An NHS watchdog has declared that fad diets such as the Atkins are useless for staying slim and blamed “coffee culture” and smartphones for undermining healthy lifestyles.

Children should not be given sweets as a treat and water should replace fizzy drinks, according to detailed guidance from the National Institute for Health and Care Excellence (Nice).

The treatments adviser has issued practical suggestions for the first time on how to avoid becoming obese. Official NHS advice will now encourage eating breakfast and having TV-free days — but warn dieters off low-carbohydrate regimes.

Everyone is at risk of getting fat and ought to change their habits to avoid the gradual weight gain that means two thirds of adults are now overweight, Nice said.

People could weigh themselves once a week and use a pedometer app to check they are walking enough, the watchdog added. A Mediterranean diet rich in vegetables, fish and whole grains should replace calorie-dense food such as cakes, sweets and cream.

Obsessive exercise and fashionable diets that aim at cutting out a single nutrient such as carbohydrates should be avoided because they do not last.

“Many of the fad diets may show initial weight loss but if you look after one year you generally find it’s exactly the same. The more extreme the diet the fewer people adhere to it,” said Professor Susan Jebb of the University of Oxford, who led the guidance.

Professor Jebb said that the detrimental side of the “coffee shop phenomena” was “people regularly drinking elaborate coffees or other drinks which have more calories than regular hot drinks, with a cake or pastry on the side. This type of eating adds calories into our diet than we just don’t need.”

Taking the stairs instead of the lift, standing up to make phone calls and limiting TV to two hours a day are all ways of living healthier, she said. A breakfast of unsweetened cereal or toast with low fat milk and fruit is the best way to start the day because it means a more balanced diet and discourages snacking.

Professor Jebb denied the guidance was nannyish: “We felt it was important to try to bring the evidence alive by giving examples. But they are just examples and not the only way to do things.

“We could just say reduce sedentary behaviour, but that’s not really helpful to people. TV-free days is just trying to be illustrative. That’s the kind of thing some families do and report as helpful.”

Parents are told to eat regular meals with their children, make sure they get enough sleep, and not to give them too much fruit juice in the belief that it is healthy. “We want to establish healthy habits from the start,” said Professor Jebb, adding that sweets should not be used as a reward for good behaviour.

“We foist calories on people with the best of intentions and maybe we should start to think about non-fat treats,” she said. “People don’t need to be encouraged to eat sweets. If we want children to have a healthy relationship with sweets, don’t establish an association with ‘what a good child you’ve been’.”

She said healthy living should not be an “extreme sport” and following even some recommendations would help.

Yesterday health chiefs announced plans for GPs to seek out the obese and put them on diet, cookery and exercise plans to prevent them getting diabetes, which costs the NHS £10 billion a year.