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Obesity (polygenic)
Also known as Body mass index · High BMI · Obesity
This common condition affects a significant portion of the adult population in the UK. Many genes interact with lifestyle choices, diet, and physical activity to determine an individual's susceptibility to obesity.
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Overview
Obesity is a medical condition defined by an excessive accumulation of body fat, which can pose risks to health. It is typically diagnosed using the Body Mass Index (BMI), calculated from a person's weight and height. A BMI of 30 or higher generally indicates obesity. In England, around 30% of adults are estimated to live with obesity [PMID:22081622]. This condition is complex, resulting from a combination of many different genes interacting with environmental factors such as diet, physical activity, and overall lifestyle.
Unlike conditions caused by a single genetic change, polygenic obesity involves the combined effects of numerous genetic variations, each contributing a small amount to an individual's overall risk. These genetic predispositions do not guarantee obesity but can influence how easily a person gains weight, how their body stores fat, and their metabolic rate. Environmental factors then play a crucial role in whether this genetic susceptibility translates into obesity.
Symptoms & clinical features
The primary clinical sign of obesity is excessive body fat, which can lead to a range of other health concerns. While a high BMI is a common measure, it doesn't always reflect body composition accurately in every individual. Other indicators might include a large waist circumference.
Individuals with obesity may experience symptoms related to the associated health conditions, such as breathlessness, increased sweating, fatigue, joint and back pain, and difficulties with physical activity. Psychological and social impacts can also be significant. These symptoms are not direct effects of the polygenic predisposition itself but rather consequences of the excessive weight.
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Affected organs
Obesity can affect nearly every major organ system in the body due to the widespread inflammation and metabolic changes it can induce. The cardiovascular system is significantly impacted, increasing the risk of heart disease and high blood pressure. The endocrine system can be affected, leading to conditions like type 2 diabetes due to insulin resistance, and influencing hormone levels. The liver can develop non-alcoholic fatty liver disease.
Musculoskeletal issues are common, particularly affecting joints that bear weight, such as the knees and hips, leading to conditions like osteoarthritis. The respiratory system can also be compromised, potentially leading to sleep apnoea. The brain's regulatory centres for appetite and metabolism can also be influenced by the genetic factors contributing to obesity.
Risks & severity
The severity of obesity varies widely among individuals, ranging from moderate to severe, with higher BMIs generally correlating with increased health risks. The lifetime risks associated with obesity are substantial. It is a major risk factor for several chronic health conditions including type 2 diabetes, heart disease, stroke, certain types of cancer (e.g., breast, colon, kidney), high blood pressure, and sleep apnoea [PMID:32560372].
The age of onset for polygenic obesity is highly variable. While sometimes becoming apparent during childhood, it often develops or worsens in adulthood, influenced by evolving lifestyle, dietary habits, and physical activity levels. The interplay between an individual's genetic background and their environment determines the trajectory of weight gain and the development of associated health complications over time.
Genetic causes
Polygenic obesity is not caused by a single genetic change but rather by the combined effects of variations in multiple genes. Each genetic variation contributes a small amount to an individual's susceptibility. Genes implicated in obesity often play roles in regulating appetite, metabolism, and fat storage. Examples of such genes include BDNF, FTO, MC4R, and TMEM18.
Specific variants in the FTO (fat mass and obesity associated) gene are consistently linked to an increased risk of higher BMI in common obesity [PMID:17435737]. The MC4R (melanocortin 4 receptor) gene is involved in regulating energy balance and appetite, with certain variations affecting how the brain senses satiety. BDNF (brain-derived neurotrophic factor) also influences appetite regulation and energy expenditure. TMEM18 (transmembrane protein 18) is another gene where common variations are associated with increased BMI. These genes represent only a few of the many genes thought to contribute to polygenic obesity, and no single gene variation is solely responsible for the condition.
Inheritance pattern
Polygenic obesity is considered to have a complex or polygenic inheritance pattern. This means that it does not follow the simple dominant or recessive patterns seen in single-gene disorders. Instead, an individual's genetic predisposition to obesity is influenced by the cumulative effect of variations in many different genes, as well as significant environmental factors.
While family history of obesity can indicate an increased risk, it reflects this complex interplay rather than direct inheritance of a specific genetic condition. Relatives of an affected individual may share some of the predisposing genetic variations and similar environmental exposures, meaning their risk may also be higher than the general population. However, having a genetic predisposition does not mean obesity is inevitable; lifestyle choices play a crucial role.
Diagnosis & testing
Diagnosing obesity typically involves clinical assessment, primarily using the Body Mass Index (BMI). BMI is calculated by dividing an individual's weight in kilograms by the square of their height in metres (kg/m²). A BMI of 30 or higher is generally classified as obese.
Genetic testing for polygenic obesity is not routinely offered through the NHS Genomic Medicine Service (NHS GMS) as part of standard care, nor are specific R-codes applicable for this polygenic condition. This is because polygenic risk scores are still primarily a research tool, and individual genetic variants only contribute a small amount to overall risk. Diagnosis and classification primarily rely on clinical measures rather than genetic testing, and a GP may refer individuals to weight management services if appropriate.
Management & lifestyle
Management for obesity focuses on lifestyle interventions aimed at weight loss and preventing or managing associated health complications. This typically involves making sustainable changes to diet, increasing physical activity, and behaviour modification. An individual's GP can provide initial advice and referrals to various NHS services, such as weight management programmes or specialist dietitians.
In some cases, medication or surgical options may be considered, but these are generally reserved for individuals with higher BMIs or significant obesity-related health issues, and always under specialist medical supervision. Management plans are highly individualised. Regular monitoring for associated conditions like type 2 diabetes, high blood pressure, and sleep apnoea is an important part of comprehensive care for individuals living with obesity.
UK care pathway
In the UK, individuals with concerns about their weight or a diagnosis of obesity can access care through their General Practitioner (GP). GPs can provide guidance and refer to NHS weight management services, which may include dieticians, exercise specialists, and psychological support. The NHS Genomic Medicine Service primarily focuses on rare, single-gene disorders, and therefore, specific R-codes for polygenic obesity are not currently available for routine diagnostic testing.
Genetic counsellors are generally involved with conditions where specific genetic variants have clear and predictable inheritance patterns or significant implications for family planning, which is not typically the case for polygenic obesity. However, for individuals with very early onset, severe, or syndromic forms of obesity, a referral to clinical genetics might be considered to rule out monogenic forms of obesity (caused by a single gene defect).
Frequently asked questions
Is obesity always caused by genetics?
No, obesity is a complex condition. While genetics can increase a person's susceptibility, environmental factors like diet, physical activity, and lifestyle choices play a significant role. It's a combination of these elements, not just one.
If I have a family history of obesity, will I definitely become obese?
Having a family history means you might have inherited some genetic predispositions, but it does not mean obesity is inevitable. Lifestyle choices can significantly influence whether this genetic risk is expressed. Many factors contribute to an individual's weight.
Can genetic testing tell me if I will become obese?
Genetic testing for polygenic obesity is not typically offered by the NHS. While research has identified gene variations associated with higher risk, each contributes only a small amount to overall susceptibility. Lifestyle factors remain crucial determinants of weight.
What is the Body Mass Index (BMI) and how is it used?
BMI is a tool used to estimate if a person has a healthy weight for their height. It's calculated using your weight (kg) and height (m). A BMI of 30 or higher typically indicates obesity, but it's one of several measures a doctor might use to assess overall health.
Are there specific genes that cause obesity?
In complex polygenic obesity, there isn't one single 'obesity gene'. Instead, many different genes, such as FTO, MC4R, BDNF, and TMEM18, each contribute a small amount to an individual's overall genetic predisposition to gaining weight or storing fat. These genes interact with lifestyle factors.
References
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- Flegal KM. Body-mass index and all-cause mortality. Lancet (London, England). 2017. PMID: 28612744
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- Suthahar N, Bergman RN, de Boer RA. Replacing body mass index with relative fat mass to accurately estimate adiposity. Nature reviews. Endocrinology. 2025. PMID: 40312540
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