SAO PAULO (Reuters) -Nu Holdings ( NU ), which runs Brazilian digital lender Nubank, posted on Thursday a $637 million second-quarter net profit, a 42% increase from a year earlier on a foreign exchange-neutral basis. The firm, which has nearly 123 million clients across Brazil - its main market - Colombia and Mexico, reported a net revenue for the April to June period at $3.7 billion, up 40% year-on-year. Chief Financial Officer Guilherme Lago told Reuters the profit increase was driven by operational leverage and revenue growth, but said the factors behind that growth are starting to change. "If in the last three to five years a major part of our growth came from adding new customers, in the next three to five years a major part of our growth revenue in Brazil will come from deepening the relationship with these customers," Lago said. Nubank's annualized return on equity, a gauge of profitability, came in at 28%, in line with the reported year-ago figure. The lender's total loan book rose 8% from the first quarter to $27.3 billion, as personal loans kept growing in Nubank's overall portfolio, which is largely made up of outstanding credit card debt. Meanwhile, the early default ratio stood at 4.4%, falling 0.3 percentage points from the first quarter. The over-90 day ratio reached 6.6%, up 0.1 percentage point in the same period, with Nubank saying it reflected both a rise in short-term delinquencies in the first quarter and a seasonal pattern.