Last night, BBC aired a documentary called “The Truth About Carbs”, where medical doctor Xand van Tulleken – a self-professed lover of carbs – sets out to discover whether carbs really are a killer as claimed by many, and responsible for record levels of obesity and type 2 diabetes.

“The film investigates cutting-edge research into the possible link between carb consumption and infertility in both women and men, and looks into whether genetic changes are taking place as we pass our eating habits on to our children.

The film then sets out to examine whether it is still possible to eat carbs, but in a way that is much healthier for us. There is a little-known type of carbohydrate called resistant starch; like fibre, it helps keep bowel cancer at bay.

But a medical exam reveals that, like most people in the UK, Xand doesn’t eat enough of this carb. So how should he go about eating more carbs like fibre and resistant starch?”

Most importantly, the programme teams up with a Merseyside GP to trial a healthy-eating plan. Originally devised to help diabetic and obese patients, this plan doesn’t count calories but asks participants to be smart with their carbs – swapping ‘sugary’ carbs for more fibre.

The results after just two weeks come as a surprise to Xand and the trial doctors.

Watch: The Truth About Carbs

The Cracker Test

In one part of the documentary, Dr Tulleken discusses a test that reveals how each of our bodies uniquely processes carbohydrates: the cracker test.

How to do the test:

1. Place a cracker in your mouth and leave it there.

2. Begin to count to 30.

3. Wait for the cracker to begin tasting sweet, then check the time.

If you notice the sweetness within 0 to 14 seconds, your body processes carbs efficiently for use as energy — you can have 250g of carbs a day (roughly four slices of wholewheat bread).

If it takes 15 to 30 seconds, you can have 175g of carbs a day (around three-and-a-half slices of wholewheat bread).

More than 30 seconds, you’re not a great processor of carbs so the body is less able to use the excess as energy, and so stores it as fat. So you can have just 125g of carbs a day (around three slices of wholewheat bread).

Dr Sharon Moalem, the Canadian geneticist who created this test, says a very slow (or no) taste change shows that your body doesn’t process carbs efficiently. This leads to perilous weight gain.

The physiological mechanisms behind this test have not been nailed down completely — it’s probably genetic, reflecting levels of amylase, an enzyme in saliva, which breaks starch down into sugar for the body to use for energy.

Some people have saliva that’s loaded with amylase (up to 50 times more than others) and break down carbs more readily, hence the sugary taste.

So, with 63 per cent of UK adults now considered overweight or obese, perhaps it’s time we started being smart about the kind of carbs we consume a lot of.