Before finding out the exact causes, we need to know few things regarding obesity. In a simple way, obesity means an excess of body fat. It results from the imbalance between the amount of energy consumed in the diet and the amount of energy expended via physical activities. When the consumed energy exceeds the expended one, the excess energy is deposited as fat in our body. And we become obese. So main factors are (1) energy intake and (2) energy expense or energy utilization. We have to keep a negative balance of energy to protect us from getting obese. If we do exercise all day and spend 1000 Kcal energy but consume foods with very high glycemic index totalling 1200 Kcal at the same day, we will still get fatty!!

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From ancient time, the environmental factor and our body physiology helped us to keep the negative energy balance. If we look at the lifestyle of our ancestor we will see that there were a scarcity of foods and they had to struggle/fight to collect their foods. Unavailability of food (less energy intake) and extreme physical activity (more energy expense) for searching the food helped to maintain a fat-free body.

But over time, due to the advancement of science, the whole scenario is now changed. People are gaining fat in their body in a consistent manner. Though there is a huge portion of the world population are still not getting proper food on daily basis, in 2006, the number of obese and overweight people in the world overtook the number of malnourished and underweight people. It is not like that only few obese people are increasing, rather all the people are getting the excess weight in a constant way. While obese people are becoming more obese, the slim people are becoming less slim. In the UK, there is a one-kilogram increase in bodyweight per adult per year!!! Why is this happening? The experts explain it introducing a nice term for the human being. They consider humans as an "obesogenic organism" who are, in the first time of human history, in an optimum obesogenic environment.

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Organism? Really? We know some organism such as bacteria, virus or parasite but humans are in the same category of those organisms!! Let's have a quick look at such organisms and their behaviours. "Aerobic organism/bacteria" are those which grows only in aerobic condition(where there is the presence of oxygen). "Anaerobic organism", on the other hand, grows in oxygen-deprived condition. When a microbiologist wants to grow an organism in an artificial culture medium, he/she has to provide a growth-favourable environment, nutrients etc specific for that organism. In a nutshell, for the rapid growth of an organism, favourable environment is the prerequisite.

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So, why human, an obesogenic organism, is becoming more obese day by day? What favourable environments this organism (we) got?

The availability of cheap and heavily marketed energy-rich foods

The increase in labour-saving device,e.g. lift, remote control etc

The increase in passive transport, e.g cars as oppose to walking, cycling etc

Those above-mentioned circumstances encourage human to eat more and exercise less.

The end result: A world filled with obese and overweight people!!

[Reference: Davidsons Principles and Practice of Medicine 22ed- page no: 101]

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