In trendy, hipster London or New York, it's all about juicing, vegan diets and snacking on kale crisps. Thousands of miles away, in Nairobi or Bogota, the middle classes are more likely to reach for roasted goat or a juicy steak. 

Later this month, world leaders are set to endorse a U.N. goal to eliminate hunger by 2030, but they will have to convince their citizens to adopt new eating habits first, experts say.

Diets must feature less red meat, which consumes 11 times more water and results in five times more climate-warming emissions than chicken or pork, according to a 2014 study.

The shift, like the U.N. Sustainable Development Goals (SDGs) themselves, must apply to both wealthy and developing nations, where consumption of ecologically unfriendly foods is growing fastest.

"Sustainable and healthy diets will require a move towards a mostly plant-based diet," said Colin Khoury, a biologist at the Colombia-based International Centre for Tropical Agriculture.

Other key changes needed are cutting food waste and combating poor nutrition, he added.

There are some signs the public is starting to take such advice on board. They include the release of an "EatBy" app that reminds consumers to use up food in the fridge, and a new social network to help people adopt a "climatarian" diet that shuns meat from gassy grazing animals, such as beef and lamb.

More than 1 million people have also signed an online petition calling on European ministers to pass laws and launch national action plans aimed at meeting a target in the SDGs to halve global food waste per capita by 2030.

ZERO HUNGER POSSIBLE

Achieving the SDGs means the international community will need to find enough food over the next 15 years for the 795 million people who go to bed hungry every night.

"I don't think it's all that ambitious to eliminate hunger," said Jomo Sundaram, assistant director-general of the U.N.'s Food and Agriculture Organisation (FAO).

That is because incomes are rising in much of the world, transport to move food is improving, and new technologies are keeping yields of many key crops on an upward trend, he said.

The previous Millennium Development Goals (MDGs), adopted in 2000, aimed to halve the proportion of hungry people worldwide, a target that was largely achieved.

U.N. officials believe that success can now be extended to put an end to hunger, which is judged according to the number of calories people consume - a system some experts say is too narrow.

Despite a rapidly rising world population, there are 216 million fewer hungry people on earth today than in 1990, the FAO reported in May.  

But with the global population expected to climb to 8.5 billion by 2030, from 7.3 billion now, and climate change predicted to ravage yields in some nations, ending hunger will require tough choices in the field and on the dinner table.

"It's not going to be easy, but if you look at the arithmetic, it is achievable," Sundaram said.

WASTED OPPORTUNITIES

The world already produces enough food for everyone, but around one third of it is discarded or spoils in transport or storage before reaching consumers, according to the FAO.

In rich countries, individuals and grocery stores are responsible for most of the waste when they throw away imperfect vegetables or products they think are no longer safe to eat.

Developing countries lose roughly a third of their edibles due to poor refrigeration systems and infrastructure bottlenecks, which prevent food from reaching the market.

"Today we could easily feed everyone – it's a distribution issue," said Michael Obersteiner of the International Institute for Applied Systems Analysis, an Austria-based thinktank.

Meeting the hunger goal by 2030 may be possible if funding were available to cut waste along the supply chain, and yields continued to climb, he said.

But by 2050, climate and population pressures - alongside an expanding global middle-class with an appetite for meat - will make it harder to keep up the momentum on zero hunger.

"Diets will have to change," Obersteiner said.

CHANGING CLIMATE, SHIFTING DIETS

Today half the world's agricultural land is used for livestock farming, he said, which is far less efficient for feeding people – and worse for the environment – than producing grain, fruit and vegetables for direct human consumption.

And as middle-income earners in developing nations rapidly boost their meat consumption, pressure is growing on farmland, forests and water supplies, Obersteiner said.

Switching from eating meat four times a week, as recommended by the UK-based Food Climate Research Network in 2008, to just once would reduce commodity prices, as less grain would go to feed animals, making food cheaper for the urban poor, he said.

It would also curb greenhouse emissions from the livestock sector, which account for roughly 14 percent of the global total, more than direct emissions from transport, according to a Chatham House paper published in December.    

But with around 1.5 degrees Celsius of global temperature rise already locked in, some regions will have to change what they grow as the climate warms, bringing more extreme weather.

"A lot of people in south and east Africa will have to move away from maize, which is the main staple at the moment," said Luigi Guarino, senior scientist with the Global Crop Diversity Trust, a plant research organization.

Lower yields for a key food source in a region where one in four still do not get enough to eat could spell disaster.

But farmers should be able to maintain or even increase production in the face of climate change if they switch to sorghum, millet and traditional vegetables like African nightshade or spider plant, Guarino said.

In addition, new "climate-smart" varieties of maize and other staple crops, bred to withstand hotter, drier weather, will be crucial for meeting the SDGs, he added.

Some scientists have also been developing food crops with extra micro-nutrients - such as orange sweet potatoes containing high levels of vitamin A - to tackle malnutrition.

Large gene banks, used to breed crops containing the best traits adapted to particular environments, together with public education to shift diets to new and more diverse foods suited to a warmer world, will be crucial, the scientist noted.

"There is no silver bullet to reaching the goal (of eliminating hunger)," Guarino said. "But even if we get 80 percent there, it's well worth it."