- Conventional scientific opinion has attributed weight gain to a net surplus of calories due to burning fewer calories than taking in.
- Opposing this view, the carbohydrate-insulin model states that diet quality matters more for weight loss than total calorie intake.
- The model posits that the intake of processed carbohydrates and starchy foods leads to changes in the levels of insulin and other hormones, subsequently resulting in increased fat deposition.
- The increased fat deposits lead to hunger and consumption of more calorie-rich foods leading to obesity.
- The model suggests that avoiding processed carbohydrates and starchy foods may be necessary to lose weight instead of restricting calories.
The World Health Organization (WHO) states that the global prevalence of overweight and obesity has increased in the past 5 decades.
There is significant consensus in the scientific community that environmental factors, especially the easy availability of highly processed foods and sedentary lifestyles, have contributed to increasing obesity rates.
However, there is much disagreement about how these environmental factors contribute to weight gain.
According to the predominant energy balance model (EBM), consuming more calories than those burned results in a positive energy balance and weight gain.
The increased caloric intake due to the easy accessibility of highly palatable and inexpensive processed food and lower energy expenditure due to reduced physical activity levels have contributed to the global increase in obesity.
In other words, the EBM suggests that successful weight loss requires reducing total calorie intake. This involves consuming fewer calories and increasing physical activity levels.
Unlike the EBM, the carbohydrate-insulin model (CIM) posits that the quality of food consumed plays a critical role in body weight management rather than total calorie intake.
Specifically, consumption of processed and starchy carbohydrates that cause a rapid increase in blood glucose levels results in their storage as fat. Increased fat accumulation sets off a feedback loop resulting in increased hunger and possible consumption of calorie-rich foods.
The CIM states that it is the increase in fat storage due to the consumption of processed carbohydrates and not increased calorie intake that leads to weight gain and is primarily responsible for elevated obesity rates.
A recent article published in theAmerican Journal of Clinical Nutrition provides a comprehensive description of the CIM, along with testable hypotheses that may help clarify the precise changes in nutrition necessary to lose weight or maintain a healthy weight.
The article’s first author Dr. David Ludwig, told Medical News Today, “If the CIM is right, then the conventional approach to weight loss, the low-calorie diet, is likely to fail for most people over the long term. We argue that people have more control over what they eat than how much. A focus on reducing processed carbohydrates, rather than calorie restriction, may be more effective by lowering the biological drive to store excessive fat.”
Flaws of the EBM
According to the EBM, a positive energy balance where a person takes in more calories than they burn is primarily responsible for weight gain. In other words, the EBM regards all calories in the same way, regardless of their dietary source.
The proponents of the CIM acknowledge that a positive energy balance is associated with weight gain, but this does not establish causation.
They argue that metabolic and hormonal changes that occur in response to the consumption of specific foods are the root cause of weight gain, with excessive calorie intake being the outcome.
Although calorie intake tends to increase during puberty, some experts think that it is the biological changes rather than positive energy balance that is responsible for the growth spurt.
Therefore, while the EBM focuses on the overall consumption of calories, it ignores the role of food quality and the subsequent metabolic processes and hormonal changes in mediating weight gain.
Moreover, reducing caloric intake tends to be successful as a weight-loss strategy only in the short term. This is due to the body adapting to the lower calorie intake, resulting in lower metabolic rate and increased hunger.
The CIM
According to the CIM, food quality plays a more significant role in weight gain than overall calorie intake.
In addition to highly processed carbohydrates, the intake of carbohydrates has been increasing since the 1980s. This is likely due to the perception that consuming fats causes weight gain.
The glycemic index (GI) rates carbohydrates according to how rapidly they raise blood glucose levels after someone has eaten them. The glycemic load is another measure that provides more comprehensive information about the surge in blood sugar levels by considering the GI and amount of carbohydrates a serving of a given food provides.
Consumption of processed and starchy foods that contain rapidly digestible carbohydrates results in a surge in blood glucose levels. Foods with a high glycemic load include processed grains, potato products, and foods with high free sugars content. Free sugars are all types of sugars that do not occur naturally in whole fruits and vegetables.
In contrast, fats and proteins have a negligible impact on blood sugar levels, whereas fresh whole fruits, minimally processed grains, legumes, nuts, and nonstarchy vegetables typically have a low or moderate glycemic load.
The rapid surge in glucose levels after consuming high glycemic load foods results in the secretion of insulin, which regulates blood sugar levels and helps the muscles, liver, and adipose or fat tissue absorb glucose.
At the same time, consuming rapidly digestible carbohydrates suppresses the levels of the hormone glucagon.
The pancreas secretes glucagon to counter low blood sugar levels that occur between meals. Glucagon secretion raises blood glucose levels by stimulating the release of glucose stored in the liver as glycogen.
During the first 3 hours after the intake of high glycemic load foods, high insulin and low glucagon levels lead to the storage of glucose as glycogen in the liver and as fat in the liver and adipose or fat tissue.
Although the body absorbs the nutrients present in high glycemic load foods in the initial 3–4 hours, the high insulin and low glucagon levels persist.
This hormonal state slows down the breakdown of the energy stores in the liver and adipose tissue needed to fuel critical tissues in the body. This results in low levels of glucose, fatty acids, and other metabolites in the blood, resembling a fast-like state.
The drop in blood metabolite levels signals the brain, indicating that the tissues are deprived of energy.
When the brain perceives this fast-like state, it provokes hormonal changes that lead to hunger and craving for high-energy foods, such as those high on the GI.
The consumption of foods with a high glycemic load leads to their accumulation as fat. This leads to a positive feedback loop, resulting in the consumption of more high glycemic load foods.
The fast-like state resulting from the consumption of high-glycemic load foods may also result in changes in the body that result in lower energy expenditure.
Addressing the scientific basis of the CIM, Dr. Ludwig said: “ There is strong evidence for some of these steps. For instance, in animals, it has been conclusively shown that all calories are not alike and that obesity can develop without increased food intake. There is evidence, but not yet proof, for this possibility in humans.”
Criticisms
The CIM has provoked a significant amount of controversy, including how insulin and carbohydrates affect weight gain.
Dr. Aaron Roseberry, associate professor at Georgia State University, told MNT, “I think there is going to be a lot of individual variability in the physiology and changes that occur in individuals with obesity as they [develop obesity]. There may be some role for insulin, along with a lot of other factors that may contribute different amounts in different individuals. This just makes it even more challenging to really identify the causes and potential treatments to help prevent weight gain and the development of obesity.”
Another criticism of the CIM is the absence of a significant difference in weight loss in some studies comparing individuals on a low carbohydrate diet with those on a low fat diet.
The authors contend that these results could be due to the long duration of these studies, during which participants may find it difficult to adhere to the dietary regimens. Furthermore, they point out that some evidence shows that a low carbohydrate diet can result in more weight loss than a low fat diet.
MNT spoke to Dr. Christopher Gardner, professor at Stanford University, CA. He is the lead author of one such study comparing the impact of a healthy low fat diet versus a low carbohydrate diet.
Describing the study, Dr. Gardner said: “We very specifically looked at insulin secretion levels in the participants to differentiate the individuals in the study who were more likely to be insulin resistant vs. insulin sensitive. We had hypothesized that a healthy low carb diet would be more helpful for those who were more insulin resistant.”
“However, we found that weight loss was no different for healthy low carb vs. healthy low fat, even when taking insulin secretion (a proxy measure for insulin resistance) into account,” continued Dr. Gardner.
The authors argue that participants in the low fat diet group in this study eliminated carbohydrates with a high glycemic load, and hence, these results do not contradict their model.
MNT also spoke to Dr. Stephen Guyenet, the author of the book The Hungry Brain. Dr. Guyenet said, “This is the most detailed and persuasive articulation of their hypothesis to date. I support low carbohydrate diets as a valid option for bodyweight management. I think their model of obesity has substantial limitations, though.” Dr. Guyenet continued:
“The paper focuses on glycemic load as a particularly important determinant of body fatness. Yet, I am unfmeritaware of experimental evidence in humans that glycemic load per se contributes to fat gain, and low-glycemic-load diets are not particularly effective for fat loss. Furthermore, it remains unknown whether the limited weight loss caused by low-glycemic-load diets is due to impacts on blood glucose and insulin themselves since these diets typically alter multiple variables simultaneously.”
The authors acknowledge that although carbohydrates and insulin play a vital role in the model, other hormones and biological processes work in association with insulin to mediate the effects of increased consumption of high glycemic load foods.
The CIM also claims to reverse the conventional wisdom that excess intake of calories leads to weight gain and posits that increased fat accumulation due to metabolic and hormonal changes results in obesity.
However, Dr. Gardner noted that both the EBM and CIM might have some merit in terms of weight loss. In other words, there seems to be a bidirectional relationship between food intake and metabolic changes.
“I also find the claim that the CIM represents a reversal in the causal pathway to be problematic. […] I find those types of statements to be more damaging than helpful in trying to clear up confusion around nutrition topics,” said Dr. Gardner.
Implications
In line with their model, the authors recommend that a person is more likely to achieve long-term weight loss by modifying diet quality rather than reducing total calorie intake.
“Trying to count calories (adding those you’ve eaten, and subtracting those that you may have burned in physical activity) is fraught with challenges in terms of accuracy, and this can be easily “gamed” so that people think they are doing the right thing, but they really are not accurately assessing these two components […] leading to poor results,” added Dr. Gardner.
The authors suggest that adhering to a diet consisting of low GI foods can lead to weight loss by reducing hunger and increasing energy levels. The authors note:
“A practical strategy is to substitute high-glycemic load foods (refined grains, potato products, concentrated sugars) with high fat foods (e.g., nuts, seeds, avocado, olive oil), allowing for moderate intake of total carbohydrates from whole kernel grains, whole fruits, legumes, and nonstarchy vegetables.”
Dr. Gardner warned that dietary advice recommending strict low carbohydrate intake sometimes involves a limited intake of healthy carbohydrates.
He said, ”I feel that a shortcoming is when this gets carried over into avoiding legumes (beans, lentils, pulses, etc.), whole fruits, and whole grains. Those are carbohydrate-rich food groups that the Dietary Guidelines for Americans, the American Heart Association, the American Cancer Society, the World Health Organization, and the Food and Agriculture Organization of the United Nations recommend. Those are the sources of good quality carbohydrates (low GI and good sources of fiber). I think recommending limiting those food sources is dangerous and will have adverse health consequences.”
Moving beyond controversies
The arguments about the validity of the CIM and the EBM have been divisive and adversarial.
“The field of obesity should embrace paradigm clash as an essential step forward. Toward this end, investigators should, first, refrain from hyperbolic claims to have disproven (or proven) alternative explanations of the obesity pandemic,” suggest the authors.
The authors also recommend “collaborations among scientists with diverse viewpoints to test predictions in rigorous and unbiased research and […] depersonalize the debate, scrupulously avoiding ad hominem argument. Rigorous research using complementary designs will be needed to resolve the debate, clarify a middle ground, or point the way to new explanatory models that better encompass the evidence.”
While there is substantial disagreement in the scientific community, there seems to be an agreement regarding the need to reduce the consumption of processed food.
Dr. Gardner noted, “I would like to see proponents of both EBM and CIM get together and make it clear that there is substantial agreement on reducing the intake of added sugars and refined grains in the diet as a major priority for addressing the obesity epidemic. I believe hearing this kind of agreement would be very beneficial for the public who is understandably confused.”
“If you want to dive into insulin secretion/resistance, fuel partitioning, ultra-processed vs. unprocessed, basal metabolic rate, lean vs. adipose tissue, ad libitum vs. isocaloric […] we can continue to discuss what small differences we might find around the periphery or fringe that could help some people to fine-tune their eating behaviors […].”
“[B]ut this pales in comparison to cutting back drastically on added sugars and refined grains […] which everyone agrees on, and which makes up 42% of the calories in the U.S. diet.”
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