07/02/2014 12:14 GMT | Updated 09/04/2014 06:59 BST

Fat or Sugar - Which Is Worse?

The recent BBC2 Horizon programme on sugar versus fat was a good idea in theory, but it turned out to be rather confusing and ultimately disappointing. The intention was to try to settle the argument as to whether fat is our real enemy (as we have been told for decades) or sugar. In case you missed it, the conclusion was that it's the combination of fat and sugar that's the problem, rather than either one individually. Personally, I would have thought this was pretty obvious to all of us. After all, I don't know many people who graze on sugar cubes or eat chunks of butter straight from the fridge, but I know a lot who find Krispy Kreme doughnuts and chocolate chip Haagen-Dazs irresistible. The presenters of the programme (both doctors apparently) implied that they didn't know this, leaving some us wondering which medical school they had graduated from (or perhaps whether they had graduated).

There is nothing new about our love of fat and sugar. We have a strong evolutionary drive to eat these foods because they are rich sources of calories which can be laid down as fat - which is simply stored energy. Our ancestors needed plenty of fat to fuel activity during periods of food scarcity and to pay for our large, expensive brains. The industrialisation of food production in the latter half of the 20th century has provided us with highly attractive, inexpensive and palatable food products full of fat and sugar and available 24-hours a day. But what is also different, is that unlike our ancestors most of us don't walk great distances to find food. Today many people don't even drive to get the food; they just click on a mouse. And it is precisely the astonishing decline in levels of physical activity that horizon failed to address, but which may be crucial in preventing obesity and its associated diseases. In fact there is an excellent example of a group of individuals who habitually eat large quantities of fat and sugar and yet who have very low rates of obesity, diabetes and heart disease.

The Old Order Amish are a Christian sect living in small rural communities in America and Canada, whose lifestyle is essentially the same as it was 300 years ago. They shun almost all forms of modern technology, including cars, electrical appliances and other modern conveniences. Labour-intensive farming is still the preferred occupation and this entails extremely high levels of physical activity.

Just how much activity was shown in a recent study of Amish farmers in Ontario, Canada. Men reported 10 hours per week of vigorous activity, 42.8 hours per week of moderate activity and 12 hours per week of walking. For women the numbers were 3.4 hours, 39.2 hours and 5.7 hours. A total of 25% of the men and 27% of the women were overweight (BMI > or = 25) but 0% of the men and just 9% of the women were obese. By contrast the rates of overweight and obesity in the general population of Ontario are 60% and 23% respectively.

Studies in the children of Old Order Amish show they are fitter, leaner and stronger than their peers in contemporary Canadian society, despite the fact that they don't have formal physical education classes in schools or any organised recreational activities. Instead they have a daily walk to school, daily chores and free time without computers or TV.

The other remarkable finding in recent studies of the Amish, concern a gene strongly associated with the development of obesity. People who carry the fat mass and obesity associated (FTO) gene are 30-60% more likely to be fat. Researchers found that while this gene contributed to overweight in the Amish community, the excess weight was only seen in individuals with the lowest activity scores - mainly women performing household chores. In those with high levels of daily physical activity, the FTO gene had absolutely no effect.

Taken together, these studies show that high levels of daily physical activity can not only protect against calorie-rich diets, but also against a strong genetic predisposition to obesity. They also confirm that since we carry the same genes as our ancestors, it is changes in lifestyle - especially the decline in physical activity - that is the real cause of the current obesity epidemic. Of course we can't all be as physically active as the Amish, but a daily walk of 30 minutes wouldn't be a bad start!