MEXICO CITY, Oct 23 (Reuters) - Mexican conglomerate Alfa swung to a loss in the third quarter, it said on Monday, dragged down by the poor performance of its petrochemical subsidiary Alpek, though slightly offset by higher volumes at food unit Sigma.

The Monterrey-based company reported a net loss of 802.88 million pesos ($46 million), compared to a net profit of 2.87 billion pesos in the year-ago period.

Total revenue for the July-to-September period of nearly 72 billion pesos was down 27% from a year earlier.

Subsidiary Alpek took a hit largely due to surging Asian polyester imports to the Americas that caused the unit's total volumes to sink 13% year-over-year, Alfa said.

"Given low visibility on when Asian imports will decline, we struggle to see when the turnaround of the (polyester) cycle will arrive," Santander analysts wrote in a research note.

Alpek's plastics and chemicals volumes also fell by double digits.

The petrochemical unit has struggled in recent quarters, causing Alfa to close a textile plant in Mexico and pause construction on an integrated PTA-PET plant in the U.S. during the quarter.

Given Alpek's weak third quarter performance, previously estimated core earnings for the year may now be on the optimistic end, Santander analysts added.

Sigma, however, saw record revenues for the quarter driven by strong sales in multiple markets as well as a "favorable" trend as it looks to reach profitability in Europe, where it saw a $120 million asset impairment from its exit from Italy.

Alfa executives have previously warned of a "conglomerate discount" for the group, or a tendency for investors to value conglomerates less than the sum of their parts.

The firm spun off telecommunications unit Axtel earlier this year, but executives noted in April that an Alpek spin-off was unlikely in the near term due to its low earnings and stock price.

Alfa's third-quarter earnings before interest, tax, depreciation and amortization (EBITDA) sank 31% to 6.36 billion pesos.

($1 = 17.4279 Mexican pesos at end-September) (Reporting by Kylie Madry and Aida Pelaez-Fernandez; Editing by Isabel Woodford, Lisa Shumaker and Jamie Freed)