Supervisors info:
Καλιώρα Ανδριάνα, Επίκουρη Καθηγήτρια, Τμήμα Επιστήμης Διαιτολογίας - Διατροφής, Χαροκόπειο Πανεπιστήμιο
Τεντολούρης Νικόλαος, Καθηγητής, Ιατρική Σχολή, ΕΚΠΑ
Κόκκινος Αλέξανδρος, Αναπληρωτής Καθηγητής, Ιατρική Σχολή, ΕΚΠΑ
Summary:
Objective: The aim of this study is to examine the effect of dietary fat on type 2 diabetes mellitus. For the fullest approach, dietary fat was divided a) based on the type of fatty acids (monounsaturated, polyunsaturated, saturated and trans) and the effect of each of them on type 2 diabetes mellitus b) based on the percentage of energy that covers fat (both total and partial fatty acids) in the total energy intake, shaping the most prevalent nutritional models (Mediterranean diet, low fat diets) and consequently the effect of these on type 2 diabetes mellitus.
Method: An extensive search was made in the databases Medline (via PubMed), Scopus, Embase and Cochrane Library for published clinical trials, reviews and meta-analyzes of both epidemiological studies and clinical trials. The publication date of the searched publications was limited between 2000 and 2018. Publications written in English were selected, while publications written in any other language were excluded.
Results: It seems that the effect of fatty acids on type 2 diabetes mellitus varies according to its type. Regarding, polyunsaturated fatty acids and especially ω3, they all seem to have beneficial effects on diabetes and its complications. However, there is literature that does not confirm the beneficial effect of supplemental fish oil intake in improving insulin sensitivity and insulin secretion, while meta-analyzes highlight the beneficial effect of fatty fish as a food. In fact, intakes of> 10g of fish oil/daily have a negative effect on glucose tolerance and insulin resistance of diabetic patients. Supplemental eicosapentaenoic acid uptake seems to improve the expression and activity of paroxonase 1 enzyme (PON1), confirming the already well-documented anti-inflammatory and antioxidant effects of this polyunsaturated fatty acid on reducing oxidative stress and thus reversing chronic complications of diabetic patients. According to the results of large-scale studies, the higher dietary intake of total fat, monounsaturated and polyunsaturated fatty acids are associated with lower risk of cardiovascular disease, cardiovascular death and death from all causes, while the higher intake of saturated fat is associated with higher risk. And while for several years saturated fat was called "bad fat", with negative effects on cardiovascular health, insulin resistance and type 2 diabetes mellitus, nowadays, the literature seems to separate it and examine the different effects of individual saturated fatty acids. Depending on the number of carbon atoms, the effect of saturated fatty acid on the body appears to change. Saturated fatty acids with even number of carbon atoms (all of 14: 0 myristic , 16: 0 palmitic, 18: 0 steatic) are positively associated with type 2 diabetes, while saturated fatty acids with odd chain (all of 15:0 pentadecanoic acid and 17:0 heptadecanoic acid or margaric acid) are negatively correlated with type 2 diabetes. Long and very long chain (20:0 arachidic acid, 22:0 behenic acid, 23:0 tricosanoic acid and 24:0 lignoceric acid) also negatively correlate with diabetes. However, their measurement in the blood should not be translated as nutritionally saturated fatty acids, as some of them derived from endogenous sources. Correspondingly, a different effect on the disease appears to exert trans fatty acids depending on the source of origin (industrially harmful, while ruminant protective), without these findings being conclusive. Regarding the literature reviewing nutrients replacements, it seems that replacing saturated fat with polyunsaturated, monounsaturated or high quality carbohydrate are associated with a lower risk of coronary heart disease, total mortality and other endpoints, while replacing refined carbohydrates/sugars with monounsaturated, polyunsaturated fatty acids or high-quality carbohydrates are associated with a lower risk. Strong criticism has recently been given to complete fat exculpation and incrimination of carbohydrates as a whole, focusing on methodological limitations involving such misleading conclusions.
Regarding the study of dietary models, all prospective studies support the protective role of Mediterranean diet in diabetes, with overall risk reduction ranging from 12-83% for highly adherent individuals, taking into account possible confounders. In fact, the PREDIMED clinical trial showed a 52% reduced risk of developing Type 2 diabetes mellitus for the Mediterranean diet group versus the reduced fat group. The majority of clinical trials examining the effect of the Mediterranean diet model on diabetic patients show its protective role in glycemic control and insulin sensitivity, as well as its superiority to control diet. Meta-analysis results show that low carbohydrate, low glycemic, high protein, DASH, and Mediterranean-type diets all lead to greater glycemic control improvement compared to corresponding control diets. The greatest benefit, however, is for the Mediterranean diet when compared to usual care or low fat diet. Several mechanisms have been proposed through which the Mediterranean diet has these beneficial effects on the disease. These include both indirect effect, via weight loss, and direct effects, via consumption of nutrients-rich foods, fiber, antioxidants, and a high ratio of monounsaturated / saturated fatty acids. These lead to improved inflammation markers, increased antioxidant capacity, and increased anti-inflammatory cytokines such as adiponectin levels, all of which improve insulin sensitivity, one of the two main mechanisms involved in the development and progression of the disease. The second mechanism is b-cell dysfunction, but to date there is no scientific evidence to guide nutrition guidelines to improve its function.
Keywords:
Dietary fat, Type 2 diabetes mellitus, Saturated-fatty acids, Trans fatty acids, Polyunsaturated fatty acids, Mediterranean diet, Low fat diet
