Complex sentence: Specifically it's the deepest layer of belly fat (the fat you can't see or grab) that poses health risks, as these "visceral" fat cells produce hormones that can affect health (e.g. increased insulin resistance and/or breast cancer risk).
Split into simpler sentences: Specifically it's the deepest layer of belly fat (the fat you can't see or grab) that poses health risks. That's because these "visceral" fat cells actually produce hormones and other substances that can affect your health (e.g. increased insulin resistance and/or breast cancer risk).