The obesity paradox is the finding in some studies of a lower mortality rate for overweight or obese people within certain subpopulations. The paradox has been observed in people with

cardiovascular disease Cardiovascular disease (CVD) is any disease involving the heart or blood vessels. CVDs constitute a class of diseases that includes: coronary artery diseases (e.g. angina, heart attack), heart failure, hypertensive heart disease, rheumati ...

and cancer. Explanations for the paradox range from excess weight being protective to the statistical association being caused by methodological flaws such as confounding, detection bias, reverse causality, or selection bias.

Description

The terminology "reverse epidemiology" was first proposed by Kamyar Kalantar-Zadeh in the journal '' Kidney International'' in 2003 and in the ''

Journal of the American College of Cardiology The ''Journal of the American College of Cardiology'' is a peer-reviewed medical journal covering all aspects of cardiovascular disease, including original clinical studies, translational investigations with clear clinical relevance, state-of-the-a ...

'' in 2004. It is a contradiction to prevailing medical concepts of prevention of

atherosclerosis Atherosclerosis is a pattern of the disease arteriosclerosis, characterized by development of abnormalities called lesions in walls of arteries. This is a chronic inflammatory disease involving many different cell types and is driven by eleva ...

and

; however, active prophylactic treatment of heart disease in otherwise healthy,

asymptomatic Asymptomatic (or clinically silent) is an adjective categorising the medical conditions (i.e., injuries or diseases) that patients carry but without experiencing their symptoms, despite an explicit diagnosis (e.g., a positive medical test). P ...

people has been and is controversial in the medical community for several years. The mechanism responsible for this reversed association is unknown, but it has been theorized that, in

chronic kidney disease Chronic kidney disease (CKD) is a type of long-term kidney disease, defined by the sustained presence of abnormal kidney function and/or abnormal kidney structure. To meet criteria for CKD, the abnormalities must be present for at least three mo ...

patients, "The common occurrence of persistent

inflammation Inflammation (from ) is part of the biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. The five cardinal signs are heat, pain, redness, swelling, and loss of function (Latin ''calor'', '' ...

and protein energy wasting in advanced CKD hronic kidney diseaseseems to a large extent to account for this paradoxical association between traditional risk factors and CV ardiovascularoutcomes in this patient population." Other research has proposed that the paradox also may be explained by

adipose tissue Adipose tissue (also known as body fat or simply fat) is a loose connective tissue composed mostly of adipocytes. It also contains the stromal vascular fraction (SVF) of cells including preadipocytes, fibroblasts, Blood vessel, vascular endothel ...

storing

lipophilic Lipophilicity (from Greek language, Greek λίπος "fat" and :wikt:φίλος, φίλος "friendly") is the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. Such compounds are c ...

chemicals that would otherwise be toxic to the body. The obesity paradox (excluding the cholesterol paradox) was first described in 1999 in overweight and obese people undergoing

hemodialysis Hemodialysis, American and British English spelling differences#ae and oe, also spelled haemodialysis, or simply ''"'dialysis'"'', is a process of filtering the blood of a person whose kidneys are not working normally. This type of Kidney dialys ...

, and has subsequently been found in those with

heart failure Heart failure (HF), also known as congestive heart failure (CHF), is a syndrome caused by an impairment in the heart's ability to Cardiac cycle, fill with and pump blood. Although symptoms vary based on which side of the heart is affected, HF ...

myocardial infarction A myocardial infarction (MI), commonly known as a heart attack, occurs when Ischemia, blood flow decreases or stops in one of the coronary arteries of the heart, causing infarction (tissue death) to the heart muscle. The most common symptom ...

acute coronary syndrome Acute coronary syndrome (ACS) is a syndrome due to decreased blood flow in the coronary arteries such that part of the heart muscle is unable to function properly or dies. The most common symptom is centrally located pressure-like chest pain, ...

chronic obstructive pulmonary disease Chronic obstructive pulmonary disease (COPD) is a type of progressive lung disease characterized by chronic respiratory symptoms and airflow limitation. GOLD defines COPD as a heterogeneous lung condition characterized by chronic respiratory s ...

(COPD),

pulmonary embolism Pulmonary embolism (PE) is a blockage of an pulmonary artery, artery in the lungs by a substance that has moved from elsewhere in the body through the bloodstream (embolism). Symptoms of a PE may include dyspnea, shortness of breath, chest pain ...

s, and in older

nursing home A nursing home is a facility for the residential care of older people, senior citizens, or disabled people. Nursing homes may also be referred to as care homes, skilled nursing facilities (SNF), or long-term care facilities. Often, these terms ...

residents. While obese people have twice the risk of developing heart failure compared to individuals with a normal BMI, once a person experiences heart failure, those with a BMI between 30.0 and 34.9 had lower mortality than those with a normal BMI. This has been attributed to the fact that people often lose weight when they have severe and chronic illness (a syndrome called

cachexia Cachexia () is a syndrome that happens when people have certain illnesses, causing muscle loss that cannot be fully reversed with improved nutrition. It is most common in diseases like cancer, Heart failure, congestive heart failure, chronic o ...

). Similar findings have been made in other types of heart disease. Among people with heart disease, those with class I obesity do not have greater rates of further heart problems than people of normal weight. In people with greater degrees of obesity, however, risk of further events is increased. Even after cardiac bypass surgery, no increase in mortality is seen in the overweight and obese. One study found that the improved survival could be explained by the more aggressive treatment obese people receive after a cardiac event. Another found that if one takes into account COPD in those with

peripheral artery disease Peripheral artery disease (PAD) is a vascular disorder that causes abnormal narrowing of arteries other than those that supply the heart or brain. PAD can happen in any blood vessel, but it is more common in the legs than the arms. When narr ...

, the benefit of obesity no longer exists. The obesity paradox is also relevant in discussion of

weight loss Weight loss, in the context of medicine, health, or physical fitness, refers to a reduction of the total body mass, by a mean loss of fluid, body fat (adipose tissue), or lean mass (namely bone mineral deposits, muscle, tendon, and other conn ...

as a preventative health measure – weight-cycling (a repeated pattern of losing and then regaining weight) is more common in obese people, and has health effects commonly assumed to be caused by obesity, such as

hypertension Hypertension, also known as high blood pressure, is a Chronic condition, long-term Disease, medical condition in which the blood pressure in the artery, arteries is persistently elevated. High blood pressure usually does not cause symptoms i ...

insulin resistance Insulin resistance (IR) is a pathological response in which cells in insulin-sensitive tissues in the body fail to respond normally to the hormone insulin or downregulate insulin receptors in response to hyperinsulinemia. Insulin is a horm ...

, and cardiovascular diseases.

Criticisms

Methodology

The obesity paradox has been criticized on the grounds of being an artifact arising from

biases Bias is a disproportionate weight ''in favor of'' or ''against'' an idea or thing, usually in a way that is inaccurate, closed-minded, prejudicial, or unfair. Biases can be innate or learned. People may develop biases for or against an individ ...

observational studies In fields such as epidemiology, social sciences, psychology and statistics, an observational study draws inferences from a sample to a population where the independent variable is not under the control of the researcher because of ethical conc ...

. Strong confounding by

smoking Smoking is a practice in which a substance is combusted, and the resulting smoke is typically inhaled to be tasted and absorbed into the bloodstream of a person. Most commonly, the substance used is the dried leaves of the tobacco plant, whi ...

has been noted by several researchers, although others have suggested that smoking does not account for the observed patterns. Since smokers, who are subject to higher mortality rates, also tend to be leaner, inadequate adjustment for smoking would lead to underestimations of the risk ratios associated with the overweight and obese categories of BMI among non-smokers. In an analysis of 1.46 million individuals, restriction to never-smoking participants greatly reduced the mortality estimates in the underweight group, as well as strengthening the estimates in the overweight and obese groups. This study concluded that, for non-Hispanic white adults who have never smoked, the BMI range of 20.0 to 24.9 was associated with the lowest mortality rates. A similar 2016 study found that, of the BMI ranges studied (which ranged from 18.5 to >30), the "normal" 18.5–22.4 BMI range combined with healthy eating, high levels of physical activity, not smoking, and no more than moderate alcohol consumption was associated with the lowest risk of premature death.Veronese, Nicola; Li, Yanping; Manson, JoAnn E; Willett, Walter C; Fontana, Luigi; Hu, Frank B. (2016) "Combined associations of body weight and lifestyle factors with all cause and cause specific mortality in men and women: prospective cohort study", ''BMJ'', v355. doi

Another concern is reverse causation due to illness-induced weight loss. That is, it may not be low BMI that is causing death (and thereby making obesity seem protective) but rather imminent death causing low BMI. Indeed, unintentional weight loss is an extremely significant predictor of mortality. Terminally ill individuals often undergo weight loss before death, and classifying those individuals as lean greatly inflates the mortality rate in the normal and underweight categories of BMI, while lowering the risk in the higher BMI categories. Studies that employ strategies to reduce reverse causation such as excluding sick individuals at baseline and introducing time lag to exclude deaths at the beginning of follow-up have yielded estimates of increased risk for body mass indices above 25 kg/m². The obesity paradox may therefore result from people becoming lean due to smoking, sedentary lifestyles, and unhealthy diets – all factors which also negatively impact health. Critics of the paradox have also argued that studies supporting its existence almost always use BMI as the only measure of obesity. However, because BMI is an imperfect method of measuring obesity, critics argue that studies using other measures of obesity in addition to BMI, such as

waist circumference The waist is the part of the Human abdomen, abdomen between the rib cage and Hip (anatomy), hips. Normally, it is the narrowest part of the torso. ''Waistline'' refers to the horizontal line where the waist is narrowest, or to the general appe ...

and waist to hip ratio, render the existence of the paradox questionable. One probable methodological explanation for the obesity paradox in regards to cardiovascular disease is

collider A collider is a type of particle accelerator that brings two opposing particle beams together such that the particles collide. Compared to other particle accelerators in which the moving particles collide with a stationary matter target, collid ...

stratification bias, which commonly emerges when one restricts or stratifies on a factor (the "collider") that is caused by both the exposure (or its descendants) of an unmeasured variable and the outcome (or its ancestors / risk factors). In the example of the obesity-cardiovascular disease relationship, the obesity is the collider, the outcome is cardiovascular disease, and the unmeasured variables are environmental and genetic factors – given that obesity and cardiovascular disorders are often associated with each other, medical professionals may be reluctant to consider both other causes of cardiovascular disease or other causes of protection against said diseases. A study from 2018 found that the reason why overweight or obese patients supposedly live longer with cardiovascular disease than people of normal weight is simply because overweight / obese patients get cardiovascular disease at an earlier age, meaning while they survive more years with it, non-obese patients don't get cardiovascular disease at all up until later in life. In fact, the obese have shorter lifespans because they get cardiovascular disease at an early age and have to live a longer proportion of their life with it. This also shows a misunderstanding regarding the paradox: While survival rate once sick is indeed higher for those with obesity than for those few non-obese that have cardiovascular disease, people without obesity usually do not get cardiovascular disease in the first place.

Ties to Coca-Cola

It has also been noted that

Coca-Cola Coca-Cola, or Coke, is a cola soft drink manufactured by the Coca-Cola Company. In 2013, Coke products were sold in over 200 countries and territories worldwide, with consumers drinking more than 1.8 billion company beverage servings ...

has promoted the hypothesis and funded researchers who agree with the hypothesis, which has raised questions about what research the company supports and why.

Weight relativism

Dixon ''et al.'' have proposed that a paradox does not actually exist, as people can be healthy at a range of sizes. As one study puts it, "There is no 'obesity paradox' to explain, if we accept the premise that varying ideal weight ranges apply to individuals over different stages of the life span, accordingly allowing us to abandon the rigid biologically implausible concept of a single 'ideal weight' (for height) or weight range."

Description

Criticisms

Methodology

Ties to Coca-Cola

Weight relativism

See also

References

Further reading