2 Genes, environment and the causes of obesity


In this section we will look at the ways in which environmental and genetic factors interact to produce the diversity that is characteristic of humans and other species. We will use variation in human body weight and changes in the incidence of obesity, first discussed in Case Report 1.

2.1 Ron revisited

In Case Report 1 you met Ron. Ron is 59 years old, has a BMI of 31 and a central obesity ratio of 0.96, indicating that he is moderately obese, with the fat concentrated in his abdomen, rather than his hips. Clearly a number of factors, some environmental, others relating to Ron's age and all interacting with Ron's genetic constitution, may help to provide an explanation. Let's look at some of these factors in a little more detail. It is clear that Ron enjoys a comfortable lifestyle with little exercise and a generous diet. Three cooked meals a day, snacks and the occasional glass of beer might easily generate an energy input of more than 12 000 kJ day−1.

What is Ron's estimated average requirement (EAR), expressed in kJ day−1?

This can be calculated from Table 1. Ron's EAR is 2550 kcal × 4.2 = 10710 kJ. (4.2 is the correction factor required to transform calories to joules.

You might wonder why Ron doesn't feel permanently sated, leading to a reduction in his food intake and body weight. Studies with volunteers living in a laboratory environment in which food intake and energy output could be continuously monitored over several weeks provide a partial answer to this paradox.