From an evolutionary biology perspective, humans and many other animals are thought to have developed mechanisms to cope with intermittent food shortages over long periods of evolution. Because periods of abundant food were limited, the ability to store energy in the liver, skeletal muscles, and adipose tissue was likely favored by natural selection.

From this perspective, obesity can be understood not merely as a consequence of overeating, but as a state that arises as a result of adaptive physiological responses, in which the body attempts to store energy when it perceives a risk of starvation.

　
The concept of “intestinal starvation” adds a new perspective to the discussion of how our bodies perceive a lack of food, moving beyond the traditional focus on energy intake alone.

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Note 1: It is unclear whether intestinal starvation is sensed throughout the entire gut or only in the small intestine, which is sometimes referred to as the "second brain."

Note 2: Because dietary fats generally require more time for digestion and gastric emptying, they may, in some individuals, help prevent the onset of intestinal starvation. However, there is substantial individual variability in the capacity to digest fats, and when fats are consumed together with refined carbohydrates, this suppressive effect may not be sufficiently achieved.

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Even if you consume a large amount of food, a diet composed mostly of refined carbohydrates and other easily digestible items (including certain protein sources and small amounts of fat)—combined with long gaps between meals, such as eating only twice a day—can induce an intestinal starvation state or a condition very close to it.

Even when overall food intake is small, a diet composed mainly of easily digestible foods such as refined carbohydrates and certain protein sources (e.g., ultra-processed foods such as simple burgers or sandwiches, or instant noodles) may still trigger intestinal starvation. 

Due to recent dieting trends, many people try to keep breakfast or lunch light in an effort to lose weight, but this approach may end up being counterproductive in the long run.

In contrast, when you eat a balanced diet including fiber-rich vegetables, and minimally processed foods three times a day, some fraction of undigested matter remains in the intestinal tract continuously throughout the day, making it less likely for intestinal starvation to be induced.

Since human genes are unlikely to change dramatically over just a few decades, changes in lifestyle and food environments since the 1970s are likely to have had a substantial impact on the recent global rise in obesity[3].

Strictly speaking, four factors (see Note 3) need to overlap for intestinal starvation to be triggered, which may explain why obesity is more likely to occur at the intersection of multiple factors, including genetic, environmental, and physiological factors.

Note3: The four factors are “what you eat,” duration of hunger, digestive ability, and continuity.

Genetic traits related to intestinal starvation include the secretion capacity and receptor sensitivity of enzymes and hormones that regulate digestion and appetite. In particular, differences in genetic background—reflecting both population-level and individual variability—that enable more rapid digestion of proteins and fats may influence the occurrence of intestinal starvation.

Of course, these biological traits may also change as a result of acquired conditions, such as obesity or gastric reduction surgery performed for weight loss.

Since the 1970s, one of the most notable environmental changes, I believe, has not merely been the increase in energy-dense foods, but rather the rise in easily digestible foods resulting from industrialized food production. Examples include refined carbohydrates, certain protein sources, and ultra-processed foods.

In vegetables and grains, the less digestible parts have been removed (e.g. refined grains), and some are now mashed or strained (e.g. mashed potatoes and potage soups). Meat and fish are often minced or ground, making them easier to digest.

<Lifestyle>

Intestinal starvation is related not only to what people eat but also to "how they eat."

Since the 1970s, changes in lifestyle have been accompanied by dramatic shifts in eating habits. In Japan, for example, the traditional rice-based breakfast has gradually been replaced by a Western-style breakfast consisting of toast, coffee, and fried (or boiled) eggs, etc.

    
In addition, many people now tend to eat light breakfasts and lunches, while consuming the majority of their daily calories at dinner. Such eating patterns may promote intestinal starvation, because our intestines are about seven to eight meters long, and all ingested food may be completely digested while traveling through the intestinal tract. 

Intestinal starvation represents an adaptive physiological response related to intestinal processing and hormonal secretion, and may prove that people who eat a balanced diet every day are likely to be less prone to weight gain. 

Although intestinal starvation may not be directly associated with the gut microbiota, the types of foods consumed have a significant impact on the intestinal environment.

Individuals who eat a wide variety of foods in a well-balanced manner are less likely to induce intestinal starvation, which may indirectly suggest a relationship that those with a healthier gut microbiota are less susceptible to obesity.

<References>

[1]Flores-Dorantes MT, Díaz-López YE, Gutiérrez-Aguilar R. Environment and Gene Association With Obesity and Their Impact on Neurodegenerative and Neurodevelopmental Diseases. Front Neurosci. 2020 Aug 28;14:863. 

[2]Khan MJ et al. Role of Gut Microbiota in the Aetiology of Obesity: Proposed Mechanisms and Review of the Literature. J Obes. 2016;2016:7353642. 

[3] Jason Fung. The Obesity Code. Greystone Books, 2016, Page 21-2.