The short-term dietary studies that demonstrated a benefit of high-fat versus high-carbohydrate diets on post-prandial blood glucose values (72,77) may, as discussed above, have been accounted for by the use of high glycaemic index carbohydrates in these studies. Jovanovic's group (78) have also stated that the addition of dietary saturated fat to a test meal produces a significantly lower glycaemic and insulin response than when the test meal contains the equivalent proportion of monounsaturated fat. The differences may be explained by slower gastric emptying when the meal contains a high saturated fat content. However, we believe that even if the glycaemic response mid-morning can be lowered by increasing the saturated fat content of the breakfast, advocating such a diet to women at future risk of diabetes and cardiovascular disease remains highly questionable, when epidemiological and clinical studies show that high-fat diets are associated with insulin resistance, p-cell dysfunction, and recurrent GDM pregnancy and future diabetes (56,59). Also the long-term effects of a high maternal saturated fat diet on cardiovascular health is unknown. Animal studies certainly suggest caution as high-fat diets in pregnant rodents can promote cardiovascular disease in the next generation (79,80). Increasing the saturation content of the diet in pregnant rats leads both to unfavourable changes in fatty acid compositions and function of the major arterial vessels. High-fat diets in pregnancy have also been associated with severe hyperemesis gravidarum, with a 5.4-fold increased risk reported for every additional 15g/day of dietary saturated fat (81).
Increasing the polyunsaturated fat (PUFA) content of the diet while restricting the saturated fat may provide an alternative approach to safely reducing the overall dietary carbohydrate content. A large epidemiological study in China reported that a high habitual intake of dietary PUFA with a correspondingly raised low dietary polyunsaturated to saturated fat ratio protected against gestational diabetes (57). It remains to be proven whether Western women would achieve a similar benefit, as their PUFA intake is highly correlated with saturated fat intake.
The potential benefits of increasing monounsaturated fat (MUFA) intake in pregnancy still need to be shown. A recent small Danish study failed to show any improvement in insulin sensitivity in late pregnancy when women with GDM eat diets high in MUFA rather than high in carbohydrates, although a favourable effect on blood pressure was reported (82). Outside pregnancy improved insulin resistance and lipid profiles have been reported when either a high-carbohydrate diet or a monosaturated-enriched diet replaces dietary saturated fat, with reductions in plasma LDL cholesterol observed (83,84). If high-MUFA diets are to be promoted over a high-carbohydrate diet, one needs to ensure that overall calorie intake leading to unnecessary weight gain does not occur (85).
A large Swedish epidemiological study has suggested that increasing dietary long-chain n-3 fatty acids (omega-3 fatty acids) by increasing fish and fish oils may provide some protection against low birth weights and pre-term deliveries (86). Similar diets in Type 2 diabetic subjects have been shown to have some favourable metabolic effects on serum triglycerides but not plasma LDL cholesterol (87,88). Other food sources of n-3 polyunsaturated fatty acids include flaxseed and flaxseed oil, canola oil, soybean oil and nuts. Population studies suggest that foods containing n-3 fatty acids, specifically eicosapentaenoic acid and docosahexaenoic acid, may provide long-term cardio-protection (89,90). There are therefore potential theoretical benefits for increasing dietary long-chain n-3 fatty acids in diabetic women both in and out of pregnancy.
In the face of no real clinical-based studies on the optimal ratio between saturated, poly, mono and fish oils for pregnancy, it is our policy to aim for a ratio of sat:poly:mono of 1:1:1, with the specific advice to eat oily fish three times a week (91). These recommendations are similar to those for people with diabetes and coronary heart disease.
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