45,000 people attended the rally on Saturday June 8, including Gates, former Archbishop of Canterbury and chair of Christian Aid Rowan Williams, and David Beckham.
Both events called on world leaders to address the issue at the G8 summit in Ireland on June 17-18.
Gates, co-chair with wife Melinda of the Bill and Melinda Gates Foundation, which tackles international development and health problems, said even if malnutrition didn’t kill, it sabotaged mental development.
“If you don’t get enough to eat the chance you’ll die from diseases like diarrhoea and pneumonia is 10 times higher,” he told viewers of the online debate, fronted by presenter Natasha Kaplinski.
“Also, particularly in your first two years, if you don’t get enough to eat your brain is not going to develop as fully as it would otherwise.”
However, he said the generosity of rich nations could make a difference. “We’ve gone in my lifetime from where 25% of all children died below the age of five; to now it’s around six percent.”
Participants in the web chat also included IFYouTube campaigners Charlie McDonnell and his mum Lindsay Atkin and Frank Kapeta, young ambassador for charity Save The Children in Tanzania.
Click here to listen to Bill Gates' speech in Hyde Park, London.