It is said that obesity is related to many factors, but why? I will quote an interesting description that is relevant to my blog.
  

The various theories fight among themselves, as if they are all mutually exclusive and there is only one true cause of obesity. For example, recent trials that compare a low-calorie to a low-carbohydrate diet assume that if one is correct, the other is not. Most obesity research is conducted in this manner.

This approach is wrong, since these theories all contain some element of truth." 
(Jason Fung. The Obesity Code. Greystone Books, 2016, Page 70.)

What we need is a frame work, a structure, a coherent theory to understand how all its factors fit together. Too often, our current model of obesity assumes that there is only one single true cause, and that all others are pretenders to the throne. Endless debates ensue.

Too many calories cause obesity. No, too many carbohydrates. No,
too much saturated fat. No, too much red meat. No,
too much processed foods. No, too much high fat dairy. No,
too much wheat. No, too much sugar. No,
too much highly palatable foods.No, too much eating out. No

It goes on and on. They are all partially correct.  (*snip*)
   

I think the author provides a keen insight into the multifactorial nature of obesity. We must first understand that being overweight is not as simple as "it happens when we consume more calories than we expend," but is caused by a complex interplay of various factors.

    
However, what I want to say is that based on my intestinal starvation idea, the various factors can be aggregated to some extent, which means that obesity can be complicated and intertwined with factors and theories we can see, and cannot be completely explained, but when we focus on the unseen workings of the intestines, the cause of weight gain would be mostly specific.

(1) Many theories that are believed to make us fat fight among themselves, as if there is only one true cause of obesity, as Dr. Fung mentioned. Researchers may be well versed in their own areas of research (e.g. resistant starch, the value of eating breakfast, the effects of carbohydrate restriction, hormones, gut bacteria, etc.), but that does not necessarily capture obesity as a whole, so each theory can stand alone.

What we need now is a framework for how each theory is intertwined, and I'd like to believe my theory can be helpful in that regard.

(2) The root cause of being overweight, I believe, is the increased set-point weight, which is caused by intestinal starvation. Intestinal starvation is caused by a combination of at least four factors, and since "what kind of food we eat" and "how we eat them (lifestyle)" are such important factors, many things seem to have something to do with weight gain－intestinal starvation can be a "confounding factor."

In other words, I believe the causes of weight gain would be mostly specific in the unseen workings of the intestines.  

When we consider that “eating a lot leads to gaining weight,” I believe you have the image of carbohydrates like bread, rice, and noodles in mind.

I’m simply saying that it is hard to gain weight if a thin person eats them properly every day.
Although a person who has already gained weight (or who is already overweight) may not lose weight by eating some indigestible foods, I consider that it may be possible to lose weight depending on how you eat them, since these foods are always discussed in dieting techniques.
  

On the contrary, digestible refined carbohydrates that are high in starch (rice, rice porridge, white bread, potatoes, etc.) and processed starches extracted from plants promote overall digestion. By eating them together with digestible proteins, they make it easier to  induce intestinal starvation. 
Those are two effects that I can think of so far.
        

If you proportionally increase digestible carbohydrates (rice, noodles, bread, etc.) in the meal, the percentage of side dishes such as fat, meat, fish, and vegetables will be relatively smaller. The density of a spoonful of oil will be lower if you eat more bread or rice portions with soup (water). 

Moreover, as seen in low-carb diets, what would happen if you decrease the intake of carbohydrates  in the meal and instead increase proteins such as meat and fish, fat, and vegetables, etc.? 

In this case, the opposite effect of the dilution effect occurs: dense nutrients are delivered to the intestines, which slows digestion and undigested food always remains in the gastrointestinal tract.
          

Starch-rich carbohydrates, when consumed with water, cause stomach bloating (I’ll call this the “balloon effect” of the stomach). And, if we eat carbs together with digestible side dishes such as stew (onion, potatoes, low-fat chicken, etc.), its holding time in the gut will be shorter since it’s easy to digest, and the food will be pushed out of the stomach fairly soon. Also, our intestines start to move actively and smoothly.

On the other hand, you may think that eating fatty foods or deep-fried foods give us stamina. It actually means that those foods fill us up better than carbs do, and its energy could be sustained during sports such as a marathon or a soccer match.
That is to say, undigested foods stay longer in our stomach and intestines, so it’s more difficult to induce intestinal starvation.

(1) Simply put, incorporating plenty of dietary fiber and foods that are slow to digest and absorb in your diet makes you less likely to gain weight compared to other meals with the same caloric content.
On the other hand, combining easily digestible foods (such as refined carbohydrates, proteins, and ultra-processed foods) in your diet makes you more likely to gain weight.

       
(2) Carbohydrates are generally classified into monosaccharides, disaccharides, oligosaccharides, and polysaccharides, depending on their chemical structure. 

Substances like cellulose (polysaccharides) which are considered dietary fiber, non-digestible oligosaccharides, and resistant starch, would not cause weight gain and should be consumed to promote health.

I also believe that simple carbohydrates like sugar may cause an increace in blood sugar levels and temporary weight gain, but they are not a cause of weight gain in the sense that they increase one’s set-point weight.

     
(3) In terms of increasing the body's set-point weight, I believe the foods that may cause obesity are easily digestible carbohydrates (polysaccharides) high in starch, such as white rice, white bread, noodles, and potatoes. 

These foods expand with water in the stomach (balloon effect) and may have a "dilution effect" or "push-out effect" in the digestive process, which might lead to intestinal starvation.

        
(4) Obesity among poverty-stricken people worldwide can be understood as the influence of cheap refined carbohydrates and unbalanced diets (lack of vegetables, etc.). Considering them, it may be easier to imagine that they are not gaining weight due to taking too many calories or sugar, but rather from consuming cheap carbohydrates as mentioned above.
[Related article] Wealthy People Get Fat? Poor People Get Fat?

First, as many of you know, even carbohydrates contain four kcal of energy per gram. So, some readers may think, "After all, isn't being overweight ultimately caused by too many calories?"

But if you think, "too many calories are the cause," you should try to reduce the total amount of calories in your overall diet, mostly focusing on fat/oil intake, which has nine kcal per gram.

On the other hand, the argument that “too many carbohydrates cause weight gain” allows you to eat any amount of meat and fatty/oily foods as long as you cut back on carbs.

In this article, I will look back on the historical argument of whether carbohydrates or calories are the cause of weight gain, and at the end of this article, I would like to share my thoughts.

In Japan, a low-carb diet was trendy around 2015, but when we look around the world, this way was repeatedly conducted since the 1800’s. Please note that there are many quoted parts. I needed to share this information with you to explain my theory.
  

"Jean Anthelme Brillat-Savarin was born in 1755. (*snip*) His passion, though, was always food and drink, or what he called the “pleasures of the table.” He began writing down his thoughts on the subject in the 1790s; Brillat-Savarin published them in a book, The Physiology of Taste, in December 1825. (*snip*)

“Tell me what you eat,” Brillat-Savarin memorably wrote, “and I shall tell you what you are." (*snip*)

Over the course of thirty years, he wrote, he had held more than five hundred conversations with dinner companions who were “threatened or afflicted with obesity,” one “fat man” after another, declaring their devotion to bread, rice, pasta, and potatoes. This led Brillat-Savarin to conclude that the roots of obesity were obvious. 

(*snip*)
"By the time, a French physician and retired military surgeon named Jean-Francois Dancel had come to the same conclusions as his countryman Brillat-Savarin. Dancel  presented his thoughts on obesity in 1844 to the French Academy of Sciences and then published a book, Obesity, or Excessive Corpulence: The Various Causes and the Rational Means of a Cure.

Dancel claimed that he could cure obesity “without a single exception” if he could induce his patients to live “chiefly upon meat," and partake “only of a small quantity of other food." 

Dancel argued that physicians of his era believed obesity to be incurable because the diets they prescribed to cure it were precisely those that happened to cause it. (*snip*)

"Until the early years of the twentieth century, physicians typically considered obesity a disease, and a virtually incurable one, against which, as with cancer, it was reasonable to try anything. Inducing patients to eat less and/or exercise more was just one of many treatments that might be considered. (*snip*)
    

<1950's>
The effects of a carbohydrate-restricted diets were then confirmed in the 1950s by Margaret Ohlson, head of the nutrition department at Michigan State University, and by her student Charlotte Young. 

When overweight students were put on conventional semi-starvation diets, Ohlson reported, they lost little weight and “reported a lack of ‘pep’ throughout... [and] they were discouraged because they were always conscious of being hungry.” 

As you can see, by cutting back on carbohydrates and eating more of other foods such as meat and greasy food, the problem of being overweight seems to be solved...but this is where the "calorie principle" comes into play. 
   

"By the 1960s, obesity had come to be perceived as an eating disorder. (*snip*)

 Adiposity 101 was discussed in the physiology, endocrinology, and biochemistry journals, but rarely crossed over into the medical journals or the literature on obesity itself. 

When it did, as in a lengthy article in The Journal of the American Medical Association in 1963, it was ignored. Few doctors were willing to accept a cure for obesity predicated on the notion that fat people can eat large portions of any food, let alone as much as they want. This simply ran contrary to what had now come to be accepted as the obvious reason why fat people get fat to begin with, that they eat too much. 

I ‘d like to talk about this controversy.

Several studies have shown that how we combine the three macronutrients (protein, fat, and carbohydrates) in the diet produces different results in body fat accumulation. It is thought that even the same one calorie has different energy used for digestion and absorption, different hormones to stimulate, and different pathways of how the calorie is metabolized in the body.

Of course I think these studies are great, but the point I'd like to add based on my theory is that "carbohydrates and fat are close to having opposite properties in their digestive processes."

First, refined carbohydrates are more easily digested than meats and fats, and the "dilution effect" or "push-out effect" they have makes our digestion go even faster and makes us feel hungrier.

If we eat an unbalanced diet that lacks vegetables, fat, and dairy products,etc., we are ultimately more prone to inducing intestinal starvation.

Plus, a diet low in carbohydrates and high in protein and fat sends dense nutrients to the digestive system, which slows overall digestion. As a result, I believe that it suppresses hunger and in turn, causes the absorption rate to decrease.

That is to say, depending on how we structure our diets, there may be a weight-loss effect even with increased caloric intake (for those who can digest protein and fat quickly, the weight-loss effect may be less pronounced) .
         

(1) From the early 1800’s through the 1960’s, several studies had shown that overweight people could lose weight without difficulty by replacing some carbohydrates in their diet with a lot of meat and fat. By that time, however, obesity was understood as an eating disorder, and this diet method was discussed only in physiology, endocrinology, etc.

(2) From the 1960’s to the late 1970’s, few physicians accepted the idea that fat people could lose weight by eating lots of meat and fat, because it obviously violated the "calorie principle.”

(3) In addition, health experts came to believe that fat in the diet caused heart disease and that carbohydrates were "heart-healthy." As a result, doctors and nutritionists began attacking low-carb diets.

(4) My thoughts: Both sides have a point, but the caloric intake does not determine everything. Different combinations of foods, even with the same calories, have different effects on weight management. In particular, carbohydrates and fat are close to having opposite properties in their digestive processes.

It is said that the cause of weight gain is the caloric intake exceeding calories burned through metabolism and activity. For this reason, I see people dieting by only reducing the amount of food they eat, and putting up with being hungry over long hours.

When I was working part-time at a restaurant in college, there was a woman who wasn’t that overweight, but she started dieting anyway. 

She wasn’t slim, but she wasn’t overweight, either. To me, she looked healthy and fit. I thought she was okay as she was. But it seemed that she started dieting because she wanted to get slim. 

The same goes for my colleague, T, who worked as a cook in the kitchen at a nursing home. When I first met him, he was a stocky guy (about 170cm tall and 70 kilos).
He wasn’t overweight but he was on a diet, saying he had gained three kilos which shattered his  previous weight level in the last year. 

There are two ways in which the intestinal starvation mechanism occurs.

(1) Eating regular or big portions of an unbalanced meal, but not eating as often (e.g. skipping breakfast and eating two meals a day) and experiencing hunger over many hours. 

(2)Eating small portions as seen in dieters or pregnant women, sometimes skewed towards digestible carbohydrates and protein, etc. Even if they eat three times a day, they often experience hunger over many hours.

In conclusion, whether you eat a good amount of food or a small portion of food, if your diet consists of mostly digestible carbs and some protein, an imbalance of food in the intestines remains the same. If you don’t eat anything else and experience hunger over many hours, it leads to the similar effect in view of creating intestinal starvation.

Eating vegetable dishes, dairy products and fat/oil, etc. is important with regards to preventing intestinal starvation, but those people in the previous examples were only conscious of caloric intake, and chose not to eat them.
        

The simple explanation is that even though exercise ultimately pushes the body to store energy, if some undigested food is always left in the intestines, as a result, intestinal starvation does not occur and the set-point weight remains the same.

I will explain this in greater detail several ways.
         

References：
[1] Jone Briffa. Escape the Diet Trap. London: Fourth Estate, 2013, Page 223.