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With the paperback version of my book coming out July 12 of 2012 (Just Tell Me What to Eat! The Delicious Six-Week Weight Loss Plan for the Real World), I've been reflecting on the myriad diets out there. There are lots of them and they make all sorts of claims - weight loss, belly fat, improved energy....
I have avoided making such claims for Just Tell Me What to Eat!, but in an interview the other day I was specifically asked about a few of the more popular claims and whether the Mediterranean diet could deliver on any of these claims. The interviewer's focus was on "belly fat" and it's clear that people are concerned about it and are looking for a diet plan to help them with it. While I don't care for hype, the fact is that a Mediterranean diet offers many of the same benefits that fad diets make their claims about, but the Mediterranean diet has the distinction of being backed by solid research - and you get to eat great food.
One of the cornerstones of the Mediterranean style diet is consuming a higher proportion of monounsaturated fats. It does appear that diets that are higher in mononusaturated fats (MUFA) are more effective at controlling abdominal adiposity (belly fat) than diets more rich in carbohydrates. As with a lot of the research on the physiology of nutrition, much of this is still in the speculative phase, but the science is quite good.
Several mechanisms for this effect have been proposed. These include changes in insulin sensitivity that have been observed during nutritional manipulations in research that compares carbohydrates to monounsaturated fats. These appear to be based on nutritionally induced differences in hormone secretion and differences in how glucose and lipids (fats) are handled.
At the same time there are differences from one body area to the next in the patterns of adipokine production (these are hormones that modulate fat storage). There are similar differences in fatty acid metabolism and these may influence the relative effects on fatty tissue deposition in certain parts of the body, including the abdomen. For instance, in one study fasting plasma leptin concentrations fell with consumption of monounsaturated fats. (Leptin signals regulation of energy storage and insulin activity.) With higher carbohydrate diets leptin is increased and that may help explain central adipose deposition.
While olive oil is the predominant fat in Mediterranean countries, there isn't all that much evidence that this is a phenomenon specific to olive oil. It does get a lot of attention because the countries that export olive oil as well as the companies that process olive oil invest in the research. There are, however, equally good (but fewer) studies indicating that this is a "class effect:" other oils high in monounsaturated fat, such as canola and grapeseed oil, show similar benefits on fat deposition.
We also know that consuming more whole grains also decreases that change in deposition of abdominal fat. As with fat consumption, I don't believe we have all the metabolic mechanisms worked out, but the research is still strong.
In one recent study in the American Journal of Clinical Nutrition, increasing whole grain intake was found to be associated with a decreased deposition of abdominal fat in adults (2010;92:1165-71). Higher intakes of refined grains were associated with more belly fat. This supports repeated research that it's not about eating a low- or high-carb diet but eating good quality carbohydrates. We do need more research to help us understand the mechanisms of how whole grain and refined grain foods influence body fat distribution.
In one large observational study researchers showed that following a Mediterranean diet high in foods of vegetable origin and unsaturated fats was associated with lower abdominal adiposity, as measured by waist circumference in both men and women (J Nutr 2009;139:1728-1737).
So yes, the Mediterranean diet can also make that sensational claim of "LOSE BELLY FAT," but the truth is that those claims are backed up by great science. The Mediterranean diet allows you to eat great food (not low carb or high protein or low fat or some other complicated fad diet) and lose your belly fat.