‘Globesity’ – A global, growing problem.

The average person’s body mass index (BMI) has steadily been on the rise since the beginning of the 20^th century, as a result of improvements to health through nutrition in diet, fitness, and sanitation. But this doesn’t explain the drastic increase in recent history, such that worldwide obesity has nearly tripled from 1975 – 2016, with now over 1.9 billion adults (18 years and older) overweight with 650 million of these people obese ¹. The number of obese people is continually increasing. Indeed it can be seen live where this website predicts the current number of obese people in the world using statistical analysis on the currently available data, which has been cited in over 800 professional journals and has been recognised as an “outstanding reference website” ².

The development of the obesity pandemic from 1975 to 2014 can be seen in this GIF constructed from data collected in a study published in The Lancet that involved over 19.2 million participants ^{3 4}.

The main conclusion of this study was that globally, we have transitioned from a world of greater underweight prevalence to a world in which more people are obese than underweight. Whilst this study used many participants in its sample, noticeably absent were children, adolescents and a sufficient number of people over the age of 70 (only 42% of the sources the study used had data for these older people). This makes the study’s findings less generalisable to the public as a whole, but rather more representative of the middle-aged population. The study has, however, justified the absence of younger people from the sample, citing their rapid growth and resultant different BMI cut-offs for diagnosis of obesity.

Mechanism – Thrifty genes and fast-food teens.

Obesity, as with other relatively prevalent diseases such as diabetes mellitus, has a genetic basis. Indeed, a study of monozygotic twins concluded that heritability (h(2)) estimates of BMI between 0.5 and 0.7 could be considered reasonable, meaning that statistics suggest that the variation in BMI of the population is 50-70% due to genetic variation between individuals, rather than environment or random chance ⁵. This 1990 report, however, used only 53 pairs, and stands in contrast to the more recent 2008 study of 5278 pairs of Finnish twins that revealed a h(2) value of 80% for males and 82% for females ⁶.

While the exact genetic influence of BMI and diabetes mellitus may still be debated, its existence is widely accepted. This led geneticist James Neel to the thrifty gene hypothesis. He discussed in his 1962 report why diseases such as diabetes and obesity with a strong genetic component but with negative effects had been favoured by natural selection. He explored how a ‘thrifty’ genotype (meaning a specific set of genes that predispose to such diseases) may have been advantageous up to 2 million years ago when food was scarce and necessity to retain fat was high, but the relatively recent abundance of food leaves those with the thrifty genotype prone to obesity ⁷.

When this is combined with the widespread availability of fast food – which is higher in fat and sugar than freshly made meals – people in the Western world, particularly teenagers (who are more prone to eating fast foods), are more at risk of obesity. A study of 4827 teenagers, who attended a focus clinic aged 13 or competed the questionnaire from which the data was taken aged 13, 48.5% ate fast food, and 13.1% were obese ⁸.

Diagnosis – Time to say goodbye to BMI?

The NHS diagnoses obesity as having a BMI exceeding 30. BMI is a formula that factors a person’s height and weight to produce a score that can be compared against a reference to determine if that person is underweight, healthy weight, overweight, or obese. BMI is incapable of differentiating between bone, muscle and excess fat. This means that muscular people can fall into an overweight or obese category despite having low fat mass.

A study of 13,601 people used bioelectrical impedance analysis (a method of estimating body composition with a focus on body fat) to correlate % body fat (BF%) with BMI to evaluate the usefulness of BMI ⁹. BF% can be used to diagnose obesity, with the World Health Organisation setting >25% and >35% in men and women respectively as the reference for obesity. They found that obesity defined by BMI was present in 19% and 25% of men and women respectively, whereas when using BF% to define obesity, the results were 44% of men and 52% of women ⁹.

Figure 1. Age and race-adjusted correlation between body mass index (BMI) and body fat percent (BF%). Age and race-adjusted correlation between BF% and BMI for men and women ⁹.

The data shows that there was a significant positive correlation between BMI and BF%, suggesting that the two different measures work similarly and will often draw the same conclusion when determining if a person is obese. However, BMI is incapable of discriminating between fat mass and lean mass, so its usefulness in diagnosing obesity may be limited.

Exercise – A solution to the problem.

It is widely known that exercise is one of the most effective methods for weight management, though there are other methods to control weight:

• Changes to diet: drinking more water, substituting processed for whole foods, and calorie restriction.
• Psychological changes: using a smaller plate, eating more slowly, and brushing teeth before a meal discourage excessive eating.
• Other changes: surgery, supplements and getting the appropriate amount of sleep.

The World Health Organisation recognises exercise as one of the most effective methods of combatting weight gain and reducing risk of obesity. They suggest 150 minutes of aerobic training per week, more information on which can be found on our endurance training page. It is important to note that resistance training and high-intensity interval training also improve health and can reduce fat mass, though due to the aforementioned limitations of BMI may actually increase the likelihood of an obese diagnosis, as a result of the muscle mass gain.

In the average time it will take to read this page, 298 will have become obese ². Read on to discover how the various types of exercise can affect weight and health.

(1010 words). Last updated: Nov, 2018.

References.

1.	World Health Organisation, “Obesity and overweight,” 16 February 2018. [Online]. Available: http://www.who.int/en/news-room/fact-sheets/detail/obesity-and-overweight. [Accessed 12 November 2018].
2.	Worldometers.info, “Obesity statistics: worldometers,” [Online]. Available: http://www.worldometers.info/obesity/. [Accessed 12 November 2018].
3.	M. Galka, “Watch how the world became obese,” 18 July 2016. [Online]. Available: http://metrocosm.com/map-world-obesity/. [Accessed 12 November 2018].
4.	NCD Risk Factor Collaboration, “Trends in adult body-mass index in 200 countries from 1975 to 2014: a pooled analysis of 1698 population-based measurement studies with 19.2 million participants,” 12 May 2016.
5.	D. Allison, “The heritability of body mass index among an international sample of monozygotic twins reared apart,” 1996.
6.	J. Hjelmborg, “Genetic influences on growth traits of BMI: a longitudinal study of adult twins,” 2008.
7.	J. Neel, “Diabetes Mellitus: A “Thrifty” Genotype Rendered Detrimental by “Progress”?”.
8.	K. Lorna, “Fast Food and Obesity: A Spatial Analysis in a Large United Kingdom Population of Children Aged 13–15,” 2012.
9.	A. Romero-Correl, “Accuracy of body mass index in diagnosing obesity in the adult general population,” 2008.