Many people say that eating too much fat or oil makes you fat, since fat has more than twice as many calories as carbohydrates or proteins. But some research shows that some people can lose weight even though they increase fat or oil in diets.
Which is true?

To make a long story short, I think both are correct. As I have mentioned so many times before in this article, there are two meanings to the phrase “gain weight,” and based on my theory, I can say that eating fat has three sides to it.

“Conventional wisdom dictates that a key to successful weight loss is to keep the diet low in calories, and a key strategy deployed here is to cut back on fat. Fat contains twice as many calories as carbohydrate or protein. It's also called fat, of course. These facts do, on the face of it, seem to incriminate fat as something inherently fattening.

As a result, past attempts at weight loss may well have had you consuming enough skimmed milk and skinless chicken breasts to last you a lifetime.

On the other hand, many individuals will have had the experience of filling up on fat-packed foods such as eggs, cream, cheese and butter on ‘low-carb' regimes (such as the Atkins Diet), only to see their own fat melt away. Such experiences should, if nothing else, cause us to question the widely held belief that the fat we put in our mouths is destined to end up in the fat stores within our body.”

He concluded :

• Low-fat diets are ineffective for weight loss.
• Dietary fat intakes are not strongly linked to body weight, and some evidence links increased fat intake with lower body weight.
• Insulin is the key driver of fat accumulation in the body. Dietary fat does not stimulate insulin secretion directly, and therefore has limited fattening potential.”

（Escape the Diet Trap, Dr. John Briffa, 2012, Pages 51,61）

I’m not a specialist or researcher. Therefore, I’d like to avoid mentioning how insulin or other hormones work in the bloodstream. But I believe I can explain why low-fat diets are ineffective for weight loss, based on my theory. I will explain in the next section.

Fat(oil) is an excellent energy source and provides nine calories per gram. It is also said that fats provide a number of benefits for your body, including regulating hormones.
But fat takes longer to digest, compared to carbohydrates or proteins. Therefore, you can see these features below:

As you can see in the diagram above regarding base weight, within the range of (B), if you eat many calories from fat with other foods containing protein and carbs, you should gain weight steadily.

People who always put in time and effort for dieting, or ripped people who cut excess fat, will get especially fat in a few days if they eat a lot of chocolates, cakes, fatty foods, or deep-fried foods.

With these images, many people reject a revolutionary suggestion of taking fat/oil in order to get slim, and that’s what makes this theory hard to accept.

As you can see in the range of (A) of the diagram above, when base weight itself increases, fat will work as a deterrent force(*). (* for those who have a higher digestive ability for dietary fat, may not experience a deterrent force).

For example, if a thin person wants to gain weight, eating a lot of fat/oil at meals will hinder gaining weight. Have you ever wondered, “why was he/she not getting fat despite the fact that they were eating high calorie foods, such as cakes, cookies, deep-fried foods, or fatty foods?”

As Dr. John Briffa whom I’ve mentioned above said, “low-fat diets are ineffective for weight loss, and increased fat intake is linked to lower body weight” is in a way correct, as though this doesn’t apply to everyone.

It’s not that carbohydrate intake is the cause of gaining weight, but rather, it’s the difference of characteristics between carbs(*) and fat usage. (*Here, I’m referring to polysaccharides such as bread, rice, and starch, but not to sugar.)

For many people, fat or oil is difficult to digest, so if you keep eating a lot of fat(oil) every three to five hours, undigested foods will remain in the stomach and the large part of the intestines for most of the day.
In turn, your appetite will settle down and relatively, the absorption ability (rate and amount) will decrease. As you can see in a low-carb diet, by decreasing carbohydrate intake and increasing fat intake, proteins, nuts, and vegetables, etc., it maximizes the diet effect.

[Related article] The Dilution Effect and the Pushing Out Effect of Carbohydrates