Obesity is a global pandemic and World Obesity Day (March 4^th, 2025) provides a timely reminder about the seriousness of this chronic disease. The International Journal of Obesity recently published a special issue reviewing 100 years of obesity research. The articles within this issue demonstrate the progress made in obesity science. Early research improved understanding of the health risks associated with obesity and the biological basis of the condition. Knowledge about the role of biology and genetics in the expression of obesity led to recognition of obesity as a disease instead of a character flaw or weak will power. It is also apparent that obesity is not a single (homogeneous) condition. Like cancer, which is a collection of diseases with differing causes, symptoms, treatment and prognosis, obesity (perhaps better described as obesities) may be a collection of diseases with unique but overlapping features. Many factors have been proposed as potential contributors to obesity. These include environmental, biological, maternal/developmental, economic, food and beverage behaviour/environment, psychological and social factors operating within and outside of the body. These factors increase energy intake and/or decrease energy expenditure.

In some cases, the cause of obesity is very specific, e.g., a deficiency of receptors in the brain responsible for regulating food intake and energy expenditure (melanocortin 4 receptors, MC4R), or the absence of leptin (a body fat sensing hormone) leading to overconsumption of food. In other instances, there is not one specific cause, but many interacting factors that elevate predisposition to obesity e.g., sub optimal levels of sleep (poor sleep hygiene), certain viral infections, gut microbial imbalance or inadequate levels of metabolically active brown adipose tissue (brown fat). While intriguing in theory, the concept of ‘obesities’ is also of practical value. Prevention and treatment approaches may depend on the type of obesity and cause-specific approaches may be more successful than the blanket approaches tried hitherto.  For obesity due to MC4R deficiency, there is an approved medication, setmelanotide.  Whereas the treatment of obesity for someone with sleep apnea may include continuous positive airway pressure (CPAP) combined with other strategies for improving sleep hygiene.  Of course, we don’t know what we don’t know, and research is needed to further identify and classify types of obesity and respective management approaches.

Weight loss strategies can be broadly classified into lifestyle modification (e.g., diet, exercise, sleep hygiene), pharmacological and surgical approaches to induce a negative energy balance.  Lifestyle modification alone for weight loss is perhaps the hardest approach.  Eating less than your brain and gut is tempting you to eat is challenging, particularly in food rich environments. This is why medications and surgical approaches for weight loss have gained importance. They empower people to achieve greater and more sustainable weight loss. This in turn reduces the severity of many obesity-related conditions including type 2 diabetes, hypertension, dyslipidaemia, knee osteoarthritis and heart failure. In some cases, remission may be possible e.g., for type 2 diabetes and hypertension. There is great excitement about the current generation of anti-obesity medications termed glucagon-like peptide 1 (GLP-1) receptor agonists. Examples include semaglutide (Wegovy) and tirzepatide (Zepbound). The International Journal of Obesity has recently published another special issue dedicated to drugs for obesity. Some pharmacological agents are now generating weight loss amounts that rival surgical approaches. This was unimaginable a few years ago. New drugs in the pipeline promise even greater weight loss although widespread uptake of such medications demands constant vigilance to monitor potential serious adverse reactions. 

Medications are not for everyone. The benefits of obesity medications need to outweigh their potential adverse effects. The utility of an obesity medication may be enhanced by carefully selecting it for specific indications using an algorithm. There are also concerns about the composition of the weight lost on obesity medications. This includes loss of body fat, water, and muscle. Obesity is an issue of excess fat deposition and fat loss is desirable. Conversely, muscle is important for strength and mobility and loss of muscle mass is undesirable. Small reductions in muscle mass in people with obesity after weight loss may not be problematic but excessive loss of muscle can lead to sarcopenia, compromising physical activity levels. This emphasizes the importance of a comprehensive approach to obesity management involving pharmacotherapy and/or weight loss surgery together with nutritional support and physical activity for the preservation of muscle mass, physical function and optimal health.

World Obesity Day provides a perfect opportunity to raise awareness about the new definition of obesity launched by The Lancet Commission on Clinical Obesity in January this year. As noted in the report, the concept of obesity as a disease remains controversial because it is generally considered to be a risk factor for other diseases. The work of the Lancet Commission changes this and establishes objective criteria for diagnosing and treating obesity. The Commission distinguishes between clinical obesity – a chronic, systemic illness due to excess adiposity – and preclinical obesity for people who have excess adiposity but no signs or symptoms of illness. As others have noted, this new definition places less emphasis on the body mass index (BMI) and more emphasis on health. It has been observed that there are other obesity classification systems which address many of the issues identified in the new guidelines. A notable example, published in the International Journal of Obesity, is the Edmonton Obesity Staging System. Nevertheless, it hoped that widespread adoption of this new definition and diagnostic criteria of clinical obesity will lead to better identification and management of obesity.  

It is important to recognise that access to care for obesity is not equal across the globe and neither is access to affordable and effective anti-obesity medications. Health inequalities are another factor driving the global obesity pandemic. It is hoped that World Obesity Day will enhance awareness about the complex and serious nature of obesity, contribute to better understanding of the disease and facilitate effective prevention and management strategies.