Q: What has been the effect from the ban on trans fats in New York City restaurants?

A: Restrictions on the use of partially hydrogenated vegetable oils at restaurants in New York City appear to have slashed the amount of trans fat that their patrons consume.

First, some background: Both saturated fat and trans fat increase blood cholesterol levels. High cholesterol increases the risk of heart disease, so health officials have long looked for ways to reduce such fats in the diet.

Trans fat has a far more negative effect than saturated fat. It’s estimated that an increase of just 2 percent of total calorie intake from trans fat — the equivalent of 40 calories in a 2,000-calorie-a-day diet, or 4.5 grams of trans fat — increases the risk of heart disease by as much as 23 percent.

Some of the trans fat we consume comes from milk, meat and other natural sources, but most of it is from partially hydrogenated oils — widely used because they improve the texture, shelf-life and flavor stability of processed foods.

When the Food and Drug Administration mandated in 2006 that trans fat amounts be listed on Nutrition Facts labels, many products were reformulated to reduce or eliminate trans fat. But meals from restaurants and other food-service establishments make up about one-third of the American diet. That’s why New York City and some other localities decided to put restrictions in place.

A study of lunches purchased at New York fast-food restaurants before and after the ban took effect found trans fat consumption decreased considerably, from almost 3 grams per meal to about a half-gram.

Interestingly, other research has found that Americans’ blood cholesterol levels have dropped from an average of 206 in 1988-94 to 196 in 2007-10, and levels of LDL (the “bad”) cholesterol have dropped from 129 to 116. While no one can be certain what is causing the decline, researchers believe the decreased consumption of trans fat certainly has played a role.

To reduce trans fat in your diet:

• Read labels. Foods with less than 0.5 grams of trans fat per serving will say “0” trans fat on Nutrition Facts labels. Also look at ingredient listings. Foods with “partially hydrogenated” oils contain at least some trans fat.

• When eating out or buying foods at bakeries or other places that might not provide a label, inquire about use of partially hydrogenated oils. And, before going to a chain restaurant, visit http://www.calorieking.com or a similar website to look up nutrition information on menu items.

• Even better: Set a weekly goal to eat out less, and prepare food at home with healthy ingredients.