D I E T
Despite the widespread availability of nutritional information in schools, doctors' offices, on the internet and on product packaging, it is evident that overeating remains a substantial problem. In the period of 1971–2000, obesity rates in the United States increased from 14.5% to 30.9%.
During the same time period, an increase occurred in the average amount of calories consumed. For women, the average increase was 335 calories per day (1542 calories in 1971 and 1877 calories in 2004), while for men the average increase was 168 calories per day (2450 calories in 1971 and 2618 calories in 2004). Most of these extra calories came from an increase in carbohydrate consumption rather than an increase in fat consumption. The primary sources of these extra carbohydrates are sweetened beverages, which now accounts for almost 25 percent of daily calories in young adults.
Dietary trends have changed with reliance on energy-dense fast-food meals tripling between 1977 and 1995, and calorie intake from fast food quadrupling over the same period. In the early 1980s, the administration of Ronald Reagan lifted regulations limiting the advertising of sweets and fast food to children, and advertisement of these products directed towards children has increased. Agricultural policy and techniques in the United States and Europe have led to lower food prices. In the United States, subsidization of corn, soy, wheat, and rice through the U.S. farm bill has made the main sources of processed food relatively cheap compared to fruits and vegetables.
There is little evidence to support the commonly expressed view that some obese people eat little yet gain weight due to a slow metabolism. What has been found, however, is that some obese people underreport how much food they consume compared to those of normal weight.
A sedentary lifestyle plays a significant role in obesity. Obese people are less active than those of normal weight. For example in Canada, 27.0% of sedentary men are obese as opposed to 19.6% of active men. Normal weight people are more fidgety then their obese conterparts; this relationship is maintained even if normal weight people eat more or the obese person loses weight.
In 2000 the CDC estimated that more than 40% of the US population was sedentary, another 30% was active but not sufficiently and less than 30% had an adequate level of physical activity. There has been a trend toward decreased physical activity in part due to increasingly mechanized forms of work, changing modes of transportation, and increasing urbanization. A study from China found urbanization reduces daily energy expenditure by about 300–400 kcal and going to work by car or bus reduced it by a further 200 kcal. Obesity rates have increased in relation to expanding suburbs. This has been attributed to increased time spent commuting, leading to less exercise and less meal preparation at home. Driving one's children to school has become increasingly popular. In the USA the proportion of children who walk or bike to school declined between 1969 (42%) and 2001 (16%) resulting in less exercise. Studies in children and adults have found an association between the number of hours of television watched and the prevalence of obesity.
Like many other medical conditions, obesity is the result of an interplay between genetic and environmental factors. Polymorphisms in various genes controlling appetite and metabolism may predispose to obesity when sufficient calories are present. Obesity is a major feature in a number of rare genetic conditions: Prader-Willi syndrome, Bardet-Biedl syndrome, MOMO syndrome, leptin receptor mutations, congenital leptin deficiency, and melanocortin receptor mutations. In a people with early-onset severe obesity (defined by an onset before ten years of age and body mass index over three standard deviations above normal), 7% harbor a single locus mutation. Apart from the above syndromes, an association has been found between an FTO gene polymorphism and weight.
The adults in the study who were homozygous for this allele weighed about 3 kilograms more and had a 1.6-fold greater rate of obesity than those who had not inherited this trait. The association disappeared, though, when those with FTO polymorphisms participated in moderately intensive physical activity equivalent to 3 to 4 hours of brisk walking. One study found that 80% of the offspring of two obese parents were obese, in contrast to less then 10% of the offspring of two parents who were of normal weight.
The percentage of obesity that can be attributed to genetics varies from 6% to 85% depending on the population examined. The thrifty gene hypothesis postulates that certain ethnic groups may be more prone to obesity in an equivalent environment. Their ability to take advantage of rare periods of abundance by storing energy as fat would be advantageous during times of varying food availability, and individuals with greater adipose reserves would be more likely survive famine. This tendency to store fat, however, would be maladaptive in societies with stable food supplies. This is the presumed reason that Pima Indians, who evolved in a desert ecosystem, developed some of the highest rates of obesity when exposed to a Western lifestyle.
Medical and psychiatric illness
Certain physical and mental illnesses and the pharmaceutical substances used to treat them can increase risk of obesity. Medical illnesses that increase obesity risk include several rare genetic syndromes (listed above) as well as some congenital or acquired conditions: hypothyroidism, Cushing's syndrome, growth hormone deficiency, and the eating disorders: binge eating disorder and night eating syndrome. However, obesity is not regarded as a psychiatric disorder, and therefore is not listed in the DSM-IVR as a psychiatric illness.
Certain medications may cause weight gain or changes in body composition; these include insulin, sulfonylureas, thiazolidinediones, atypical antipsychotics, antidepressants, steroids, sulfonylureas, certain anticonvulsants (phenytoin and valproate), pizotifen, and some forms of hormonal contraception.
While genetic influences are important to understanding obesity, they cannot explain the current dramatic increase seen within specific countries or globally. Though it is accepted that calorie consumption in excess of calorie expenditure leads to obesity on an individual basis, the cause of the shifts in these two factors on the societal scale is much debated. There are a number of theories as to the cause but most believe it is a combination of various factors.
Stressful Working Lifestyle
The correlation between social class and BMI varies globally. A review in 1989 found that in developed countries women of a high social class were less likely to be obese. No significant differences were seen among men of different social classes. In the developing world, women, men, and children from high social classes had greater rates of obesity. An update of this review carried out in 2007 found the same relationships, but they were weaker. The decrease in strength of correlation was felt to be due to the effects of globalization.
Dead Line Stress ?
Many explanations have been put forth for associations between BMI and social class. It is thought that in developed countries, the wealthy are able to afford more nutritious food, they are under greater social pressure to remain slim, and have more opportunities along with greater expectations for physical fitness. In the developing world the ability to afford food, high energy expenditure with physical labor, and cultural values favoring a larger body size are believed to contribute to the observed patterns. Attitudes toward body mass held by people in one's life may also play a role in obesity. A correlation in BMI changes over time has been found between friends, siblings, and spouses.
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