A roomful of tweed-clad ladies posed questions yesterday to one of their heroes, the author of “The South Beach Diet,” Arthur Agatston. Can eating lots of fish be dangerous? What about wine? Is nibbling healthy? How much exercise is absolutely necessary?

Dr. Agatston had answers for all. Avoid canned tuna (it has the most mercury). Limit wine intake to two glasses a day. Do nibble, with several small meals a day (“The amount of overweight people who skip breakfast is amazing,” Dr. Agatston said). And 20 minutes of daily exercise has more impact on your health than what you eat does.

The lunchtime gathering was a fund-raising event for City Harvest, the nonprofit that annually distributes 19 million pounds of food to 800 community meal programs in New York City. The organization also teaches healthful food preparation and nutrition.

Four board members of City Harvest, Carol Atkinson, Joy Ingham, Emilia Saint-Amand, and Topsy Taylor, served as chairwomen of the event. Amy Atkinson and Renee Fishman were the junior chairwomen, but the majority of the guests were over 40. So the sponsor of the event, More magazine, had a perfect target demographic in the crowd. Each guest received a free issue of the magazine, which under its editor, Peggy Northrop, offers a sassy view of women at midlife (the actress Kim Catrall is on the February cover).

In honor of the luncheon’s guest speaker, the Pierre prepared a meal adhering to principles of the South Beach Diet. The tables were pre-set with vegetable-juice cocktails and trays of raw vegetables and low-fat cheese. The entree was black bass on a bed of fennel with carrot and cauliflower purees on the side. The sweet end was a lemon souffle made with Splenda. As for taste … let’s just say no one comes to fund-raising events for the food anyway.

Whether busting out of their suit jackets or getting lost in them, guests found strength in knowing Dr. Agatston goes off his own diet. His first lapse was a slice of apple pie, while on vacation in North Carolina. He is also a chocoholic – but he says dark chocolate is good because it has antioxidants.

On the whole, the event was motivating, so who knows how slim the ladies will be at the next City Harvest benefit – the Practical Magic Ball on April 6. The event has a new set of chairwomen, including Heather Mnuchin (already impossibly thin) and Pamela Kaufmann. One of the honorees is the founder of the Square Feet to Square Meals fund-raising campaign, Ellie Manko. The program has raised more than $100,000 by asking real-estate brokerage firms and landlords to pledge $.005 for each square foot of space leased or sold.