Abdominal obesity
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Central obesity |
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Classification and external resources |
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Abdominal obesity, colloquially known as belly fat or clinically as central obesity, is the accumulation of abdominal fat resulting in an increase in waist size. There is a strong correlation between central obesity and cardiovascular disease.[1]
Visceral fat, also known as organ fat or intra-abdominal fat, is located inside the peritoneal cavity, packed in between internal organs and torso, as opposed to subcutaneous fat which is found underneath the skin, and intramuscular fat which is found interspersed in skeletal muscle. Visceral fat is composed of several adipose depots including mesenteric, epididymal white adipose tissue (EWAT) and perirenal fat. An excess of visceral fat is known as central obesity, the "pot belly" or "beer belly" effect, in which the abdomen protrudes excessively. This body type is also known as "apple shaped", as opposed to "pear shaped", in which fat is deposited on the hips and buttocks.
Causes
The immediate cause of obesity is net energy imbalance — the organism consumes more usable calories than it expends, wastes, or discards via elimination. The fundamental cause of obesity is not well understood, but is presumably a combination of the organism's genes and environment. There is a growing consensus that, in humans, central obesity is related to the excessive consumption of fructose.[2][3][4] Other environmental factors, such as maternal smoking, estrogenic compounds in the diet and endocrine disrupting chemicals may be important also.[5]
Hypercortisolism, such as in Cushing's syndrome also leads to central obesity. Many prescription drugs, such as dexamethasone and other steroids, can also have side effects resulting in central obesity[6], especially in the presence of elevated insulin levels.
Because fat in the midsection contains the greatest amount of cortisol receptors, fat is created and stored in the midsection, specifically in fat cell deposits deep in the abdomen
Diagnosis
While central obesity can be obvious just by looking at the naked body (see the picture), the severity of central obesity is determined by taking waist and hip measurements. The absolute waist circumference (>102 centimetres (40 in) in men and >88 centimetres (35 in) in women) and the waist-hip ratio (>0.9 for men and >0.85 for women)[8] are both used as measures of central obesity. A differential diagnosis includes distinguishing central obesity from ascites and intestinal bloating. In the cohort of 15,000 people participating in the National Health and Nutrition Examination Survey (NHANES III), waist circumference explained obesity-related health risk better than the body mass index (or BMI) when metabolic syndrome was taken as an outcome measure and this difference was statistically significant. In other words, excessive waist circumference appears to be more of a risk factor for metabolic syndrome than BMI.[9] Another measure of central obesity which has shown superiority to BMI in predicting cardiovascular disease risk is the Index of Central Obesity (waist-to-height ratio - WHtR), where a ratio of >=0.5 (i.e. a waist circumference at least half of the individual's height) is predictive of increased risk.[10]
An increasing acceptance of the importance of central obesity within the medical profession as an indicator of health risk has led to new developments in obesity diagnosis such as the Body Volume Index, which measures central obesity by measuring a person’s body shape and their weight distribution.
Index of Central Obesity
Index of Central Obesity (ICO) is the ratio of waist circumference and height first proposed by a Parikh et al in 2007[11][12] as a better substitute to the widely-used waist circumference in defining metabolic syndrome.[13] The National Cholesterol Education Program Adult Treatment Panel III suggested cut off of 102 cm and 88 cm for males and females as a marker of central obesity.[14] The same was used in defining metabolic syndrome.[15] Misra et al. suggested that these cutoffs are not applicable among Indians and the cutoffs be lowered to 90 cm and 80 cm for males and females.[16] Various race specific cutoffs were suggested by different groups.[citation needed] The International Diabetes Federation defined central obesity based on these various race and gender specific cutoffs.[citation needed] The other limitation of waist circumference is that it can not be applied in children.[dubious – discuss]
Parikh et al looked at the average heights of various races and suggested that by using ICO various race- and gender-specific cutoffs of waist circumference can be discarded.[13] An ICO cutoff of more than 0.5 is suggested as a criteria to define central obesity.[citation needed] Parikh et al further tested a modified definition of of metabolic syndrome in which waist circumference was replaced with ICO in the National Health and Nutrition Examination Survey (NHANES) database and found the modified definition to be more specific and sensitive.[13]
This parameter has been used in the study of metabolic syndrome[17][18] and cardiovascular disease.[19]
Body Volume Index
BVI is based upon the principle that excess abdominal weight, measured by part volume as a percentage of total volume, constitutes a greater health risk. Recent validation has concluded that total and regional body volume estimates correlate positively and significantly with biomarkers of cardio-vascular risk and BVI calculations correlate significantly with all biomarkers of cardio-vascular risk.[20]
Health risks
Excess adipose tissue on a male
Central obesity is associated with a statistically higher risk of heart disease, hypertension, insulin resistance, and Diabetes Mellitus Type 2 (see below). Belly fat is a symptom of metabolic syndrome, and is an indicator used in the diagnosis of that disorder.[21][22][23]
Central obesity can be a feature of lipodystrophies, a group of diseases which is either inherited, or due to secondary causes (often protease inhibitors, a group of medications against AIDS). Central obesity is a symptom of Cushing's syndrome[24] and is also common in patients with polycystic ovary syndrome (PCOS). Central obesity is associated with glucose intolerance and dyslipidemia.
Relationship with diabetes
There are numerous theories as to the exact cause and mechanism in Type 2 Diabetes. Central obesity is known to predispose individuals for insulin resistance. Abdominal fat is especially active hormonally, secreting a group of hormones called adipokines that may possibly impair glucose tolerance.
Insulin resistance is a major feature of Diabetes Mellitus Type 2 (T2DM), and central obesity is correlated with both insulin resistance and T2DM itself.[25][26] Increased adiposity (obesity) raises serum resistin levels,[27][28][29][30] which in turn directly correlate to insulin resistance.[31][32][33][34] Studies have also confirmed a direct correlation between resistin levels and T2DM.[27][35][36][37] And it is waistline adipose tissue (central obesity) which seems to be the foremost type of fat deposits contributing to rising levels of serum resistin.[38][39] Conversely, serum resistin levels have been found to decline with decreased adiposity following medical treatment.[40]
Relationship with Alzheimer's Disease
A US study reported in May 2010 Annals of Neurology examining over 700 adults found evidence to suggest higher volumes of visceral fat, regardless of overall weight, were associated with smaller brain volumes and increased risk of dementia.[41][42][43]
Measurement
Silhouettes and waist circumferences representing normal, overweight, and obese
There are various ways of measuring abdominal obesity including:
- Absolute waist circumference (>102 cm in men and >88 cm in women)[14]
- Waist–hip ratio (the circumference of the waist divided by that of the hips of >0.9 for men and >0.85 for women)[8]
- Index of Central Obesity[11]
- Sagittal Abdominal Diameter[44]
In those with a BMI under 35, intra-abdominal body fat is related to negative health outcomes independent of total body fat.[45] Intra-abdominal or visceral fat has a particularly strong correlation with cardiovascular disease.[8]
Sex differences
Female sex hormone causes fat to be stored in the buttocks, thighs, and hips in women. Men are more likely to have fat stored in the belly due to sex hormone differences. When women reach menopause and the estrogen produced by ovaries declines, fat migrates from their buttocks, hips and thighs to their waists;[46] later fat is stored in the belly.[47]
Prevention and treatments
Adjunctive therapies which may be prescribed by a physician are orlistat or sibutramine, although the latter has been associated with increased cardiovascular events and strokes and has been withdrawn from the market in the United States,[48] the UK,[49] the EU,[50] Australia,[51] Canada,[52] Hong Kong,[53] Thailand[54] and Mexico.
In the presence of diabetes mellitus type 2, the physician might instead prescribe metformin and thiazolidinediones (rosiglitazone or pioglitazone) as anti-diabetic drugs rather than sulfonylurea derivatives. Thiazolidinediones may cause slight weight gain but decrease "pathologic" abdominal fat, and therefore may be prescribed for diabetics with central obesity.[55]
Weight loss may not be an effective intervention for obesity: as Bacon and Aphramor wrote, "The majority of individuals regain virtually all of the weight that was lost during treatment, regardless of whether they maintain their diet or exercise program"[56]. The Women's Health Initiative ("the largest and longest randomized, controlled dietary intervention clinical trial"[56]) found that long-term dietary intervention increased the waist circumference of both the intervention group and the control group, though the increase was smaller for the intervention group.[57]
Sit-ups myth
There is a common misconception that spot exercise (that is, exercising a specific muscle or location of the body) most effectively burns fat at the desired location, but this is not the case. Spot exercise is beneficial for building specific muscles, but it has little effect, if any, on fat in that area of the body, or on the body's distribution of body fat. The same logic applies to sit-ups and belly fat. Sit-ups, crunches and other abdominal exercises are useful in building the abdominal muscles, but they have little effect, if any, on the adipose tissue located there.[58]
Slang terms
Excess abdominal fat on a male.
Several colloquial terms used to refer to central obesity, and to people who have it, refer to beer drinking. However, there is little scientific evidence that beer drinkers are more prone to abdominal obesity, despite it being known colloquially as "beer belly", "beer gut", or "beer pot". One of the few studies conducted on the subject did not find that beer drinkers are more prone to abdominal obesity than nondrinkers or drinkers of wine or spirits.[59][60] Chronic alcoholism can lead to cirrhosis, symptoms of which include gynecomastia (enlarged breasts) and ascites (abdominal fluid). These symptoms can suggest the appearance of central obesity.
"Love handles" and "spare tyre" (or "spare tire") are colloquial terms for deposits of fat around a person's midsection, especially visible on the sides over the abdominal external oblique muscle. Love handles are visible deposits on each side of the abdomen or lower back (that a hypothetical lover might grab to pull the subject into an embrace); a spare tire appears to encircle the abdomen (thus resembling an automobile tire).
"Muffin top" is a term used for a person whose midsection spills over the waistline of his or her trousers in a manner that resembles the top of a muffin spilling over its baking pan.
"Pot belly" is another colloquial term used to describe a person who has an excessive amount of abdominal fat. This is especially pronounced and visible over clothing and may be indicative of other health related problems.
