Obesity, the Root of All Illness: Its Social Cost Exceeds Even Smoking

Dr. Lee Obesity has emerged as a worldwide problem affecting even developing nations. According to the 2024 Community Health Survey released by the Korea Disease Control and Prevention Agency, the obesity rate in 2024 was 34.4 percent—roughly one in three people are obese.

Susan Doctor, are obesity and being overweight different?

Dr. Lee Yes. Past being overweight comes obesity. Obesity refers to a body mass index (BMI) of 25 or higher, while being overweight means a BMI between 23 and 24.9. So the obesity rate is the proportion of people aged 19 or older with a BMI of 25 or higher.

Susan Ah—so we can think of it as normal (BMI under 23), overweight (23–24.9), and obese (25 and above), in that order.

Dr. Lee That's right. In Korea, the male obesity rate peaked at 55.4 percent in the thirties, while for women it kept rising from the twenties through the seventies.

Emily Adult obesity is a problem, but childhood obesity seems even more serious. My older child is in elementary school, and when I go to pick them up after class, I see so many obese children.

Dr. Lee Yes, exactly. Unlike before, childhood obesity is rising sharply. Children who are obese often remain obese into adulthood. As of 2021, the obesity rate among children and adolescents was 19.3 percent—one in five. For boys it was even higher, at 25.9 percent. The social cost of obesity in 2021 exceeded 15.6 trillion won, surpassing smoking (11.4 trillion) and drinking (14.6 trillion).

Zoe Goodness—more than the cost of drinking? But isn't obesity our body's way of storing energy needed for survival, in case food runs short? If it follows a law of nature, isn't it inevitable?

Dr. Lee You could think of it that way, but chronic obesity can be the cause of many diseases. The most representative is metabolic syndrome. When obesity sets in, blood lipid levels rise, glucose use becomes difficult, and blood sugar climbs. To excrete the elevated glucose, diabetes develops. High blood sugar also slows blood flow, raising the risk of cardiovascular disease.

When fat is stored in the trunk, it is called central obesity; when accumulated in the abdominal cavity, it is called visceral obesity. Trunk or central obesity expands the waistline, and this is more dangerous than fat stored in subcutaneous tissue. The reason is that visceral lipids affect the liver, making liver disease more likely. Moreover, excess nutrients in the body can act as toxins. The kidneys, which detoxify, then come under strain, increasing the risk of kidney disease. We must not overlook that adipocytes release inflammatory signals, making systemic inflammation easier to develop.

Zoe Ah, so that's why obesity is called the source of all disease. When I gain weight from overeating, I get tired and develop skin inflammation. I've also heard obesity is a global problem.

Dr. Lee Yes. Not only in developed countries but also in developing ones, metabolic syndrome including obesity is rising rapidly, burdening national healthcare systems. According to the World Obesity Federation, as of 2022, 43 percent of adults aged 18 and over—2.5 billion people—were overweight, and 890 million were obese. The global cost of obesity is projected to reach 5,220 trillion won by 2045.

Susan Could you remind us once more about the diseases linked to obesity?

Dr. Lee According to current scientific reports, obesity raises the risk of heart disease, hypertension, cerebrovascular disease, diabetes, hyperlipidemia, fatty liver, gallstones, gastroesophageal reflux, menstrual irregularity, polycystic ovary syndrome, infertility, reduced libido, depression, degenerative arthritis, gout, colon cancer, pancreatic cancer, prostate cancer, and breast cancer—and many other conditions besides. Beyond being a cause of countless illnesses, obesity also lowers self-esteem because of concerns about appearance.

Susan Doctor, we can't live without eating. If we can't stop eating, it seems impossible to avoid obesity—especially in our modern world of abundance.

Dr. Lee A great question—that is exactly today's topic. The wonderful method of improving obesity while still eating is what we'll discuss today. Fortunately, recent research shows that obesity can be readily improved simply by choosing the right kind of diet. Today we'll review several research papers on this.

"Effect of an Ad Libitum High-Protein Ketogenic Diet on Hunger, Appetite, and Weight Loss in Obese Men" (Johnstone et al., 2008)

Dr. Lee The first paper we'll examine is "Effect of an Ad Libitum High-Protein Ketogenic Diet on Hunger, Appetite, and Weight Loss in Obese Men" (Johnstone et al., 2008). The study design left intake amount to the participants' own choice—allowing them to eat freely—while controlling only the macronutrient ratios so that fat content was elevated. Mean age was 38 years, and 17 obese men were given two diets, both high in protein (30 percent). For 30 days they followed a 4 percent-carbohydrate ketogenic diet, and for the other 30 days a 35 percent-carbohydrate diet, in a crossover design.

Zoe Doctor, why use a crossover design where the same group switches diets at different times?

Dr. Lee Because in a crossover study each participant serves as their own control, reducing the number of subjects required. Comparing two or more diets within a single group also lets us estimate the effects more precisely.

Zoe Ah, I see—so it reduces the variability that comes from having different groups. One more question: shouldn't the food amount be fixed? Why allow them to eat as much as they want?

Dr. Lee A fine question. The reason is that satiety can differ between diets. Allowing free intake also lets us estimate satiety per unit of food consumed for each diet.

Zoe Ah—I see. Thank you.

Dr. Lee Let's continue. As shown in Figure 8-1(b), the 4 percent-carbohydrate keto group consumed fewer calories than the 35 percent-carbohydrate normal-diet group. Mean hunger scores (panel c) were also lower on the keto diet than on the normal diet. Subjective satisfaction—the "happiness" measure (panel d)—was actually higher on keto than on the normal diet.

Susan If satiety was higher and intake lower on the ketogenic diet, then I imagine weight loss should also have been greater in the keto group.

Dr. Lee Indeed. As Susan naturally inferred, the experimental results showed greater weight loss on the ketogenic diet than on the normal diet (Table 8-1). For body weight, fat mass, lean mass, and total body water, all four decreased more on the keto diet than on the normal diet.

Susan Doctor, how should we interpret why total body water dropped more on keto than on the carbohydrate diet?

Dr. Lee It was mentioned earlier that storing carbohydrates requires more water than storing fat. Naturally, on a ketogenic diet, as carbohydrates and metabolic waste decrease, water content drops as well. Furthermore, the keto diet activates autophagy and immune-cell phagocytosis, efficiently clearing waste from the body. As impurities decline, the water needed to maintain proper concentrations also decreases. Blood glucose and insulin levels also fell more in the keto group.

Emily Doctor, I'm curious—how can keto produce satiety while eating less? Does it mean the ketogenic diet affects mental cravings?

Dr. Lee Yes, exactly. The ketogenic diet has been reported to affect the cells governing appetite. We can examine this in the next study.

"Effect of Ketogenic Diet on Obesity and Other Metabolic Disorders: Narrative Review" (Baylie et al., 2024)

Dr. Lee The next paper is "Effect of Ketogenic Diet on Obesity and Other Metabolic Disorders: Narrative Review" (Baylie et al., 2024). The authors explain that ketone bodies send various hormonal signals to the brain, naturally lowering appetite and promoting satiety. They also describe antioxidant and anti-inflammatory effects, and note that ketone bodies themselves can influence gene regulation.

Emily Could you tell us more about the appetite-related hormones?

Dr. Lee First, the nervous system can be divided into afferent and efferent pathways. Signals traveling from the periphery to the central nervous system are afferent. Signals running from the central nervous system to the periphery are efferent. Various peripheral signals are transmitted to the center, which integrates and sorts them and then sends final decisions back to the periphery. As this feedback proceeds smoothly, the body operates as a unified organism.

Various hormones released by adipocytes, the stomach, pancreas, ileum, and colon are integrated in the arcuate nucleus of the hypothalamus, generating an afferent signal. The arcuate-nucleus neurons then synthesize the input and send out efferent signals to the periphery. The two core actions of efferent signaling are nutrient synthesis and breakdown—that is, food intake and energy expenditure. In the arcuate nucleus, "POMC/CART neurons" lower appetite and reduce body weight, whereas "NPY/AgRP neurons" promote food intake and increase body weight.

Zoe So does the ketogenic diet release appetite-suppressing hormones and activate the appetite-lowering "POMC/CART neurons"?

Dr. Lee Yes, exactly. A representative hormone is leptin. Leptin is synthesized in adipocytes and reduces body weight. It stimulates the appetite-suppressing "POMC/CART neurons" while inhibiting the appetite-promoting "NPY/AgRP neurons." Yet on a ketogenic diet, appetite is also suppressed. Other studies have shown that leptin levels either fall or remain unchanged on keto, suggesting that the appetite suppression of the ketogenic diet involves a mechanism beyond leptin.