According to the World Health Organization, obesity claims at least 2.8 million lives globally each year. A sample size of 34,648 subjects analysed by the Global Obesity Observatory found that the prevalence of obesity among adults in India was 18.7 per cent in the year 2019-20.
The disease is recognised as one of the major risk factors for Type 2 diabetes, certain types of cancers, heart diseases, stroke, and other diet-related non-communicable diseases. Recently, the US’ Centers for Disease Control and Prevention mentioned that obesity may triple the risk of hospitalisation due to Covid-19, as being overweight is linked to the impairment of the immune system. In the United States, between the beginning of the pandemic and 18 November 2020, there were more than 9,00,000 adults hospitalised with Covid, 30 per cent out of whom were obese, reports the Journal of the American Heart Association.
But what causes obesity? And is counting calories the best way to tackle it?
Debunking older models
Research revealed that losing even a small amount of body weight can drastically reduce the risk of multiple health hazards. The global population is striving to attain sustainable weight loss by adopting suitable lifestyle modifications that involve the conventional ‘eating less-spending more’ approach. However, the century-old ‘calories in, calories out’ or the energy-balance model (EBM) to explain the causation and management of obesity was recently challenged by an alternative paradigm presented by Dr. David Ludwig, professor at Harvard Medical School, in the form of the ‘carbohydrate-insulin’ model (CIM). The latter proposes a bold hypothesis that negates the EBM and claims that the current obesity epidemic is driven by large consumption of high glycemic load (GL) diet, particularly refined carbohydrates found in ultra-processed, easily digestible carbs such as white sugar, refined wheat and starch flour, cake, pastry, or chips.
In the paper, Dr. Ludwig and colleagues argue that the EBM doesn’t explain the biological causes of weight gain. This debate poses two primary challenges to the nutrition science community—finding the rationale behind carbohydrate-insulin hypothesis backed by sound scientific evidence, and the credibility of conventional energy balance models in achieving sustainable weight loss.
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Carbohydrate-insulin model gets it better
The Carbohydrate Insulin Model asserts that changes in food quality cause weight gain. In other words, hormonal and metabolic changes occur due to the body’s response to what we eat rather than how much we eat. The model explains adiposity by insulin-glucagon interplay in response to high-GL foods. Insulin secretion increases and glucagon secretion decreases when we consume such foods. Higher insulin levels signal fat cells to store more calories, leaving a small number available to fuel tissues. This phenomenon makes the brain think that the body isn’t getting enough fuel to function, which in turn leads to hunger. CIM advocates manage obesity by understanding the hormonal response and metabolic changes in response to the nature of food and not just its calorie content. The EBM, however, fails to register the compositional distinction of different foods and focuses on their calorie values alone.
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Calorie deficit is needed for weight loss
From the biological perspective, losing weight requires a calorie deficit. There is no getting around it. Excess calories from foods get stored either in muscle as glycogen as fat in adipose tissues. Continuous consumption of surplus calories and not utilising them leads to weight gain, even if we eat the healthiest foods found on the planet.
Calories ‘in’ – calorie intake via foods is self-explanatory. Calories ‘out’ is the combination of resting energy expenditure (REE) defined as the energy needed for the basal metabolism of the body at rest, physical activity, and thermal effect of food (TEF) or diet-induced thermogenesis.
Studies consistently reported that calorie deficit caused weight loss irrespective of the source. In a 2017 comprehensive review, Scott Howell and Richard Kones cited two highly rigorous metabolic ward studies and concluded that both these studies failed to support the CIM hypothesis, which predicted that lowering dietary carbohydrates (CHO) significantly should decrease insulin levels, thereby leading to fat release from adipocytes and ultimately resulting in expedited fat loss.
An endocrine society scientific statement argued that it is calorie intake rather than changes in energy expenditure or internal metabolic environment that is largely responsible for the impact of diet on obesity risk. Calories are calories, regardless of macronutrient content, so habitual consumption of highly palatable and energy-dense diets predisposes one to excessive weight gain. In a 2021 review, Yves Schutz and colleagues mentioned that low-carbohydrate, ketone-based diets are more ‘academic’ than practical, as these diets are nutritionally imbalanced and cause serious health hazards in the long run.
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Health is more than just calorie debate
The EBM matters for weight loss. However, not all calories have a similar impact on hormones and metabolism. For example, two simple sugars, glucose and fructose, provide the same number of calories per gram but get metabolised through two different pathways. High fructose-containing diets lead to insulin resistance and elevated triglyceride levels. Satiety, hunger, and fullness also vary depending on the type of nutrients. Protein and fibre are found to be more filling than simple carbohydrates. ‘Calorie in-calorie out’ theory fails to take nutrient density into account, and thus lacks credibility in promoting overall good health.
Nutrition should be whole and sustainable
Good nutrition is beyond weight loss and theories. A diverse diet is nutrient-dense, rich in essential vitamins, minerals, and antioxidants that ensure multiple health benefits. There can be N-number of theories and models that try to explain obesity and therapeutic approach to weight loss, but individualisation, local availability of foods, food preference, food intolerance, assessing medical-physiological conditions, and sustainability are the key factors to plan a practical diet prescription. Low-carb diets are well-established nutrition therapies in controlling weight or blood glucose levels. However, the ultra-low forms of these diets give mixed results in defining good health outcomes and compliance. They are also marketed casually as a quick fix for weight loss, and thus need strict monitoring and a scientific approach at all times.
Dr Subhasree Ray is Doctoral Scholar (Ketogenic Diet), certified diabetes educator, and a clinical and public health nutritionist. She tweets @DrSubhasree. Views are personal.
(Edited by Humra Laeeq)