You can drink as many green smoothies as you like, but no immune-boosting magic potion is going to protect the average junk-food junkie from catching a cold or the flu.

Especially if the person has low vitamin D, is sleep deprived and fails to wash their hands regularly, says Dr Rosemary Stanton, a speaker at a discussion about the ability of superfoods to fight flu.

One of the latest fads is the green smoothie breakfast, she says.

“I have no problem with people having a smoothie made from a whole range of green vegetables and some yoghurt for breakfast,” she told the Australian Science Media Centre discussion.

But she does have a problem if that means children don’t have any vegetables for dinner with their fatty and salty instant noodles.

The best way to boost the immune system is to eat a good general diet with plenty of vegetables and fruit, legumes, nuts, seeds, seafood and wholegrains, she said.

The other essential ingredients are vitamin D from a sensible amount of winter sun as well as enough sleep and exercise.

“We also need to wash our hands before eating and after going to the toilet.”

Fruit and vegetables should preferably be in season.

Citrus such as oranges, mandarins, lemons and grapefruit are in season in Australia in winter. So are brassica vegetables such as broccoli, kale and cauliflower.

Rolled oats and tea and coffee are also beneficial, says Dr Stanton, a visiting fellow at the School of Medical Sciences at the University of New South Wales.

Muti-vitamins probably won’t do any harm.

But they don’t have any particular benefit, “except to the person selling them”, she said.

Many superfood claims are based on myths, said Deakin University Associate Professor Paul Lewandowski.

One example is the idea that chocolate is healthy.

“Dark chocolate is not going to harm you if eaten in moderation, but there is no hard evidence that it will lead to long-term health benefits or boost your immune system.”

Although wine in moderation has benefits, these do not include boosting the immune system.

“It’s unlikely that it will protect people from a specific disease in the long term,” he said.