The easiest ways to explain is:
• A serving size is the amount of food listed on a product's food label. It varies from product to product and is chosen by the manufacturer.
• A portion is how much food you choose to eat at one time. It varies from person to person and is chosen by the individual.
Sometimes the serving size and portion size match; sometimes they do not.
For example, a small bag of pretzels may say that it contains two servings, so if you choose to eat the whole bag (i.e. ‘your portion size’), you'd have to double the calorie, fat, and carbohydrate information per serving to know how much you're consuming.
Dr Wansink states that "When it comes to portion control, you can count on your brain not being very interested and your body not being very well calibrated" - which means that we often overeat in 100 or 200 calorie increments, which over time adds up to weight gain.
According to Dr Wansink, most of us know what we're supposed to eat, and how much of it but we just ignore what we know and instead eat ‘mindlessly’.