SAO PAULO (Reuters) - Brazilian digital lender Nubank's adjusted net profit more than doubled in the third quarter from year earlier on a foreign-exchange neutral basis, it said on Wednesday, beating analysts' estimates with record returns.

Nubank, which is backed by Warren Buffett, reported an adjusted net profit for the quarter ended in September of $592 million, above the $559 million expected by analysts polled by LSEG.

Chief Financial Officer Guilherme Lago told Reuters that the profitability increase was driven mainly by more active customers and higher average revenue per active customer, while costs of services and clients changed little.

Nubank's annualized non-adjusted return on equity, a gauge of profitability, stood at a record 30%.

Nubank's credit portfolio increased some 47% year-on-year to $20.9 billion, while the 90-day-plus default ratio for loans in Brazil, its main business, came in at 7.2%, up from 6.1% a year earlier and 7% in the second quarter.