It's almost impossible to accurately estimate the amount of anything in food calories or fat, but if it looks greasy and smells bad, don't eat it.

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My Tuesday Q&A for New York University's Washington Square News:

When you go out to eat, how can you estimate the amount of butter and grease that is used to cook vegetables? How does this detract from the nutritional value of the food?

If you are eating out, guessing the amount of anything in food calories or fat is next to impossible. You cannot guess accurately unless you are in the kitchen watching what goes into your food, looking up the composition of each ingredient and adding up the nutrients. If you want to try this, the U.S. Department of Agriculture food composition tables are at ndb.nal.usda.gov.

I like a little butter or olive oil on my vegetables. Fat brings out taste and makes vegetables taste delicious.

Fat does other good things to vegetables. Without some fat in your diet, you will not be able to absorb and use beta-carotene and other fat-soluble nutrients.

From a quantitative standpoint, fat provides twice the calories per unit weight than do either protein or carbohydrate. A tablespoon of fat provides about 100 calories. A tablespoon of sugar gives about 45 calories.

That kind of fat is important to health. All food fats -- no exceptions -- are mixtures of saturated, unsaturated, and polyunsaturated fatty acids but proportions differ. Animal fats like butter are more highly saturated than salad oils.

As for quality, grease sounds pejorative so I assume you mean oils that have been repeatedly reused. Those are best avoided, as are those that have been partially hydrogenated, a process that introduces heart-unhealthy trans fats.

How can you tell fat quantity and quality? If a food looks greasy and smells bad, don't eat it. It's unlikely to be good for you.

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This post originally appeared on Food Politics, an Atlantic partner site.