Understanding Serving Sizes: Grams vs. Cups - What's Most Accurate?

Question
What is the best way to determine a serving size? For example, a 1/2 cup of beans (130 g) = 100 calories. Should I go for the 1/2 cup, or the 130 g? What is the most accurate? I ask because when I measure out 1/2 cup of beans, it doesn't equal 130 g... So, I guess I don't know which one actually equals the 100 calories. I'd appreciate knowing if I always go by weight, or by measurements in cups, etc. THANKS!

Answer
Hi Kris,

Servig sizes is something that has to be specified evry time they mention it. However, USDA  defines it this way:

"A serving is a medium-size piece of fruit; 3/4 cup (6 fl. oz.) of 100 percent fruit or vegetable juice, 1/2 cup cooked or canned vegetables or fruit, 1 cup of raw leafy vegetables, 1/2 cup cooked dry peas or beans, or 1/4 cup dried fruit."

Of course, grams and calories usually vary a lot because believe it or not, calorie count in foods is a very vague thing depending on too many factors to be unified.

Reading:

Servings
http://dietandbody.com/5_A_Day_Basics.html

Calorie Counting
http://dietandbody.com/did-you-know/2006/09/calorie-counting-is-best-weight-loss...

Aren't All Calories Equal?
http://dietandbody.com/article1175.html

Tanya Zilberter