Lifting weights or training aerobically at moderate to high intensity levels, our primary source of energy is from carbohydrates. The Atkins diet minimises carbohydrate levels, limiting supplies of our necessary body fuel and reducing the body's ability to generate power output. Hence, performance levels appear poor. When carbohydrate levels are low, your body looks for alternative energy sources. Fats are not readily available as a fuel at higher intensities, so the body turns to protein. Using this for energy can result in loss of muscle tissue and reduced metabolic rate. This means you can actually end up putting fat on.