Malnutrition is responsible for 45% of the global deaths of children under the age of five, research published in the Lancet medical journal suggests.
Poor nutrition leads to the deaths of about 3.1 million under-fives annually, it says.
An international team reviewed different causes of malnutrition in pregnancy and childhood.
They say the first 1,000 days of life - from conception to two years - have lasting consequences for health.
Malnutrition - which includes being overweight or obese as well as under-nourished - also has an economic impact.
According to a recent United Nations report, malnutrition is estimated to cost the world $3.5tn (£2.3tn) - or $500 for every person - in healthcare and lost productivity.
A team led by Prof Robert Black, of Johns Hopkins Bloomberg School of Public Health, Baltimore, US, reviewed evidence on maternal and child under-nutrition and obesity in low-income and middle-income countries since 2008.
The team also assessed national and international progress on nutrition programmes.
Prof Black and colleagues say while some progress has been made in recent years, they estimate that more than 165 million children were affected by stunting (low height for age) and 50 million by wasting (low weight for height) in 2011.
An estimated 900,000 lives could be saved in 34 countries if 10 proven nutritional interventions were scaled-up to 90% of the world, they say.
"The nutritional consequences of the months during pregnancy and the conditions during the first two years of life have very important consequences for mortality and for adult chronic disease," Prof Black told BBC News.
"The early nutritional deficit results in developmental consequences for the individual and that has implications for their ability to succeed in school and ultimately in society to have the most productive jobs."
The researchers warn that countries will not break out of poverty unless nutrition becomes a global priority.
Dr Richard Horton, editor-in-chief of the Lancet, said: "If maternal and child nutrition can be optimised, the benefits will accrue and extend over generations, which is why we must work together now to seize this opportunity."
Experts working in development are gathering in London this weekend for a summit on nutrition hosted by the UK and Brazilian governments.
This will be followed by the annual summit of leaders from the G8 countries.
The Enough Food for Everyone IF campaign is calling for $1bn per year in additional aid money to be spent on malnutrition by 2015.
Enough Food for Everyone IF spokesperson Anita Tiessen said:
"These new figures confirm our worst fears - that hundreds of thousands more children are dying from malnutrition than we previously thought.
"We have made incredible progress in tackling child deaths around the world, but malnutrition remains the Achilles' heel in our efforts to prevent millions of needless deaths each year. We must urgently prioritise tackling hunger if we are to continue the progress we have made."