Mumbai, India - At 18 months old, Zaina* was still unable to walk. Her worried mother suspected the problem was caused by the constant bouts of diarrhoea her daughter had when she was younger.

"She couldn't keep any food down and even now her arms and legs are very weak," said Badia Shah*. Eventually, Zaina needed emergency high-calorie food packs to treat what doctors called severe acute malnutrition, a potentially life-threatening condition caused by an extreme lack of nutrients.

Zaina will grow up to become part of the world's largest workforce - India's - which appears set to overtake China's by 2030. But with close to 40 percent of Indian children stunted because of a lack of adequate nutrition, the country risks missing out on what experts call a "demographic dividend" - economic growth boosted by workers who can generate ever-larger amounts of goods and services.

Shah and her family live in Shivaji Nagar, one of the most deprived slums in India's financial capital, Mumbai. Here, more than half of the children are stunted - a proxy indicator for cognitive and physical underdevelopment. Life expectancy is just 39 years, lower than the world's worst-performing country, Sierra Leone, where it is 52 years, according to the World Bank.

The residents of Shivaji Nagar complain that authorities have long neglected them. Their population has grown but vital infrastructure and employment opportunities remain absent.

The only maternal health facility in the area was demolished in 2014 and is yet to be replaced, according to local NGOs. A lack of piped water means families spend on average 12 percent of their income on supplies from private water tankers, funds that could go towards more nutritious, but expensive, types of food.