The prevalence of obesity around the world is monitored by the WHO through the Global Database on BMI that gathered data from surveys or population studies, where weight and height are measured or self-reported [10].

In 2008, 35 % of adults aged 20+ were overweight and 11 % were obese (meaning that 205 million men and 300 million women were obese). The worldwide prevalence of obesity had nearly doubled since 1980 (10 % of men and 14 % of women were obese in 2008, compared with 5 % for men and 8 % for women in 1980). The prevalence of overweight and obesity was highest in the WHO Regions of the Americas (62 % for overweight in both sexes and 26 % for obesity) and lowest in the WHO Region for Southeast Asia (14 % overweight in both sexes and 3 % for obesity). In the WHO Region for Europe, for the Eastern Mediterranean, and for the Americas, over 50 % of women were overweight; for all three of these regions, roughly half of overweight women are obese (23 % in Europe, 24 % in the Eastern Mediterranean, 29 % in the Americas). In all WHO regions, women were more likely to be obese than men, and in the WHO regions for Africa, Eastern Mediterranean, and Southeast Asia, women had roughly double the obesity prevalence of men (Fig. 3.1).

The prevalence rates of overweight and obesity increase with income level of countries up to upper middle income levels. The prevalence of overweight in high income and upper middle income countries was more than double that of low and lower middle income countries. For obesity, the difference more than triples from 7 % obesity in both sexes in lower middle income countries to 24 % in upper middle income countries. Women’s obesity was significantly higher than men’s, with the exception of high income countries where it was similar.

The current epidemic of obesity has been reported in most, but not all, regions of the world.

The more recent US data are from the National Health and Nutrition Examination Survey, 2011–2012. More than one-third (35 %) of adults were obese and the prevalence of obesity was higher among middle-aged adults (40 %). The overall prevalence rates of obesity did not differ by gender, but they differ within ethnic groups: for example, among non-Hispanic black adults 57 % of women were obese, compared to 37 % of men. The highest prevalence rate of obesity was among non-Hispanic black adults (48 %), the lowest among non-Hispanic Asian adults (10.8 %). The prevalence of obesity among adults did not change between 2009–2010 and 2011–2012. In the early 1960s, the prevalence of obesity was 11 % among men and 16 % among women, and it changed relatively little until 1980. Data from NHANES II (between 1976 and 1980) and NHANES III (between 1988 and 1994) demonstrate that the prevalence rates of obesity increased considerably, to about 21 % in men and to about 26 % in women. By 2003–2004 the prevalence had increased to almost 32 % in men and 34 % in women [11].

Overall, most countries have rising trends of obesity. Only 2 of the 28 countries in the Global Database on BMI showed a falling trend in the prevalence of obesity in men (Denmark and Saudi Arabia), and 5 of the 28 countries showed a falling trend in the prevalence of obesity in women (Denmark, Ireland, Saudi Arabia, Finland, and Spain) [10].

However, the secular trend on the prevalence of obesity must be considered with caution: a continuous variable, such as body weight, is used to classify dichotomous variables, such as obesity and overweight. This could imply that an average modest weight gain might lead to a relevant increase in the incidence of overweight and obesity. However, in the USA it has been reported that the average increase of BMI has been very relevant, changing from 25.6 in 1976–1980 to 27.9 in 1999–2004 in men and from 25.3 to 28.7 in women, respectively (this corresponds to an increase of more than 7 kg in weight for men and women of average heights) [12].

Overweight and obesity lead to adverse metabolic effects on blood pressure, cholesterol, triglycerides, and insulin resistance. Risks of coronary heart disease, ischemic stroke, and type 2 diabetes mellitus increase steadily with increasing body mass index (BMI). Raised body mass index also increases the risk of sleep apnea, musculoskeletal disease, infertility, dementia, and cancer of the breast, colon, prostate, kidney, endometrium, and gall bladder. Recent analyses show that, in spite of advances in cardiovascular prevention and treatment, it is likely that the overall health burden associated with excess body weight will increase over time, particularly through an increasing prevalence of all other more disabling conditions associated with it [2].

Many epidemiologic studies report a U-shaped relationship between total mortality and BMI, with significant increased mortality at either extremes of BMI, but lower mortality in the overweight category [13]. The relation between overweight and total mortality is controversial, because some studies report an inverse relationship, but methodological issues might be responsible for this “obesity paradox,” for example, reverse causality, given that elevated mortality rates at low BMI might be due to weight loss associated with occult or preexisting diseases, to smoking, or, in older population, to the development of frailty. Moreover, potential over-controlling by adjustment for weight-related conditions (such as diabetes and hypertension) might mislead and decrease the association of BMI and mortality [14, 15]. A recent long-term prospective study of older men indicates that a good overall health prognosis is associated with maintaining normal weight over the life course. It also demonstrates that age is an effect modifier in the association between overweight and risk of death and disability: midlife overweight is associated with a higher mortality rate, whereas in late life, the associations become more complex. Those who lose weight after being overweight in midlife not only have a higher risk of death but also have a higher risk of developing frailty and incident mobility-related disability in late life [16]. One important consideration is that higher BMI values may also be due to higher lean body mass, and a further indication comes from the Cardiovascular Health Study, where in men and women 65+, higher waist circumference was related to higher mortality risk, after controlling for BMI, while high BMI was associated with lower mortality, when controlling for waist circumference, probably because it represented the protective effect of lean mass [17]. During recent years, the obesity paradox has been a popular topic in the research of chronic diseases, such as cardiovascular disease, diabetes, and cancer. The suggestion that overweight or obesity could have beneficial effects may even have raised questions about the need for weight control programs. However, as reported in recent studies, the apparent paradox may be due to relatively short follow-up times and, especially, the inability of most studies to account for weight trend during the life course.

Weight should be rigorously controlled since birth. A study comparing data from large nationwide surveys has shown that the combined prevalence of obesity and overweight in children was high in the USA (25 %), moderate in Russia (16 %), and low in China (7 %) [18]. Previous studies indicate that in many developed countries children obesity has reached levels similar to those in the USA and that it is increasing in developing countries (e.g., in Brazil it has tripled from 1970 (4 %) to 1990 (14 %)). To the contrary, recent trend seems to stabilize in US children, and this might be due to the aggressive campaign against obesity and unhealthy dietary patterns. Data from other countries have shown a decline or stabilization of obesity levels, especially in children. For example, in Germany a study found a significant decline in overweight or obesity in children aged 4–7 years and a stabilization in children from 8 to 16 years of age, between 2004 and 2008 [19]. An Italian survey (Okkio alla salute) shows that 22.2 % of children aged <14 years are overweight, while 10.6 % are obese, with higher percentages in the southern regions. However, in Italy, as well as in the USA, prevalence seems to be stabilized in the very recent years [20].

In conclusion, we believe that the high prevalence rate of overweight and obesity, in spite of stabilized trends observed in the recent past in some developed countries, has to be considered with great concern, particularly in children. A high percentage of obese children and adolescents nowadays present complications which, until a decade ago, characterized only adulthood: insulin resistance, type 2 diabetes, dyslipidemia, hepatic steatosis, hypertension, that sometimes cluster in the metabolic syndrome, and all of them are associated to cardiovascular events, cancer and premature death as an adult. [21]. Obese children are also at higher risk of precocious puberty, polycystic ovary syndrome, nighttime sleep apnea, orthopedic complications, and psychological and social disturbances [22]. Nor should it be forgotten that obese children have a higher probability of becoming obese adults [23].