Based on literature, since there is a strong association between NAFLD and obesity, type 2 diabetes mellitus (T2DM), hypertension, dyslipidemia, metabolic syndrome, and increasing incidence of non-communicable diseases worldwide, NAFLD has become a new challenge for public health (
3). The prevalence of NAFLD is reported between 10 and 30% all over the world (
4), and a systematic review estimated NAFLD to be 15% - 20% in Asia (
12). The prevalence of NAFLD was 43.8% in a study in the north of Iran (
13), and 15.3% in another study in the south of Iran (
14). In this study, 95 (19.9%) of participants were diagnosed with NAFLD among whom, 13 (2.7%) were lean and 82 (17.2%) had BMI ≥ 25. Overall, starchy foods and potato intake were significantly different between the groups. In non-lean group, potato intake was higher among NAFLD compared to non-NAFLD and in lean group, total starchy foods intake was higher in those with NAFLD compared to the counterpart group. The study showed that the chance of NAFLD would increase by an increase in carbohydrate, potato, and fat intake.
According to our results, sex, age, marital status, and BMI were associated with NAFLD. In previous studies, gender (
13,
15-
18), age (
16,
18,
19), and weight or BMI (
15,
16,
19) were associated with NAFLD. However, some studies have shown that there are no gender differences in the development of fatty liver (
19).
Some studies have investigated nutritional risk factors in NAFLD. In our study, there was a significant relationship between intake of fat and carbohydrate and incidence of NAFLD. However, the sources of fat such as vegetable oil, meats, dairy, and fast-foods were not significantly associated with NAFLD although total fat diet showed a significant relationship with the disease. Polyunsaturated fatty acids were also significantly associated with NAFLD (P = 0.02) but there was no significant association between saturated fatty acids, monounsaturated, cholesterol, and omega-3 fatty acids, and NAFLD in this study.
Western diets through increasing fat intake have been associated with the incidence of NAFLD and its severity. Increased intake of SFA and less consumption of PUFA, especially n-3 PUFA, are common eating behaviors among patients with NAFLD as literature shows that SFA can worsen NAFLD progression because of its disturbance in lipid and glucose homeostasis (
3).
Fish oil is rich in eicosapentaenoic and docosahexaenoic acids, the main n-3 PUFAs, that have shown to have protective effects (
3). In our study, these fatty acids were not significantly associated with NAFLD. Perhaps their low consumption due to high price for these fatty acids can explain why we did not see any difference between the two groups. It is also a fact that fish consumption is more common in some parts of Iran which are closer to the sea (Caspian Sea and Persian Gulf); but in other regions, fish is not a common food in daily diet of people and it is rarely consumed. Although the mean daily consumption of fish was higher in our study than previous studies (
20), a study in Iran revealed that the gap between the current fish consumption and the amounts necessary for maintaining healthy diet is still far from satisfactory (
21).
At present, the impact of the amount and type of dietary proteins, which contribute to the development of obesity and its subsequences, is not understood well. Therefore, it is not possible to determine its effect on NAFLD because of the lack of clear evidence (
3). In our study, its association with NAFLD was not significant, as well.
Consumption of dietary fructose which is primarily available in beverages has increased simultaneously with the worldwide increase of obesity, diabetes, and NAFLD. Some studies have suggested that there is a direct link (
3) between the consumption of fructose in patients and NAFLD, which was nearly 2 to 3 times higher compared to controls (
22). However, in our study, there was no association between fructose and NAFLD. Syrups and products containing fructose, which are common in the Western diet, are not consumed as much in Iran, and fructose in Iranian diet often comes from fruits.
In another research, the amount of soft drinks intake was almost twice in NAFLD group (P = 0.03), and they ate 27% more meat (P < 0.001) (
23). However, we did not find any association between the consumption of soft drinks and lean meat and NAFLD.
Dietary carbohydrate increases blood insulin level and triglyceride concentration leading to the formation of fat in liver (
3). In our study, there was a link between carbohydrates and NAFLD. It was also revealed that each gram increase in carbohydrate consumption cant increase the chance of NAFLD as 1.99 times (P = 0.004). Based on previous studies in Iran, carbohydrate intake is much higher than the recommended amount (providing 45% - 65% of required energy). Food and agriculture organization (FAO) estimated it about 69%. (
24), and according to the national food consumption survey, more than 60% - 70% of the Iranian energy intake is obtained from carbohydrates (
25). In this study, there was no association between NAFLD and bread and rice consumption; but potato consumption was meaningfully associated with the disease. However, reviewing the food balance sheet of FAO (
26) for Iran and other countries that have similar staple food to Iran, such as China and India, which are the main producers of rice, wheat, and potato in the world, has shown that energy intake per capita of cereals in Iran is 1,497 kilocalories which is higher than the corresponding values in China and India (1,440 and 1,394 Kcal, respectively). Energy intake per capita of potato is also higher in Iran (125 Kcal/capita/day compared to 80 in China and 46 in India). Cereals are refined in Iran; thus, they have higher amounts of glycemic index and glycemic load (
25), which may lead to the development of NAFLD.
Similar to our results, in earlier studies, type and amount of carbohydrates were mentioned as important factors in the development of fatty liver in Central Asia when compared to fat intake (
25). Despite the weak impact of fat on NAFLD in our study, because of the introduction of processed and high fat foods in Iranian diet, it should be given a greater attention than ever before.
Nowadays, the number of lean NAFLD patients is on the rise. Studies tend to review and determine predictors of lean NAFLD. In the present study, we compared nutrient intake in lean and non-lean NAFLD participants.
However, no significant difference was found among these groups regarding nutrients and food items. Only potato and total starchy foods intake was significantly different between the groups. In Younossi et al. (
27) study, food items, micronutrients and macronutrients among lean NAFLD subjects and lean healthy controls were investigated, but then again, no differences were found in macronutrients intake (carbohydrate, fat, and protein), vitamins, and minerals between these groups. Only copper, beta-tocopherol, galactose, pectin, and phytic acid were different between the groups and the authors suggested that these results may be an indication that NAFLD in lean patients has occurred due to other metabolic abnormalities. Others have postulated that the cause of their NAFLD could be due to genetic characteristics, impaired intestinal motility, and some other metabolic disturbances that are not related to weight status (
27).
Given the rising prevalence of obesity and NAFLD and the importance of a multi-sectorial approach toward correction of poor nutritional behaviors in the community, as its necessity has been regarded in other community-based problems (
28), the integration of these interventions in primary health care (PHC) and recently established family physician program (
29) of Iran should not be overlooked. Besides studying NAFLD in obese patients, lean NAFLD patients should not be neglected. We suggest more studies with adequate sample size assessing dietary intakes, dietary pattern, and diet quality in lean-NAFLD people to find if there is any association between NAFLD and diet.
Due to the significant association between fat and refined starches and fatty liver, and because of ever increasing consumption of processed and high fat foods, it seems these items should be given greater attention than ever before. Most cereals are almost completely refined in Iran; thus, increasing supply of whole grains and paying attention to training and health promotion programs in this area can be a priority for policy-makers in Iran to prevent the rising trend of NAFLD in population.
4.1. Implication for Health Policy Makers/Practice/Research/Medical Education
The pathophysiology of NAFLD can be impressed by diet and nutrients, and assessing the impact of dietary nutrients can help prevent or treat the disease. Refined carbohydrates and starch intake especially potato, in both lean and non-lean groups with NAFLD, should be also taken into consideration.