What if humans cast aside processed foods and saturated fats in favour of the sort of diet our ape-like ancestors once ate? Nine volunteers gave it a go… and were glad they did so.

Being locked in the zoo and offered bananas to eat is the kind of extreme diet scenario to wake some of us screaming in the night. But that was how a group of volunteers opted to try to cut their blood pressure and cholesterol levels.

They set up home in a tented enclosure at Paignton Zoo, Devon, next to the ape house, in an experiment filmed for TV. The idea, says Jill Fullerton-Smith, who helped organise the trial, was that modern diets, often dominated by processed foods and saturated fats, cause costly health problems.

For example, nearly half Britain’s 117,000 annual deaths from coronary heart disease are linked to high cholesterol, according to the British Heart Foundation. And while the government urges everyone to eat five portions of fruit and veg a day, obesity is still rising.

So could an experiment on ordinary people’s lives deliver the healthy eating message?

Nine volunteers, aged 36 to 49, took on the 12-day Evo Diet, consuming up to five kilos of raw fruit and veg a day.

EVO DIET: WHAT THEY ATE

-5kgs or 2,300 calories of fruit, vegetables, nuts and honey

-On a 3-day rotation, typically: Broccoli, carrots, radishes Cabbage, tomatoes, watercress Strawberries, apricots, bananas Mangoes, melons, figs, plums Satsumas, hazelnuts

The regime was devised by nutritionist and registered dietician Lynne Garton and King’s College Hospital. It was based on research showing such a diet could have health benefits for cholesterol levels and blood pressure, because it is made up of the types of foods our bodies evolved to eat over thousands of years.

Ms Garton looked for inspiration to the plant-based diet of our closest relatives, the apes, and devised a three-day rotating menu of fruit, vegetables, nuts and honey. The prescribed menu was:

• safe to eat raw; • met adult human daily nutritional requirements; and • provided 2,300 calories – between the 2,000 recommended for women and 2,500 for men,

Volunteers could also drink water. In the second week, standard portions of cooked oily fish were introduced – a nod to a more hunter-gatherer lifestyle.

Among the volunteers was Jon Thornton, 36, a driving instructor from Sheffield, who had never eaten vegetables, from childhood upwards.

Weighing in at almost 19-stone, his typical diet read like the children’s book, Mr Strong. Breakfast was four slices of toast; at 10am a bacon sausage and egg sarnie followed; fish and chips for tea and a Chinese take-away before bed.

That was before his wife signed up Mr Thornton for the experiment. Over 12 days he lost 5.7kg (12.5lbs), and reduced his cholesterol by 20%. His blood pressure also fell.

Despite nearly backing out at the start – the first day’s food arrived in a cool-box, was raw and he was distinctly uncomfortable with the idea of broccoli – he was converted to eating vast portions of fresh fruit and veg.

“I didn’t feel any loss of energy, I didn’t feel ill at all,” he says. “It’s not a diet you’d recommend as a diet itself, but it worked to bring my cholesterol and blood pressure down.”

Harmony in camp

With so much food bulk and plenty of calories the subjects did not go hungry – indeed most failed to finish their daily ration. And once they were over the withdrawal from caffeinated drinks and some foods, says Ms Garton, they enjoyed good energy levels and mood.

So the “moments of unhappiness and grumpiness” that the TV crew was primed to capture failed to happen. The proved to be a motivated group, although the one odorous side-effect from all that roughage couldn’t be ignored.

Overall, the cholesterol levels dropped 23%, an amount usually achieved only through anti-cholesterol drugs statins.

The group’s average blood pressure fell from a level of 140/83 – almost hypertensive – to 122/76. Though it was not intended to be a weight loss diet, they dropped 4.4kg (9.7lbs), on average.

The regime provided an education for all, and a permanent change for some.

“The main lesson that they took away was to eat more fruit and veg,” says Ms Garton. They also cut salt intake from a group average of 12g a day, to 1g (against a guideline maximum of 6g) and reduced saturated fat – which makes cholesterol – from 13% to 5% of calories (recommended, 11%).

At the same time, they increased the soluble fibre which binds cholesterol in the gut, so that it is expelled, and increased the intake of plant sterols – which help to lower cholesterol.

For Jon, life has changed since he was “released” from the zoo. He has gained a little weight but now says he only eats when hungry and knows good food can help health and longevity. He can play football because his knees no longer hurt under the extra weight and he goes cycling.

He even managed to hold out at the most tempting time of year. “For the first time in 36 years this year I had vegetables with my Christmas dinner,” he says. “Usually, I say no to them and have a few extra roast potatoes instead.”