Your body tries to keep your blood sugar in a set range. If you eat carbs your blood sugar spikes so your body produces insulin to knock it down. The problem is if you repeatedly spike your blood sugar, by eating sugary snacks your cells start to become insulin resistant. When that happens you need a higher insulin level to push the sugar into your cells, but also the other way around, you need a lower insulin level in order for them to release the energy again. This means people who are insulin resistant your fat cells just don't release the energy, and they hold onto more of it. If your fat cells don't release the energy you eat more because you are hungry!

The reason a high fat diet works is because you can't spike your blood sugar eating fat. So people who are insulin resistant this starts to return to normal.