IBD has traditionally been thought of as a disease of the Western hemisphere, however there is an increasing incidence in Japan, Hong Kong, Korea and Eastern Europe [
62,
63]. Although still rarer, an increasing incidence of IBD is also being identified in South Africa, South America and Saudi Arabia [
64,
65,
66]. The dramatic rise in incidence of IBD, particularly in South Asia, India and Japan, where traditionally there was a low incidence, suggests that environmental factors, such as the Western diet pattern, play an important role in disease pathogenesis [
67,
68,
69]. This hypothesis is further confirmed by the increasing incidence of the disease in immigrants to the Western hemisphere. Migration from a country with a history of low-incidence to a country of a higher incidence increases the risk of developing IBD, particularly in the first generation children [
70]. Diet composition has long been suspected to contribute to IBD. Thus, dietary patterns and nutrients are important environmental factors to consider in the etiology of IBD [
22,
71].
3.1. The Western Diet Pattern
The diet of today is considerably different from the traditional diet of previous generations, when the prevalence of IBD was considerably lower. The Western diet pattern is dominated by increased consumption of refined sugar, omega-6 polyunsaturated fats and fast food, combined with a diet deficient in fruit, vegetables, and fiber [
72]. Much of today’s food supply has been processed, modified, stored and transported great distances, in contrast to the traditional diet, where food that was produced locally was consumed shortly after harvest. This shift to the Western diet pattern is hypothesized to increase pro-inflammatory cytokines, modulate intestinal permeability, and alter the intestinal microbiota promoting a low-grade chronic inflammation in the gut [
73]. A diet that contains pro-inflammatory foods is an important risk factor in the development of UC. A case-control study completed in Iran with newly diagnosed UC patients (
n = 62 UC patients, 124 controls) found that subjects that had a higher dietary inflammatory index (pro-inflammatory diet) had an increased risk of developing UC (Odds Ratio (OR): 1.55, 95% Confidence Interval (CI): 1.04–2.32) [
23]. The authors concluded that encouraging intake of more anti-inflammatory dietary factors, such as plant-based foods rich in fiber and phytochemicals, and reducing intake of pro-inflammatory factors, such as fried or processed foods rich in trans-fatty acids, could be a potential strategy for reducing risk of UC. This was one of the first studies that has examined dietary inflammatory index as an outcome for developing UC. Several large scale studies have attempted to elucidate the dietary components that are associated with IBD risk [
22,
24,
26,
27]. Overall, this suggests that the Western diet pattern is a risk factor for IBD.
3.5. Dietary Fat Intake as a Risk Factor for IBD
There have been conflicting data on the association between dietary fat intake and the development of IBD, as many of the studies are retrospective and use small sample sizes. However, a very large, long-term, prospective study (
n = 170,805) completed over 26 years did not observe a significant association with increased risk of developing CD or UC with total dietary fat intake, saturated fatty acids (SFA) and monounsaturated fatty acids (MUFA) [
26], which has been well supported by other research studies [
74,
75,
76]. A growing body of scientific evidence indicates that the Mediterranean diet pattern has been associated with significant improvements in health status [
77,
78] and decreases in inflammatory markers in humans [
79]. The protective effect is hypothesized to be derived from the balance in fats, which includes incorporating MUFA, SFA and fish intake [
80]. While a few studies do show that MUFAs are beneficial during colitis, studies on the effects of SFA and PUFAs on gut health are controversial.
Dietary
n-6 PUFA, in particular linoleic acid, have been implicated in the etiology of IBD. Dietary
n-6 PUFAs are essential fatty acids present in high amounts in red meat, cooking oils (safflower and corn oil) and margarines. A prospective cohort study (
n = 203,193) conducted over four years found that intake of linoleic acid was associated with an increased risk of UC (OR: 2.49, 95% CI: 1.23 to 5.07,
p = 0.01) [
27]. Further analysis of the European Investigation into Cancer and Nutrition study (
n = 260,686) over five years found an increased risk of UC with a higher total PUFA intake (trend across quartiles OR = 1.19 (95% CI: 0.99–1.43)
p = 0.07) [
74], which was also supported by a systematic review (
n = 2609 patients with IBD) that examined pre-illness intake of nutrients and subsequent development of UC [
22]. A case-control study in CD found that increased total PUFA consumption was positively associated with CD risk (OR: 2.31, 95% CI: 1.12–4.79) [
30].
The Nurses’ Health Study cohorts (
n = 170,805 women with 269 incident cases of CD and 338 incident cases of UC) reported high, long-term intake of trans-unsaturated fatty acids was associated with a trend towards an increased incidence of UC (HR 1.34, 95% CI: 0.94–1.92) but not CD [
26]. An increased relative risk of developing IBD has also been associated with frequent intake of fast foods (fast foods are high in trans-unsaturated fatty acids) [
81,
82]. The relative risk associated with the consumption of fast foods at least two times a week was estimated at 3.4 (95% CI: 1.3–9.3) for CD and 3.9 (95% CI: 1.4–10.6) for UC [
82]. Frequent fast food intake, defined as more than once a week, was significantly associated with a risk of UC (43%, OR: 5.78, 95% CI: 2.38–14.03) and CD (27%, OR: 2.84, 95% CI: 1.21–6.64) [
81].
It has been speculated that the intake of long-chain
n-3 PUFAs (docosapentaenoic acid, eicosapentaenoic acid, docosahexaenoic acid), known as omega-3s, may be of benefit to patients with IBD. The beneficial effects are believe to be derived from the anti-inflammatory properties of
n-3 PUFAs; however, clinical and experimental studies have shown conflicting results [
83]. Meta-analyses have failed to show benefit with supplementation with fish oils in the maintenance of remission in CD and UC [
84,
85,
86]. Dietary intake of
n-3 PUFAs were inversely associated with risk of UC, whereas no association has been found with CD [
26]. The European Investigation into Cancer and Nutrition study (
n = 203,193) found a negative association with the development of UC with increasing dietary intake of the
n-3 PUFA, specifically docosahexaenoic acid (OR: 0.23, 95% CI: 0.06 to 0.97) [
27], and is supported by the European Investigation into Cancer and Nutrition -Norfolk study (
n = 26,639) (OR: 0.43, 95% CI: 0.22–0.86) [
57]. Two case-control studies in CD report that a diet with regular consumption of fish had a protective effect on the development of CD (OR 0.52, 95% CI: 0.33–0.80,
p = 0.003) and (OR 0.46, 95% CI: 0.20–1.06,
p = 0.02) [
29,
69].
The total ratio of
n-3 PUFA:
n-6 PUFA found in the diet has been hypothesized to be an important consideration. One prospective cohort [
87] and one case-control study [
29] report that a high
n-3PUFA:
n-6 PUFA ratio in the diet is inversely associated with the risk of IBD. In support of this explanation, a dietary intervention trial that focused on increasing the
n-3 PUFA:
n-6 PUFA ratio was found to be effective in maintaining disease remission in patients with both UC and CD, through increasing
n-3 PUFA intake [
88]. Overall, it does not appear that full fat diets should be avoided, however fat including diets rich in olive oil, dairy products and fish but not fish oil pills should be consumed while avoiding large intakes of vegetable oils rich in
n-6 PUFA.
In summary, several epidemiological studies provide compelling evidence for the role of food in IBD pathogenesis. Furthermore, the rise in incidence of IBD in countries that previously have had a very low incidence suggests that industrialization and adoption of the westernized diet may be a risk factor in the development of IBD. Reduced consumption of fruits and possibly vegetables, resulting in a reduced overall intake of fiber, with high intake of meats, fast foods and trans-fatty acids appears to be associated with an overall increase in the risk of developing IBD [
71].