in the comments, kurt harris makes the point to distinguish that when carbs or fat were introduced in those diet, it was usually introduced in the form of a toxic neolithic agent of disease- "carbs" usually means wheat and "fat" usually meant omega 6 seed oils. i have been trying to and i think the conversation looks a lot different when you stop thinking in terms of strict macronutrients and think of food quality.

There are definitely other benefits to eating lower carb. I retain water BADLY when I eat too many wheat based carbs. (This does not happen with fruit/vegetable carbs.) I am talking 5/10 lbs over just a few days if I go on a high carb binge.

Generally, I eat moderate carb, my goal is 75 grams or less per day, but no gluten based carbs and that is enough to keep my water retention at bay.

I am working on a post on them, but the Ache are a tribe in the Amazon that eat high fat and plenty of carbs too. Based on bone depositions, it's possible that this is similar to the diet of peoples in the Middle Paleolithic. So no, I don't think there is much evidence that humans either have to eat low-fat or low-carb. What seems to make people all over the world fat, at different rates depending on genetic variation, is industrial food.

when discussing low carb/high carb the only thing mentioned is weight loss. Couldn't there be other benefits when eating low carb (or high carb, for that matter) besides weight?

This is a good question. Low carb diets definitely have some benefits for people who are obese or have certain pathological disorders.

On the other hand high carb diets generally have higher nutrient density and allow the storage of a significant amount of energy(high carb, high calorie diets increase glycogen stores to ~1000g). HC VLF diets also have therapeutic uses in treating heart disease, arthritis, obesity and even diabetes. HC diets also tend to be a lot cheaper and maybe more sustainable overall with the right agriculture choices?

I generally lean towards the opinion that higher carb diets are better overall unless the person has severely deranged glucose metabolism or can't tolerate carbs for whatever reason. I could be wrong though, I would love to hear some more benefits of a VLC diet because I personally don't have a lot of experience with it and haven't done a whole lot of research on the subject.

In regards to your main question there are some studies that allude to a problem mixing carbs/fats in the same meal but I think this is based on the assumption that the fat and carbs enter the bloodstream at the same time. I don't know if that's necessarily true under normal physiological circumstances though.

If you're running close to your minimum weight, a switch in either direction would cause a weight gain out of all proportion to the amount of food eaten. Going from high fat to high carb, there's a water/glycogen gain. Going high carb to high fat there's a food retention gain (due to slowed digestion - I'm still living with the effect of a big piece of prime rib I ate a week ago). Either of these can result in a five pound overnight weight gain.

Neither of these events results in fatness, and the gain goes away in a few days. Real fat gain takes more time - months to years of steady overeating on any diet. Post Atkins regain comes to mind as an example on one side, "meat and potatoes" on the other.

I add carbs and fat together, I gain weight, I add meat and fats together and I lose weight. I think fat works better with certain food groups and badly with other food groups.

I go only on my own experience here, but in addition to weight loss, I experience a reduction in inflammation both when on a HCLF diet and also on a LCHF diet. And while I have been on a lower-carb diet for about a year, I can almost time the breakouts as a function of the amount of carbs I had the previous day, especially if wheat was involved and, yes, to a certain extent, fruit and tubers. But what I look for is that if one eats a true whole-foods diet, that is, regular foods and not food that has been trimmed, skinned, degermed, expeller-pressed, etc., it is much easier to do the LCHF version instead of a HCLF version. The protein that I really need enough of is usually coupled with the fat. Carbs and fat aren't coupled together in real foods, only in our culinary combinations.