WASHINGTON — The number of poor people in the United States in 2009 was millions higher than previously known, with 1 in 6 Americans — many of them 65 and older — struggling in poverty due to rising medical care and other costs, according to preliminary census figures released yesterday.

Under a new revised census formula, overall poverty in 2009 stood at 15.7 percent, or 47.8 million people. That’s compared to the official 2009 rate of 14.3 percent, or 43.6 million, that was reported by the Census Bureau in September.

Across all demographic groups, Americans 65 and older sustained the largest increases in poverty under the revised formula — nearly doubling to 16.1 percent. As a whole, working-age adults 18 to 64 also saw increases in poverty, as well as whites and Hispanics. Children, blacks, and unmarried couples were less likely to be considered poor under the new measure.

Due to new adjustments for geographical variations in costs of living, people residing in the suburbs, the Northeast, and West were the regions mostly likely to have poor people — nearly 1 in 5 in the West.

At the same time, government aid programs such as tax credits and food stamps kept many people out of poverty, helping to ensure the poverty rate did not balloon even higher during the recession in 2009, President Obama’s first year in office.

The new numbers will not replace the official poverty rate but will be published alongside the traditional figure this fall as a “supplement’’ for federal agencies and state governments to determine antipoverty policies. Economists have long criticized the official poverty measure as inadequate because it only includes pretax cash income and does not account for medical, transportation, and work expenses.

“Under the new measure, we can clearly see the effects of our government policies,’’ said Kathleen Short, a Census Bureau research economist who calculated the revised poverty numbers.