Soul-searching question after the Chinese New Year: Have I gained weight again? Insulin says I am not to blame

The Spring Festival holiday is over, have you gained weight again? Regarding the mechanism of obesity, we instinctively think that it is because we eat too much and the intake exceeds the consumption. This is the traditional "energy balance model". But in fact, the mechanism of obesity is not so simple. In recent years, scientists have proposed the "carbohydrate-insulin model". People eat too much food with a high glycemic index, which leads to increased insulin secretion, prompting the intake of sugar to synthesize into glycogen and fat, leading to obesity. However, this new model also has flaws. To lose weight healthily, it seems that the simple saying "control your mouth and move your legs" is still reliable, but the simplest is often the most difficult.

Written by Gu Shuchen

I gain three pounds every holiday season, and three kilograms during the Spring Festival. I have been trying hard to lose weight for half a year, but it is already the Spring Festival. Nowadays, fat has become the public enemy. On the one hand, it seems that everyone is losing weight: there must be someone around you who is running, doing aerobics, dieting, or fasting, trying to lose weight or control the growth of waist circumference; on the other hand, people are getting fatter and fatter.

Obesity has become a major global public health problem. The World Health Organization points out that excessive fat can lead to a variety of serious health problems, such as cardiovascular diseases such as heart disease and stroke, type 2 diabetes, osteoarthritis, and endometrial cancer, breast cancer, and colon cancer. Obesity has become a global epidemic. Data from the World Health Organization show that the number of obese people in the world has almost tripled since 1975. Currently, more than 1.9 billion adults are overweight, of which 650 million are obese. China is already the country with the largest obese population. However, in the public's view, obesity as a disease is still vague. The new crown pandemic will definitely pass, but when will we see the light at the end of the tunnel for the obesity pandemic? To solve this problem, we first need to know how people get fat.

A better model for the mechanisms of obesity?

The traditional "energy balance" concept holds that people get fat because they consume more calories than they burn, and if they consume less, they can stay slim. But is it really that simple? Do you sometimes feel that you are gaining weight even if you drink cold water?

On September 14, 2021, a research team composed of 17 internationally renowned experts published a study in The American Journal of Clinical Nutrition that broke the traditional concept. They believed that the "energy balance model" has fundamental flaws and ignores the impact of food on hormones and metabolism. The "Carbohydrate-Insulin Model (CIM)" can better explain the mechanism of obesity and guide weight loss.

Their core idea is that obesity is not simply caused by eating too much, but by consuming too much high GI (glycemic index), high GL (glycemic load) and high-carb foods (especially refined, rapidly digestible carbohydrates); these foods cause higher insulin secretion, thereby changing the body's metabolism, ultimately leading to fat storage, weight gain and obesity. In a low-fat diet, food will contain more high GI, rapidly digestible carbohydrates, which will promote fat storage and lead to obesity. Reducing the intake of such foods will help achieve the goal of weight loss [1].

The so-called GI (Glycemic Index) is an indicator for evaluating the effect of food on postprandial blood sugar. It is calculated based on 100g of food, and GI70 is a high GI food. According to the data of the "Guidelines for the Diagnosis and Treatment of Type 1 Diabetes in China", the GI of 100g of rice, the staple food that we Chinese usually eat, is about 83, and 100g of Fuqiang flour steamed bread is about 88. This type of food is digested quickly after entering the gastrointestinal tract, has a high absorption rate, and releases glucose quickly, which can cause blood sugar to rise rapidly. Therefore, they are also called "fast carbohydrates", that is, fast-digesting carbohydrates. Corresponding to this are low-glycemic index foods, which include fruits, vegetables, beans and whole grains, that is, they stay in the gastrointestinal tract for a long time after eating, have a low absorption rate, and release glucose slowly, which can prevent postprandial blood sugar from rising. They are also called "slow carbohydrates."

Glycemic load (GL) is also used to measure the level of blood sugar. Compared with observing the effect of carbohydrate-containing foods on blood sugar levels, glycemic load adds the factor of consumption. High glycemic load foods include sugary foods and drinks, white bread and rice.

In addition to a small amount of basal insulin secretion that is independent of eating, another large amount of insulin secretion is caused by the stimulation of increased blood sugar after eating. Therefore, the amount of insulin secretion caused by the intake of high-GI fast carbohydrates is also higher. Insulin can act on multiple organs and tissues in the human body, but one of its core responsibilities is to participate in the storage of calories in food to cope with possible energy shortages in the body. When we eat high-GI carbohydrates, insulin secretion increases, and under the action of insulin, blood sugar is used to synthesize glycogen and fat, causing the body's blood sugar level to drop. This insulin secretion caused by eating can immediately terminate the decomposition of fat, carbohydrates and proteins originally stored in the body. At the same time, because the body has been saving calories, it will cause the brain to "mistakenly believe" that the body has not obtained enough energy, and the basal metabolism may slow down in this process to save energy. The result is that even if excess fat has been gained, we may still feel hungry.

According to the CIM model, excessive fat storage is caused by postprandial insulin secretion, so the corresponding "obesity solution" is the so-called low-carbohydrate, high-fat or "ketogenic" diet, which is to replace carbohydrates with dietary fat, thereby reducing postprandial insulin secretion. The theoretical model of this study is not new, and low-carb diet/ketogenic diet weight loss methods have already swept the world. However, many experiments have also reached conclusions that are contrary to this model. In the May 2021 issue of Science magazine, there is an article Carbohydrates, insulin, and obesity that critically discussed the regulatory relationship between carbohydrates, insulin and obesity. The article believes that the main cause of widespread obesity in humans is still uncertain, and the CIM theory cannot well explain some recent experimental results [2].

Figure: Carbohydrate-insulin model on the left, energy balance model on the right [2]

Don’t be superstitious about low-carb diets

In a mouse diet experiment, researchers fed the mice a diet in which the protein content remained constant, while the fat and carbohydrate content varied between 10% and 80%; the carbohydrates consisted of "high GI" carbohydrates such as a mixture of corn starch, maltodextrin, and sucrose. The experiment lasted for 12 weeks, roughly equivalent to 9 years in humans [3]. If the CIM predicts that as the carbohydrate content of the diet increases, postprandial insulin will also increase, and the mice will eventually develop obesity. However, the experimental results showed that despite the higher postprandial insulin levels of the mice on the high carbohydrate diet, they also gained less weight and body fat because they consumed fewer calories in total.

Another experiment based on human diet also obtained the opposite result to CIM. In this experiment, the researchers came to the conclusion by changing the diet of 20 adult hospitalized patients and analyzing their metabolism after one month. These patients were randomly assigned to a diet consisting of 10% carbohydrates and 75% fat, or 10% fat and 75% carbohydrates, and were told to eat as much as they wanted until they felt full [4]. Because the total calories consumed by participants in a high-carbohydrate diet were lower than those in a low-carbohydrate, high-fat diet when they were in the same state of fullness, the participants who ate high-carbohydrate diets not only did not gain weight, but their body fat content was significantly reduced. Other experimental studies have found that compared with a low-carbohydrate diet, a high-carbohydrate diet significantly increases satiety after 10 to 15 weeks.

Furthermore, a study that observed individuals randomly consuming low-carbohydrate and high-carbohydrate diets in their daily lives also found that there was no sustained difference in energy intake between the two diets over a year of follow-up [5], and their long-term average weight loss was almost the same. Therefore, individual differences in post-meal insulin secretion do not predict who loses the most weight on each diet.

Although CIM is controversial in recent experiments, it does not deny the important role of insulin in regulating body fat. Some mice with knockout genes related to insulin secretion or people with drugs that inhibit insulin secretion will have less body fat. Diabetic patients usually lose weight before diagnosis, and after increasing endogenous insulin secretion or exogenous insulin treatment, patients often regain weight. All of this proves that insulin plays an important role in body fat regulation. Although some of the above experiments have their own flaws, such as the metabolism of mice is not exactly the same as that of humans, and the human test time is too short. But these experiments should not be ignored. High insulin does not necessarily lead to weight gain directly, nor can it accurately predict future weight changes. It is not appropriate to directly explain whether obesity will occur by differences in insulin secretion or insulin action. We should comprehensively consider the multi-effects of insulin on different organs, including those insulin functions that are independent of carbohydrate intake, so as to better understand the role of insulin in obesity and improve our understanding of the causes and treatment of obesity.

A little commonplace

At present, modern medicine has not been able to provide a permanent treatment for obesity, but this does not mean that we are helpless. Short-term low-carb ketogenic diets have indeed helped many people lose weight successfully, but long-term use of low-carb diets also has certain disadvantages, such as headaches, constipation, muscle cramps, diarrhea, and ketosis. A few people may also suffer from sudden cardiovascular diseases.

For ordinary people, losing weight is not only about getting a slim body, but also about getting healthy. A reasonable diet, a regular lifestyle, and moderate exercise are the keys to healthy weight loss.

References

[1] Ludwig DS, Aronne LJ, Astrup A, de Cabo R, Cantley LC, Friedman MI, Heymsfield SB, Johnson JD, King JC, Krauss RM, Lieberman DE, Taubes G, Volek JS, Westman EC, Willett WC, Yancy WS, Ebbeling CB. The carbohydrate-insulin model: a physiological perspective on the obesity pandemic. Am J Clin Nutr. 2021 Sep 13;114(6):1873–85.

[2] Speakman JR, Hall KD. Carbohydrates, insulin, and obesity. Science. 2021;372(6542):577–8. Hu S, Wang L, Togo J, Yang D, Xu Y, Wu Y, Douglas A, Speakman JR. The carbohydrate-insulin model does not explain the impact of varying dietary macronutrients on the body weight and adiposity of mice. Mol Metab. 2020 Feb;32:27-43.

[3] Hall KD, Guo J, Courville AB, Boring J, Brychta R, Chen KY, Darcey V, Forde CG, Gharib AM, Gallagher I, Howard R, Joseph PV, Milley L, Ouwerkerk R, Raisinger K, Rozga I, Schick A, Stagliano M, Torres S, Walter M, Walter P, Yang S, Chung ST. Effect of a plant-based, low-fat diet versus diet an animal-based, ketogenic diet on ad libitum energy intake. Nat Med. 2021 Feb;27(2):344-353.

[4] Shimy KJ, Feldman HA, Klein GL, Bielak L, Ebbeling CB, Ludwig DS. Effects of Dietary Carbohydrate Content on Circulating Metabolic Fuel Availability in the Postprandial State. J Endocr Soc. 2020 May 26;4(7):bvaa062.

[5] Guo J, Robinson JL, Gardner CD, Hall KD. Objective versus Self-Reported Energy Intake Changes During Low-Carbohydrate and Low-Fat Diets. Obesity (Silver Spring). 2019 Mar;27(3):420-426.

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