Why was there such a big rise in obesity in the 20th century? I would point mainly to two things: the rise in sugar content (high in almost all processed foods) and a lengthening of the eating window in the day so that people now eat almost from the moment they wake up to the moment they go to sleep at night. (See “Stop Counting Calories; It's the Clock that Counts” on why this matters.)

In the blog post “For, Then Against, High-Saturated-Fat Diets,” Slate Star Codex argues that it might be the polyunsaturated fat content of processed food (primarily vegetable oil) as much or more than the sugar content of processed food that led to the rise of obesity.

Why might polyunsaturated fat contribute to obesity? One possibility is that polyunsaturated fat might foster inflammation. That is an intriguing mechanism; focusing on reducing inflammation as a weight-loss tool sounds like a great thing to try. Now that I have said that, I’ll bet I start noticing a lot of articles and research about inflammation and weight gain and loss.

The other possibility for how polyunsaturated fat could contribute to obesity is that it has a lot of calories compared to how satiated it makes people feel. This, too, is a mechanism that would be interesting far beyond just thinking about polyunsaturated fat. I’ll bet that sugar has one of the lowest ratios of satiation to calories of food ingredients. But polyunsaturated fat could be low on this ratio.

“For, Then Against, High-Saturated-Fat Diets” discusses the claim that saturated fat has a high ratio of satiation to calories. The evidence discussed seems to suggest that one shouldn’t lump all saturated fat into one category for this: some types of saturated fat seem to have a high ratio of satiation to calories, others not so much.

I like Slate Star Codex’s focus on the question of why obesity is so much higher now than it used to be. This discussion is lacking however, by not talking about the timing of eating. (See “Stop Counting Calories; It's the Clock that Counts.”) Evidence has been accumulating that eating all the time can lead to obesity and disease and conversely that time-restricted eating can reduce obesity and disease.

But even if you believe in fasting (not eating, but drinking water) as a weight-loss tool as I do, the ratio of satiation to calories is a very interesting ratio to measure. Eating foods high on the satiation to calorie ratio for one’s last meal before fasting could make fasting easier, for example.

Let me conclude by saying where I am now in relation to different types of fat. I act on the assumption that avocados and olive oil are healthy (monounsaturated fats). I consume quite a bit of coconut milk (with one type of saturated fat) not being sure that it is healthy, but not yet seeing any big red flags in what I have read. I eat a fair amount of butter and cream. Experientially, they seem quite satiating—perhaps satiating enough to have a high satiation to calorie ratio. When it comes to meat and milk, I have other worries that have nothing to do with their saturated fat content. See “Meat Is Amazingly Nutritious—But Is It Amazingly Nutritious for Cancer Cells, Too?” and “'Is Milk Ok?' Revisited.”

Don’t miss my other recent post on this topic: “Journal of the American College of Cardiology State-of-the-Art Review on Saturated Fats.”

For annotated links to other posts on diet and health, see:

• Miles Kimball on Diet and Health: A Reader's Guide