What decides our physical form?
Our bodies come in all shapes and sizes. We can thank forces as diverse as our evolutionary history, our genes, our upbringing and our environment for our body shape. Increasingly, in modern society these factors are combining to drive up our weight.
Our ‘natural’ size is governed by several factors, including our genetic make-up. We are beginning to find out about some of these genes.
A crucial phase in our development is our time in the womb. Our mother’s diet and health will impact on our birth weight. (See our Building a baby article and video for more.) With food relatively plentiful (in high-income countries), small babies tend to go through ‘catch-up’ growth in infancy. Although this may have benefits when young, it appears to store up problems in later life – increasing the risk of cardiovascular and other metabolic diseases – as small babies may be born ‘pre-programmed’ for a life of low nutrition.
Growth of the fetus also depends on an interplay between the genes inherited from the mother and those from the father – the paternal genes acting to increase fetus size, the mother’s trying to limit fetal growth (see our issue on ‘Sex and Gender’ for more on this).
Taller, bigger, fatter?
In most developed countries people have been getting taller. For the past couple of hundred years improvements in nutrition and treatment of childhood diseases have caused average height to increase significantly. In the 1870s the average British man was around 5 feet 5 inches (167 cm) tall, whereas by the 1970s he was 5 feet 10 inches (177 cm). (There is little historical data for women.)
Now, rather than getting taller, we just seem to be getting fatter. Arguably, limited nutrition restricted our potential growth in the past. Once that potential has been reached, additional nutrition doesn’t make us taller; it is stored as fat.
Fat is deposited in various places around the body, and in slightly different ways in men and women.
Overweight people are sometimes divided according to whether they are ‘apple-shaped’ or ‘pear-shaped’, which depends on where fat is predominantly deposited in their bodies – around the abdomen or the buttocks and thighs.
Some studies have suggested that being an apple (rather than a pear) shape puts you at a greater risk of having a heart attack. However, more recent studies have raised questions about this link. It may simply be that extra weight is bad for your health, wherever you carry it. As these contradictory studies show, the links between weight and health are not always straightforward, and measures of obesity, such as body mass index, need to be interpreted carefully.