Representatives from 190 countries were expected to attend the Second International Conference on Nutrition (CIN2), organised by the UN's food agency (Fao) and World Health Organization (WHO) and boasting appearances from leaders including Pope Francis.

"We are registering unacceptably high and persistent levels of malnutrition in the world," Leslie Amoroso, a Fao nutrition expert, told journalists ahead of the conference, which will draw together over 100 health and agriculture ministers.

"The human costs of malnutrition are very high: 805 million people are undernourished and 161 million children have delayed growth," she said, adding that it was "a global problem which demands coordinated action from numerous sectors."

A lot of progress has been made already, with the number of undernourished people in the world dropping by over half in the past two decades, from one billion people in 1992 when the first conference (CIN1) was held, to 805 million in 2014.

But malnutrition is not just about hunger: two billion people suffer from deficiencies in nutrients such as vitamin A, iron and zinc – a condition known as "hidden hunger" by experts – while 42 million children and 500 million adults are overweight or obese.

As poor nutrition "acts like a brake on development," investing in healthy food is proven to "improve productivity and economic growth, reduce health insurance costs and foster education and intellectual capacity," Amoroso said.

Royalty, philanthropists, the Pope

Star guests at the conference will include Queen Letizia of Spain, King of Lesotho Letsie III, philanthropist Melinda Gates and economist Jeffrey Sachs, as well as Pope Francis, a fervent campaigner against hunger who is expected to give a speech on Thursday.

Delegates will adopt a "Rome Declaration on Nutrition" and "Framework for Action", which Fao said were based on a consensus reached by over 200 national governments after consultations with civil society organizations and the private sector.

The declaration focuses not only on access to healthy food but also the growing problem of inactive over-eaters, finding that "dietary risk factors, together with inadequate physical activity, account for almost 10 percent of the global burden of disease and disability."

It stresses that "food should not be used as an instrument for political or economic pressure," and that food price volatility can have a knock-on effect on food security and nutrition and therefore "needs to be better monitored and addressed for the challenges it poses."

The action framework presents 60 recommendations, including developing and implementing national plans and policies to better nutrition, as well as upping related investments.

It stresses the need for universal health coverage, because "for health systems to be able to deliver improvements in nutrition, it is essential that there is access to health services for all, including the most marginalized and most vulnerable."

It also urges governments to ensure universal access to safe drinking water and protect children from infections, such as diarrhoea, malaria and intestinal worms.

The framework said it backed commitments made by the WHO to reduce deaths from NCDs – diet related non-communicable diseases – by 25 percent by 2025, as well as reducing salt intake by 30 percent and halting the increase in obesity prevalence in adolescents and adults.