The nation’s eight largest health insurance companies collectively lost 836,000 members in the first six months of the year, as commercial enrollment sagged.

Those commercial losses were offset by increased enrollment in products sold to seniors and increased Medicaid business, according to Mark Farrah Associates, a Kennebunk, Maine-based provider of market data.

Meanwhile, Administrative Services Only (ASO) enrollment showed its first quarterly decline since June 2007.

The marketing company said the difficult economy continues to fuel rising medical expenses and declining membership for most plans; however, profitability is showing signs of improvement for some of the top plans in the country.

As employers have shed employees and sought other means of cuts to their budgets, health plans have suffered. Fewer employees and less of a commitment to health coverage has hurt the industry, researchers said.

For the eight largest U.S. health insurers, aggregate membership was down to 132.4 million members, from 133.2 million members at year-end 2008. For these insurers, combined Medicare Supplement and Medicare Advantage enrollment increased by 405,000 members in the same period, offsetting some of the 1.45 million in membership declines experienced in the commercial sector. Medicaid enrollment increased 211,000 for the same period, the company said.

ASO enrollment decreased 0.9% and risk enrollment fell 0.2% for these plans between the second quarter last year and second quarter this year, the research found.

Between first and second quarter 2009 ASO enrollment declined 4.6%, the first quarterly decline since June 30, 2007. Of the top plans, all but Aetna, HCSC and Health Net reported decreases in ASO covered lives.

The eight largest health plans – Aetna, CIGNA, Health Care Service Corporation, Health Net, Humana, Kaiser Permanente, UnitedHealth Group and WellPoint – cover 59% of the total insured population in the nation, marking a 2.3% increase from second-quarter last year to this year.