The hormone insulin is what regulates fat storage. When someone eats carbs, this spikes blood sugar, the levels at which it rises depends on how easily digested they are. Refined carbs and sugar are easily digestible so it gets to the bloodstream a lot quicker causing a spike in blood sugar, which causes the body to secrete the hormone insulin to reduce blood sugar levels. Normally, it can either be immediately used as energy, converted to glycogen for the muscles and the rest stored in fat cells. But over time, the body can become resistant to the effects of insulin, known as insulin resistance. An expanding weight line is an indication of Insulin resistance. This is why some have no problem eating carbs while others would be better off restricting their carb intake. In a study done by Stanford University, called A to Z, 311 women for 1 year on the most popular diets but essentially it was high carbs vs low carbs vs traditional diet. At first glance, the low carb diet produces the most weight loss and best improvement in health risk factors, but when comparing people who were insulin sensitive, there wasn’t much difference between low carb vs high carb diet but if someone was insulin resistant the low carb diet worked dramatically better than the high carb diet. Hope this helps!