A crucial part of of maintaining a new eating plan is eating in such a way that you can feel fuller longer and still take in less, a concept that we health care people called "satiety." One of the ways that people can achieve satiety is to make sure that they are actually getting adequate fat in their diet, as fat is one of the things that slows down the emptying of our stomachs. If the emptying of our stomach is slowed, it will take longer to get to our bloodstream, and there will be a smaller increase in the production of hormones like insulin which can stimulate the appetite (that was, once again, a really simple explanation of a very complicated process). It has also been proposed that one of the reasons people report increased satiety when they switch to a carbohydrate controlled plan, such as Atkins or Paleo, is because people have added fat bck in to their diet and therefore the above stated happens. They are, of course, probably eating a lot less carbohydrate per day (I had one friend who figured out he was eating the equivalent of 13 cups of pasta per day before he went on Atkins), so they are not getting the same elevations in insulin, etc. Historically people on these type of plans have tried to avoid trans-fats and overloading on omega-6, but otherwise the diet contains more saturated fat (and monounsaturated) then they probably were eating before.
I have to admit that I have been guilty of thinking "fat slows down the emptying of the stomach so all good fats must increase satiety" and stopping right there. I had a few other studies/reminders, however, remind me that satiety seems to be more than just the action of slowing down the emptying of the stomach. Last week I attended a lecture given by Dr. Deborah Clegg from UT Southwestern, and she discussed a study she had authored in which palmitic acid, a type of saturated fat, did not seem to promote satiety as well as unsaturated fats. In the lecture she talked about how this actually makes sense from an evolutionary (aka human as an opportunistic omnivore) standpoint; if you eat whatever you can when you can get it, and what you can get is animal fat (which contains saturated fat), then it wouldn't make sense for you to fill up after a few bites. Now, a few caveats about the study; first of all we can all see that it was done in rats and not in humans, so there is something here to speculate about for human studies later. Second of all, the authors of study acknowledged that people in the United States tend to eat a lot of carbohydrate with their fat, so the rats were given a controlled amount of carbohydrate as well as different infusions of fat. (Oddly enough I was able to figure that one out by reading this dialogue between one of the study authors and a blogger who had said some very caustic things about the study before it was pointed out that the media reports and the studies are not the same thing). Third, I will admit that my reading of the study was tempered by reading a more recently released human study in which olive oil had a higher satiety value over butterfat (and it was mixed in yogurt, which also contains a certain amount of carbohydrate as well).
So let's summarize--when people eat a high fat and high carbohydrate diet all of that carbohydrate seems to stimulate you to eat more and all that fat makes it taste so good that it makes that tasty food harder to resist. When the amount of carbohydrate is controlled (and the amount in the yogurt could have been anywhere from 7 grams to 12 grams--about 1/2 a small appple to a whole small apple--still less than a donut or a hamburger bun), the olive oil seems to provide more satiety than the butterfat.
So what do we do with this information? First of all we realize that something has to give. If there was a choice between the newer version of the Atkins diet (meaning you eat animal protein, fat from a variety of sources, and vegetables) and the so-called Standard American Diet (animal protein coupled with excessive carbohydrate and trans fats), it looks the Atkins diet would be a better choice for satiety. (Keep in mind that in the first phase you are eating in such a way to promote ketosis, which also suppresses the appetite). If you throw a more Meditteranean diet in there--animal protein, lots of olive oil, vegetables and frt, only whole grains if you eat them, this might actually work out to be a good compromise (of course you won't get the appetite suppressing ketosis, and you don't get free reign to eat all the brown rice you want either). I think it's also important to keep in mind that someone who is eating animal fat from feedlot sources is going to be getting fat that has a different fatty acid composition that grass fed beed and foraging pigs; so if you aren't paying the extra money for the grass-fed stuff you can use the lean cuts and use olive oil to cook with, etc. Third, we should keep in mind that what worked for a group of people that had limited access to food might not work the same for people who have easier access to food, and maybe we just need to have a nice variety of good fats to help promote satiety and not just eat animal fat. Fourth, we also have to keep in mind that scents/smells/tastes may also impact our satiety (as is speculated with the olive oil). Fifth, we have to acknowledge that some of us eat past the point of satiety and we still need to work on behavior change and controlling our environment.
Take home message--less carbohydrate and adequate fat might promote physical satiety, but it looks like it couldn't hurt to include olive oil as a regular fat source if you are trying to lose weight.