The simple answer is in these three words: Fiber, vinegar and lipid.
When glucose level (blood sugar level) rises sharply with a meal, insulin is secreted into our bloodstream. Insulin has storing sugar as fat function. Excessive insulin secretion can result in weight gain.
Among the three macro-nutrients (proteins, lipids, carbohydrates) the carbohydrate is equal to sugar and can easily lead to blood glucose level raise. Most of us tend to avoid lipids because they’re high in calories. But the truth is, it takes time for our bodies to digest and absorb them, so it’s hard for them to raise your blood sugar level. Vinegar also has suppressing blood sugar level function.
Thus when we eat salad with dressing before main meal, we help suppress a sudden rise in blood glucose level from the meal. As a result, even with the same amount of food consumed, the way calories are absorbed is changed.
One more benefit of having a salad, especially with Japanese food such as sushi, is that one of the component from the fresh greens called chlorophyll helps diminish and clear the fishy aftertaste in your mouth. That’s another secret of why our sushi give you a refreshing effect after the meal ☺