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One of my favourite fitness mantras is: “Obesity doesn’t run in the family. The problem is that nobody runs in the family." A March study published online in the journal Nature Genetics proves this adage wrong. You can, in fact, possibly blame your genes for obesity.

The study by Mario Falchi, Philippe Froguel and colleagues at the Imperial College London, UK, in collaboration with other international institutions, showed the relationship between body weight and a gene called AMY1. This gene is responsible for salivary amylase, an enzyme in our saliva which comes in contact with food when it enters the mouth. It is the one that begins the process of starch digestion, which then continues in the gut.

Starch is broken down into sugar and then digested. Foods with high starch content include grains (wheat, rice, barley, oats), potatoes, corn and beans. Grain is the basic ingredient in bread, cereal, pasta, crackers, biscuits, cookies, cakes, etc.

This study looked at the levels of AMY1 present in the DNA of thousands of randomly picked people in the UK, France, Sweden and Singapore. People with low levels of the gene were at greater risk of obesity than people with high levels of AMY1.

There is a consensus in the scientific world that cereals and grains only became staple human diet in the last 10,000 years. On the other hand, we have been eating meat for as long as Homo sapiens have existed, i.e., more than 200,000 years. So it’s not surprising that the human intestine bears a greater resemblance to that of a carnivorous lion than that of a vegetarian chimpanzee.

Even though the AMY1 finding is recent, Timothy Noakes highlighted in his book, Challenging Beliefs: Memoirs of a Career, that there are two types of humans—those who can metabolize carbohydrates normally and who are, therefore, carbohydrate-tolerant (CT), and those who lack this capacity and are, therefore, carbohydrate-resistant (CR). If body mass index (weight in kilograms divided by height in centimetres squared) is more than 25, then there is a high probability of a person being CR. The extent of CR can be determined by having your fasting blood glucose, insulin, glucose tolerance test and HbA1c concen-trations measured.

In prehistoric times, humans had low levels of AMY1 and were CR. Having fewer AMY1 genes or being CR was a genetic advantage in prehistoric times, when humans were living on a diet which was high in protein and fat content, but low in carbohydrate content. When they did find carbohydrate-rich foods like honey and fruits, they were able to store excess energy derived from these carbohydrates as fat.

As the human diet became heavier in starch, the levels of the AMY1 gene increased, and led to people becoming CT.

So if someone today has low levels of AMY1, what was once an advantage turns into a disadvantage and exposes him/her to a higher risk of putting on weight. This might only be a theory, but it’s a plausible one.

Among CR people, besides low levels of AMY1, there is a perpetual state of elevated blood glucose concentrations when they eat a carbohydrate-heavy diet. To check this, there is excessive secretion of insulin. But insulin is unable to drive glucose in the bloodstream into the liver and muscle cells. Instead, it moves that glucose into fat cells. It’s important to clarify that these “fat" cells have nothing to do with the “fat" in diet. Persistently high insulin levels further prevent the release of stored fat from fat cells. It might surprise most that increased carbohydrate intake also changes blood fat (cholesterol and triglyceride) concentration to one that favours the development of heart diseases.

In people with low levels of AMY1, or CR, consumption of food items made from flour (roti, bread, pasta, biscuits) raises the chances of obesity. Consume these items in small amounts.

Though I am an advocate of exercise and increased physical activity, I realize just working out is not enough to prevent obesity in CR-type people who eat a high-carbohydrate diet.

So, if high carbohydrate levels are the main trigger of this domino effect in CR people, what is the solution? A diet low in carbohydrate content.

When CR people eat a diet low in carbohydrate but higher in fat and protein content, it reduces their chances of gaining weight and improves their chances of staving off diabetes.

Not all starch is bad. The starch that is digested slowly is much better. Beans, lentils, brown rice, barley and amaranth work well.

The point is not to eat less, but eat smart and keep moving.

Rajat Chauhan is an ultra marathon runner and a doctor specializing in sports and exercise medicine and musculoskeletal medicine, and founder of Back 2 Fitness. He is also associate editor, British Journal of Sports Medicine.

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