Intake of dietary fats and fatty acids and the incidence of type 2 diabetes: A systematic review and dose-response meta-analysis of prospective observational studies

Background The role of fat quantity and quality in type 2 diabetes (T2D) prevention is controversial. Thus, this systematic review and meta-analysis aimed to investigate the associations between intake of dietary fat and fatty acids and T2D, and to evaluate the certainty of evidence. Methods and findings We systematically searched PubMed and Web of Science through 28 October 2019 for prospective observational studies in adults on the associations between intake of dietary fat and fatty acids and T2D incidence. The systematic literature search and data extraction were conducted independently by 2 researchers. We conducted linear and nonlinear random effects dose-response meta-analyses, calculated summary relative risks (SRRs) with their corresponding 95% confidence intervals (95% CIs), and assessed the certainty of evidence. In total, 15,070 publications were identified in the literature search after the removal of duplicates. Out of the 180 articles screened in full text, 23 studies (19 cohorts) met our inclusion criteria, with 11 studies (6 cohorts) conducted in the US, 7 studies (7 cohorts) in Europe, 4 studies (5 cohorts) in Asia, and 1 study (1 cohort) in Australia. We mainly observed no or weak linear associations between dietary fats and fatty acids and T2D incidence. In nonlinear dose-response meta-analyses, the protective association for vegetable fat and T2D was steeper at lower levels up to 13 g/d (SRR [95% CI]: 0.81 [0.76; 0.88], p(nonlinearity) = 0.012, n = 5 studies) than at higher levels. Saturated fatty acids showed an apparent protective association above intakes around 17 g/d with T2D (SRR [95% CI]: 0.95 [0.90; 1.00], p(nonlinearity) = 0.028, n = 11). There was a nonsignificant association of a decrease in T2D incidence for polyunsaturated fatty acid intakes up to 5 g/d (SRR [95% CI]: 0.96 [0.91; 1.01], p(nonlinearity) = 0.023, n = 8), and for alpha-linolenic acid consumption up to 560 mg/d (SRR [95% CI]: 0.95 [0.90; 1.00], p(nonlinearity) = 0.014, n = 11), after which the curve rose slightly, remaining close to no association. The association for long-chain omega-3 fatty acids and T2D was approximately linear for intakes up to 270 mg/d (SRR [95% CI]: 1.10 [1.06; 1.15], p(nonlinearity) < 0.001, n = 16), with a flattening curve thereafter. Certainty of evidence was very low to moderate. Limitations of the study are the high unexplained inconsistency between studies, the measurement of intake of dietary fats and fatty acids via self-report on a food group level, which is likely to lead to measurement errors, and the possible influence of unmeasured confounders on the findings. Conclusions There was no association between total fat intake and the incidence of T2D. However, for specific fats and fatty acids, dose-response curves provided insights for significant associations with T2D. In particular, a high intake of vegetable fat was inversely associated with T2D incidence. Thus, a diet including vegetable fat rather than animal fat might be beneficial regarding T2D prevention. Author summary Why was this study done? Type 2 diabetes is one of the most common noncommunicable diseases worldwide, with a substantial global health burden and healthcare costs. The role of dietary fat and fatty acid intake in type 2 diabetes prevention is under debate. It was our aim to investigate the associations of dietary intakes of total fat, animal fat, vegetable fat, and various types of fatty acids with type 2 diabetes incidence in an updated systematic review and dose-response meta-analysis of prospective observational studies, and to evaluate the certainty of evidence. What did the researchers do and find? This systematic review and dose-response meta-analysis included 23 prospective observational studies in adults (>= 18 years) on the associations between intake of dietary fats and fatty acids and type 2 diabetes incidence. In linear dose-response meta-analyses, no or weak associations between intake of dietary fats and fatty acids and type 2 diabetes incidence were found. In nonlinear dose-response meta-analyses, a decrease in type 2 diabetes incidence was associated with higher quantities of vegetable fat in the diet, as well as with lower doses of polyunsaturated fatty acids, including the plant-based fatty acid alpha-linolenic acid. A harmful association of saturated fatty acids with T2D incidence was not confirmed. What do these findings mean? These findings add to recent evidence that contradicts the long-held belief that diets high in fat increase the risk of type 2 diabetes. Additionally, they highlight the importance of the fat source, and specifically reveal an inverse association between plant-based fat intake and type 2 diabetes. The findings are limited by very low to moderate certainty of evidence.