I get a fair amount of traffic here from time to time thanks to my posts about Zoe Harcombe and the Harcombe Diet. Zoe's dietary philosophy boils down to parroted Taubesian Pseudophysiology, but one of the things she advocates, that I've discussed here before, is the strategy of separating carbs and fats and having them at different meals. She makes a lot of comments about how calories don't count and all that too ... so the Google searches on her name and/or CICO or the separating carbs & fats thing tend to land folks here at the Asylum. As a result, apparently just enough folks know enough about my stance on carbs & fats to misrepresent my position in discussions about the web. More than once I've been described as subscribing to "that old food combining nonsense" or things along that line. I've long since passed the point where I could answer to these various misconceptions everywhere they are found, but in this case, it's happened enough times that I thought I'd set the record straight on this issue.
I believe that when it comes to fats and carbs, with the exception of the small amount of fat that goes to sustaining cellular structure and biological function, the two are relatively interchangeable as dietary sources of energy. Within a relatively wide range of dietary macronutrient ratios, our fuel burning ratio (the respiratory quotient, RQ) remains relatively consistent depending on activity levels. This has been born out in metabolic ward studies. So let me begin with what I don't believe. I don't believe that if you eat 25g protein with 400 more calories at a meal it matters much whether the fat vs. carb breakdown of those calories is. If one eats 400 cals fat or 400 cals starch or 200 cals of each, if that's 200 cals too many, then ultimately you've stored 200 cals worth of lipid in adipose tissue at the end of the day. If anything the fat is stored more efficiently as fat and you get a few more net calories than the carbs, but it all balances out except at the very extreme ends of the spectrum.
So, if you eat 200 cals fat and 200 cals carb together, it is true that most of the 200 cals of fat will be deposited in your fat tissue. All the while, however, fat is constantly going into and being released from the fat tissue, so it's not like it's a "fat sentence" for you for this to occur. Later in the day when the carbs from the meal are gone, 200 cals of fat will be mobilized and burned provided you need it. This is why I find the arguments of Taubes and Eades and Harcombe and a whole host of others so specious -- they all focus on what's going on in the hour or two after you eat and ignore what goes on the other 16+ hours of the day. So I don't believe in this "starving cells" thing, nor do I believe in any metabolic advantage of fat vs. carb (though carb comes out ever-so-slightly with the advantage there).
I believe in separating carbs and fats between meals in one's diets as a STRATEGY, not some rule. It's something I've been doing a lot more of lately as my carb intake has increased, though I have some carb or some fat at almost every meal. Lately my macro ratios are rather Zone-like, but I almost never eat a Zone-friendly meal. But enough about me.
1. Food Reward/Palatability: Whether it's FR or palatability or some less well characterized phenomenon, the combination of fat + carb is intrinsically appealing to many people and many foods people overeat fall in this category: ice cream, chocolate bars, cakes, donuts, burgers & fries, cream mashed potatoes, bread with butter, pasta alfredo, pizza, just about every chip/snack food, etc.etc. (sorry, don't want to needlessly tempt anyone further). Low carbers always blame the carbs for overeating these foods when fat is every bit the devilish macro. If you eat just boiled potatoes, you almost can't overeat. But add butter and you'll likely eat more of the buttery potatoes. The same goes for just about everything. I know these are rodents, but the CAF rat study really makes the point on this rationale. Give the rats a high fat chow or a high sugar chow and they eat about the same or a little more than if you give them a high starch chow. But give them human junk foods and they eat more ... a lot more ... and pay the price. They also eat more of just those foods as they shy away from the chow almost entirely.
2. Calorie Density & Sensing: While fatty foods have high calorie density, most are packaged with enough protein in whole foods to prevent overeating. Common LC stalling culprits such as cheese and nuts are, interestingly enough, exceptions to this rule. But carbs by themselves have a low calorie density ... unless you add fat. You have to eat 50% more of a medium potato to equal the tablespoon of butter you might put on it. If you keep fat away from your carbs, at those meals you can eat quite a lot without eating too many calories. Our signaling seems to work well for sensing carb calories and well enough for fat calories (although see the aforementioned argument about protein) for satiety. It doesn't seem to work well for sensing calories when these two energy sources are combined.
3. Ancestral/Evolutionary Reasons: If one believes at all in how we evolved -- or even adapted if evolution offends you -- one thing we can all agree upon is that ancient humans ate "real" foods, likely as "whole" as possible. And they probably ate somewhat opportunistically. While they likely had the brains to store non-perishable (mostly) carbs, it was for that proverbial rainy day or for when Grok was too tired to go hunt. Grok didn't fry his wild boar in coconut fat with a side of tubers smothered in ghee. The added fat thing is definitely not "paleo", and it is hard to imagine that Grok did any food combining. Freshly hunted game was eaten before it spoiled, vegetation eaten as gathered or stored and drawn from when the hunt was unsuccessful. There are really no foods found in nature where carbohydrates and fats are BOTH present at "high" levels. Nuts, larger seeds and some legumes probably being as close as we come to that in the veggie world, assorted dairy being the only source from the animal kingdom I can think of. I don't think postprandial metabolisms are designed for large mixed macro meals.
4. Protein Deficiency: If one doesn't deliberately eat protein at every meal, consumption of fat + carb foods together often leads to too low a protein consumption by percentage. Once protein needs have been reached for satiety, too many calories have been eaten.
5. Comfort Foods: When one thinks of comfort foods, they are almost invariably fat + carb. From hearty stews to ice cream. When we eat these foods, hunger is not the only reason for eating them. Therefore they are easy to overeat.
So there you have it. I'm not saying that one shouldn't eat fats + carbs together, only that doing so ad libitum often leads to overeating. So my strategy to guard against overeating ad libitum is to eat essentialy low fat or low carb at any one given time. If I'm going to eat a high fat + carb food, which I do on occasion, I am careful to portion it out accordingly. Never, never, eat ice cream from the carton, potato chips from the family sized bag or whatever. But I'm not claiming any metabolic magic to food combinations or that separating these macros keeps the fat I eat for breakfast from being locked away all day if I had a slice of toast with my bacon and eggs.