No matter whether young or old, male or female, athlete or non-athlete, an energy deficit is necessary to be able to lose body fat at all. This applies regardless of the diet chosen. The fat tissue stored by the body provides an energy reserve that the organism can fall back on when it receives too little nutritional energy, in other words less than it needs. In this case one speaks of a negative energy balance, so the body loses energy. In the best case, from the energy reserve stored in storage fat.
If the individual has good metabolic flexibility, the energy deficit generated during the negative energy balance diet is completely compensated by mobilizing the body's storage fat. However, the ability to burn fat can be greatly reduced, for example through years of high-carbohydrate nutrition with frequent meals, which means poorer metabolic flexibility. In this case, there may be a lack of energy during the diet, since the compensation of the energy deficit does not work well despite actually full energy stores. The ability to burn fat can, however, be regained, in particular by taking longer breaks in the carbohydrate intake or by reducing the carbohydrate content of the overall diet. All usable carbohydrates are relevant here. Long food breaks (e.g. intermittent fasting) or a change in diet to a low carb or ketogenic diet are well suited for this. The ketogenic diet is the most extreme form of carbohydrate reduction, as only about 30 g of usable carbohydrates are consumed per day. It is so effective in terms of activating the mobilization and utilization of fat that the organism produces water-soluble ketone bodies from fatty acids in order to be able to transport the energy carriers in the blood to the tissues where they can be used for energy. This metabolic state is also called ketosis in its physiological form and, in addition to activating the fat metabolism, has other health benefits.