Well, a couple things to consider:

• Many of the foods that are rich in saturated fat are also healthy to eat for other reasons besides the fact that they contain sat fat.

• Eating foods that are high in carbohydrate (for most people, especially those on the SAD) means eating mostly unhealthy foods. There's a difference between someone who gets their carbs from good sources.

• Excessive carbohyrdate to fat conversion can cause your body to actually store the fat instead of burn it. Many people run into this problem. Of course, if you consider all the other aspects and causes of obesity, as noted in recent articles from Stephan G and Jack Kruse and Chris Kressser etc... you know that the brain is highly involved as well.. but STILL, chronically eating loads of carbs and requiring your body to convert to fat can cause problems for a variety of reasons.

• Just because the body converts it, doesn't mean it's smartest to force it to do so overabundantly. The better line of thinking is probably... if the body converts carbs to sat fat anyway, why not take the hint and just eat more sat fat?

Of course, everyone responds differently to specific macronutrient ratios. By now, we should all know this. Some do quite well on much higher carbs. Some do quite well on mostly fat from pure foods. I don't think eating loads of sat fat is necessarily required for good health. I think it's more accurate to say that if you are specifically trying to avoid sat fat foods, you are likely to run into trouble with what you are eating unless you are really educated correctly. Most people are definitely not, and this is where the masses run into major health problems.

The human body very rarely creates fat from carbohydrates as it is an inefficient process. Fats of many lengths can be created via fatty acid synthase as it adds two carbons from acetyl CoA at a time. I suppose that acetyl CoA could originally come from carbs...

What's far more common however is a switch in mitochondrial energy substrate brought about by a glucose-(and amino acid to a lesser extent)induced insulin spike, which inhibits lipolysis via HSL. Under normal fasted circumstances, muscle mitochondria are fueled by our rather large stores of energy as fat, but in the fed state we shift to glucose as a fuel. The net result is that we are releasing and burning less fat from our adipocytes when we eat carbohydrates especially during times of greater activity. It may simply be semantics since the result is the same, but I think it's an important point.

I personally don't believe that saturated fat is a necessary dietary component in the same way that DHA/AA are because we have such huge stores of it that we are releasing constantly into the bloodstream and can manufacture in our cells.

Not sure if anyone has mentioned the cell membrane argument.

Cell membrane composition differs significantly in a heavy butter vs heavy soybean oil diet: http://www.ncbi.nlm.nih.gov/pubmed/17002871

In this experiment, carb percentages were similar between groups. The difference in composition means that membrane fluidity and perhaps membrane receptor activity would differ between groups.

Bottom line is that I want my cell membranes to look like the cell membranes of my ancestors. Eating some saturated fat would presumably contribute to that, as it increases the LCFA composition of the membrane (although this was not directly addressed in the above study compared to a higher carb isocaloric diet).

I think the reason that it's better to get saturated fat from dietary fat rather than from dietary carbohydrate has to do with the hormonal effects that come along with the fat or carbs when ingesting them. Even if you can get the fat you need from the carbs, you probably don't want the insulin surge and resulting cascade of inflammatory prostaglandins, the fuel partitioning that favours fat storage, etc, etc. It's not so much the material -- the body tries to get what it needs from what you give it -- but the side-effects.

• External fat comes with vitamins A,D,E and K2, which are all important. There are other beneficial liposoluable components, like cannabinoids.

• Simple sugars compete with Ascorbate. High load reduces leucocite phagocytic index by 50% over next 5 hours. Its different when liver is optidosing glucose then what you do when you load it up at once and produces spikes of insulin and blood glucose. Insulin is triggered even by very small amounts of sugar, while beta cells are completely lethargic to fat.

• On the level of digestive system, carbs produce extra fermentation in the gut. This could and does produce dysbiosis. Extra flatulence is damaging on several levels. On the same level fat makes more endotoxins enter the blood, which probably only provides training grounds for immune system and it is probably kept on higher level.

• There is no such thing as pure saturated fat in the nature. There are procentages. This means you also ingest DHA/EPA/ALA/MCT/CLA/GLA and friends. All those require resources to be build up in the body and some can not be made by the body. This influences cell membrane and responsiveness to hormones.

• Liver sparing, as it does de novo lipogenesis. This means it is less capable of doing more important things like toxin clearance. Although the same could be said for gluconeogenesis it must be done anyway. This paper shows that amount of glucose determines lipogenesis 10fold but only a little for gluconeogenesis. So, high fat diet spare liver from working a lot I think. (there are many other papers with the same conclusion btw).

+1 for this statement right here:

In the end, you should eat what your body needs, not force your body to try to convert things you don't need into things you do need.

dietary fat is not the same thing as body fat. I think that is what confuses people.