDI, hPDI, and uPDI, respectively (Additional file 1: Table S15). Furthermore, when we excluded each one of 17 food groups at a time from PDIs and adjusted for the excluded food group intake, the adjusted HRs for each 10-point increment in PDIs were not substantially altered. However, higher intakes of nuts, tea, and coffee were associated with lower NAFLD risk, but higher intakes of sugar-sweetened beverages, fish, and sea foods were associated with higher NAFLD risk (Additional file 1: Table S16). These results indicated that associations between PDIs and NAFLD risk might largely be driven by higher intakes of coffee and tea and lower intakes of sugar-sweetened beverages, fish, and sea foods.

In the present longitudinal study, we found that greater intake of plant-based diets, particularly healthful plant-based diets was associated with lower NAFLD risk and liver fat content, while a higher uPDI was associated with increased NAFLD risk and higher liver fat content. Compared to the lowest quintile, participants in the highest quintile of overall PDI, hPDI, and uPDI had a 22% lower, 26% lower, and 24% higher risk of NAFLD, and 0.51% lower, 0.71% lower, and 0.72% higher liver fat content, respectively.

The longitudinal evidence on associations between plant-based diets and NAFLD risk is scarce, and only several cross-sectional studies reported inconsistent associations between PDIs and NAFLD [12‐15]. A study of 18,345 participants in the National Health and Nutrition Examination Survey (NHANES) showed 21% lower, 24% lower, and 34% higher odds of NAFLD for overall PDI, hPDI, and uPDI comparing the highest tertile to the lowest, respectively, in which NAFLD was diagnosed based on the fatty liver index (FLI) [13]. However, in another study of 3900 participants in NHANES, only hPDI showed a significant association with lower transient elastography-diagnosed NAFLD prevalence, and the positive association between uPDI and NAFLD was largely modified and insignificant after further adjusting for BMI [15]. Besides the sample size, the NAFLD diagnosis method might also account for the aforementioned inconsistent findings. Though FLI was frequently used in several large population-based studies to define fatty liver disease (FLD), the more accurate non-invasive measurement of liver steatosis was MRI-PDFF [26]. In another study of 578 participants, the association of three PDIs and the likelihood of MRI-diagnosed FLD was not significant, which might contribute to the limited sample size [14]. In our preregistered analysis, the associations of higher overall PDI and hPDI and lower uPDI with lower NAFLD risk remained significant in fully adjusted model and sensitivity analyses, and the longitudinal study design, the larger sample size, and a more accurate assessment method for intrahepatic liver content increased the confidence in our findings.

The interplay between genetic risk and PDIs has not been reported, and only one study has examined the interaction between the whole diet quality and the overall genetic risk of NAFLD [18]. Based on the Framingham Heart Study, Ma et al. reported that improved diet quality (represented by Mediterranean diet score and AHEI) modified the genetic risk of NAFLD on the liver fat content increase. Though not PDIs, the richness of plant-based foods in the Mediterranean dietary pattern and the good correlation between AHEI and hPDI [30] hinted that PDIs might interplay with NAFLD genetic risk. In our analysis, the significant multiplicative interaction between PDIs and NAFLD-PRS on the risk of NAFLD was observed in hPDI in a sex-specific manner, which might be partly due to the higher NAFLD risk observed in men rather than in women [31].

Several food groups might account for the observed associations, including whole grains, tea, coffee, sugar-sweetened beverages, and red meat. The associations of nuts, tea, and coffee consumption with lower NAFLD risks in our study were in line with previous findings [32, 33]. Those associations might contribute to the higher intake of dietary fibers, flavonoids, caffeine, phytosterols, and plant proteins following a plant-based diet rich in healthy plant-based foods [34], which are all shown to have effects on improved insulin resistance, decreased central obesity, and improved gut microbiome, and hence reduce NAFLD risk [35‐39]. The significant mediating effects of BMI on PDIs-NAFLD associations observed in our study also partly supported the aforementioned mechanisms. Besides beneficial foods and nutrients, the positive association between a higher intake of sugar-sweetened beverages and NAFLD risk was in agreement with a previous umbrella review of meta-analysis [40], and the accompanying intake of fructose has proved to promote liver fat accumulation [41]. In addition, previous evidence showed that red meat consumption was associated with increased NAFLD risk [42], but not white meat [43]. In PDIs, red meat and white meat are both grouped into meat, and the inconsistent associations between these two types of meat might explain the marginally significant association of meat with NAFLD risk in our analyses.

The longitudinal study was based on a relatively large sample with a long follow-up period. We used a validated dietary recall method and every dietary assessment information to quantify participants’ dietary intake, which limited the measurement bias. Several limitations should be mentioned. First, the dietary assessment was based on 24-h recall, which might be subjected to recall bias and lead to misclassification. However, the misclassification would likely bias our results toward the null. In addition, the representation of long-term dietary habits was limited, while the results were not substantially changed when we limited our analyses to those who completed at least twice dietary assessments. Second, NAFLD cases were ascertained based on primary care, in-hospital record, and death registry data, which might potentially underestimate the true NAFLD incidence. However, it is unlikely that the underdiagnosed NAFLD cases would be diet-specific. Assuming that the specificity of outcome detection is perfect and sensitivity is lower than 100% in both exposure groups, outcome misclassification would produce little bias in estimating the hazard ratio [44]. Third, though we have controlled the majority of confounders, the potential confounding factors are still likely. Fourth, insulin resistance status in UK Biobank was not available, which interfered with the interpretation of its effect on associations of PDIs with NAFLD risk and liver fat content. However, further adjusting for waist circumference, a marker of central obesity and closely associated with insulin resistance, did not largely change the results. Last, our analyses were conducted among Europeans, limiting our findings’ generalization to other ethnic groups.

Our results suggested that higher intakes of overall and healthful plant-based diets were associated with lower NAFLD risk regardless of genetic susceptibility. Conversely, an unhealthful plant-based diet was associated with increased NAFLD risk. Our findings highlighted the importance of the quality of plant-based food when adhering to a plant-based dietary pattern to prevent NAFLD in the entire population.