Only a handful of governments have issued guidelines promoting healthy diets that can help tackle two of the most urgent challenges of today: securing good nutrition for all and protecting the environment, according to a new UN study published Thursday.

The report, entitled The Plates, Pyramids, Planet, was compiled by the UN Food and Agriculture Organization (FAO) in collaboration with the Food Climate Research Network (FCRN) at the University of Oxford.

It concluded that a plant-based diet has advantages for health and the environment.

Yet only four countries -- Brazil, Germany, Sweden and Qatar -- promote diets and food systems that are not only healthy but sustainable, the study said in a press release.

The Netherlands and the United Kingdom have since followed suit.

"Growing numbers of people now understand that diets rich in whole-grains, legumes, fruits and vegetables -- with reduced consumption of meat and smaller quantities of high-fat and high-sugar foods -- are good for our bodies," said lead author Carlos Gonzales-Fischer of FCRN.

He said that there is ample evidence showing that such diets have much lower environmental impacts than the unhealthy and unsustainable eating patterns that are increasingly prevalent today.

"So by eating well for our own personal health, we're also doing right by the planet -- in essence, it's a win-win," he added.

Anna Lartey, director of FAO's Nutrition and Food Systems Division, stressed that Sustainable Development Goal 2 makes a clear link between the needs for healthy nutrition and sustainable agriculture. "It's time that dietary guidelines reflect that relationship," she said.

More than 80 governments already issue dietary advice, and the number is rising. However, most governments have yet to issue national dietary advice, and this gap is particularly apparent in low income countries -- only five in Africa have such guidelines.

And most existing guidelines still fail to consider the environmental impacts of dietary choices.

Sweden is providing detailed advice on which plant-based foods are to be preferred, recommending for example root vegetables over salad greens.

Brazil's guidelines stand out for emphasizing the social and economic aspects of sustainability, advising people to be wary of advertising, for instance, and to avoid ultra-processed foods that are not only bad for health but are seen to undermine traditional food cultures.

The study said that, to have a real effect on food consumption, dietary guidelines need to have clear links to food policies that are actually implemented, such as school and hospital meal standards and advertising and industry regulations.

The report's overarching suggestion is that countries that already have dietary guidelines should begin to consider a process of incorporating sustainability into them. The countries that do not already have them are in a unique position to develop integrated guidelines from the outset.