Overweight and obesity are defined as abnormal or excessive fat accumulation that presents a risk to health. A body mass index (BMI) over 25 is considered overweight and over 30 is obese. The issue has grown to epidemic proportions, with over 4 million people dying each year as a result of being overweight or obese in 2017 according to the global burden of disease.
Body mass index (BMI) is a calculation that takes a person’s weight and height into account to measure body size. In adults, obesity is defined as having a BMI of 30.0 or more, according to the Centers for Disease Control and Prevention (CDC).
Rates of overweight and obesity continue to grow in adults and children. From 1975 to 2016, the prevalence of overweight or obese children and adolescents aged 5–19 years increased more than four-fold from 4% to 18% globally.
The last century has witnessed fundamental developments in our understanding of the biological basis of obesity. Many lines of investigation demonstrate that body weight is controlled by complex and interconnected systems involving multiple organs, hormones, and metabolic pathways. Common genetic variants acting on these systems may explain >20% of the population-level variation in BMI.
During the last century, 2 models addressing environmental causes of obesity have emerged. In the dominant energy balance model (EBM), energy-dense, tasty, modern processed foods drive a positive energy balance through increased intake and thereby result in fat deposition. In other words, 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 of obesity, which means that this model suggests that successful weight loss requires reducing total calorie intake. This involves consuming fewer calories and increasing physical activity levels.
In the second model, the carbohydrate-insulin model (CIM) postulates 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.
This model states that it is the increase in fat storage due to the consumption of processed carbohydrates and not increased calorie consumption intake that leads to weight gain and is primarily responsible for elevated obesity rates.
An article recently published in the American Journal of Clinical Nutrition provides a comprehensive description of this model, along with testable hypotheses that may help clarify the precise changes in nutrition necessary to lose weight or maintain a healthy weight.
According to the EBM, a positive energy balance where a person takes in more calories than they burn is primarily responsible for weight gain, without taking into account the dietary source.
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.
Moreover, reducing caloric intake tends to be successful as a weight-loss strategy only at the beginning. This is because the body adapts to the lower calorie intake, which results in lower metabolic rate and increased hunger. According to the CIM, food quality plays a more significant role in weight gain than overall calorie intake.
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 information about the surge in blood sugar levels considering the GI and amount of carbohydrates a serving of given food provides.
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.
Based on their study results, the authors recommended that a person is more likely to achieve long-term weight loss by modifying diet quality rather than reducing total calorie intake and that adhering to a diet consisting of low GI foods can lead to weight loss by reducing hunger and increasing energy levels. One example of this is to substitute high-glycemic foods like refined grains, potato products and concentrated sugars with high fat foods like nuts, seeds, avocado and olive oil, allowing for moderate intake of total carbohydrates from whole grains, whole fruits, legumes and non-starchy vegetables.
David S Ludwig, et al. The carbohydrate-insulin model: a physiological perspective on the obesity pandemic, The American Journal of Clinical Nutrition, 2021;, nqab270, https://doi.org/10.1093/ajcn/nqab270