Image copyright BBC/Lion Television/Joe Taylor Image caption Not all carbs are created equal...

Low-carb diets have been around for a while. But did you know the colour of the carbs you eat also matters - and as Dr Xand van Tulleken explains, it's the beige ones you really have to watch out for.

Dr Faisal Maassarani is on a mission. The GP, from Kirkby, on Merseyside, wants to get his patients to eat fewer carbs and thereby improve their health.

But there are a huge number of obstacles for Dr Maassarani. He works in one of the poorest regions of the country, with high unemployment, high obesity rates and low levels of education.

The plan seemed simple to the point of naivety. He assembled a group of seven overweight and obese patients, with health problems like type 2 diabetes, high cholesterol and high blood pressure and carried out a few basic tests.

He then enlisted chefs from a local cookery school to prepare a feast for them, to show that healthy food could be tasty, affordable and filling.

Dr Maassarani also read them the riot act on the possible consequences of obesity and diabetes (gangrene, strokes, heart attacks, ulcers and more).

Then he was going to leave them to it for two weeks (really no time at all to address chronic health problems) and allow them to alter the kinds and amounts of carbs in their diets in whatever way they decided, and then repeat the tests.

It seemed to me he was on a hiding to nothing.

Media playback is unsupported on your device Media caption The cracker test could show how well you digest carbs

How we digest carbs

As we discover in The Truth About Carbs, carbohydrates are one of the ways our bodies get energy from food.

There are three types - starch, sugar and fibre.

Starch is what we usually understand carbs to mean - foods like bread, pasta, potatoes and rice - but these "beige" varieties aren't good for you.

Neither are "white" carbs - found in sugary foods such as fizzy drinks, sweets and processed and refined foods including cakes and biscuits.

Most of the starch and sugar in these beige and white carbs are broken down into glucose for energy, and if you eat too much, the glucose is stored as fat.

Diet tips

Reduce:

Flour, rice, potatoes, pasta, breakfast cereals and other processed grains

White or highly processed brown bread

Replace with:

Cauliflower rice, celeriac, sweet potatoes

Rye or pumpernickel bread

Include:

Eggs, meat, fish, nuts and legumes

And half of each dish should be green or brightly coloured: fresh vegetables, salad greens, tomatoes and aubergines

Take the BBC Food test: How carb-clever are you?

But there is another type of carb - dietary fibre, found in fruit and vegetables - what we might call "green carbs". It keeps you full, slows stomach emptying, and is usually the part of the plant that supplies you with vitamins and minerals.

It's good for your teeth and gums and good for your guts, keeping everything moving and feeding your gut bacteria.

And then there is resistant starch - found in high-fibre foods such as lentils, beans and unprocessed whole grains - is also hard to digest in a good way: it gets right down into your lower digestive tract (your colon) where its main job is not to feed you but rather to feed your gut bacteria.

Healthy gut bacteria are linked to a wide range of benefits, both physical and mental.

And one other tip - reheating can also turn bad carbs into good - if you reheat starches like pasta or toasted bread from the freezer, the molecules reconfigure themselves and become more resistant, allowing them to travel further in your gut and feed your microbiome.

Diabetes 'barely detectable'

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Two weeks on in Kirby, it was amazing to see that all seven patients had altered their diets as requested and opted for more green carbs.

They weren't feeling hungry, or finding it inconvenient, and they had started a chat group to help each other with recipes.

It was their measurements that astounded me: the diabetics had improved their blood sugar so dramatically that one of them was in part-remission.

In two weeks his type 2 diabetes, which he had had for 17 years, was barely detectable.

And six out of the seven lost half a stone or more.

The Truth About Carbs, with Dr Xand van Tulleken, is on BBC One at 20:00 on Wednesday 6 June.