You can either add sugar/carbohydrates with sports drinks in order to add energy, but then your body will only use the sugar as a source of energy without burning the body's stored fat, no fat burning. If you get tired, you probably have poor fluid balance rather than no more energy (true for training sessions up to 90 minutes).

Water has no important salts and minerals, fluid is important, but you need a combination of fluid and electrolytes. Water doesn't give

The body's best fuel is fat, an energy source with better energy value than for example sugar. The body needs energy during physical activity and releases fat cells which are burned to create energy. This process is what we call fat burning.

The body stores some fat in muscles and the liver, which you use first. The amount in muscles and liver is however limited and will run out, and then you are completely dependent on the fat cells being transported in the blood to other cells that use fat for fuel/energy. The more energy your body needs, the more fat is released from the fat cells.

Transport of fatty acids in the blood
The blood functions as the body's subway system with transport of nutrition, heat and gases such as oxygen and carbon dioxide. The blood corpuscles are also carried by the blood, and these give the blood a certain sluggishness (viscosity). High viscosity means that the blood is flowing sluggishly and vice versa.

In order for the fat burning to function, the free fatty acids have to be able to be transported easily from the fat cells to the fat burning cells. Oxygen is also necessary so that the fat can be used as fuel. This is why a good fluid balance is important. Your blood will only flow easily if you have enough fluid in it.

Absorption by the cells
Several cells in the body absorb fat so they can use it as fuel. Those that do it the best are the working muscle cells and the liver.

The most critical stage is absorption of the free fatty acids by the mitochondria. These are often called the cells' power plants and are the part of the cell that turns the fat into an energy form that we can use in the cells and for movement, ATP. Once it is in the mitochondria, the fat can finally be burned and fat burning is complete.

Breakdown of fat from the cells in the body's fat is called lipolysis. During this process, fatty acids, glycerol and ketone bodies are released, among other things. All of these are carriers of energy.

Water – "transporter" of fat. Release of fatty acids into the blood is not just dependent on the fat cells' inclination to release or take up fatty acides. The blood itself plays a major role. Blood has the capacity of transporting quantities of nutritional elements, minerals, blood corpuscles, oxygen, etc., but in order for the transport of nutritional elements to be optimal, the blood must be able to flow easily. If you don't drink enough, the blood becomes sluggish and the transport of nutritional elements becomes less effective.

Imagine a bowl of water where you are going to dissolve some salt. When you pour salt in the water, it dissolves in the water, but when enough salt has been poured in, the solution is saturated and the water cannot take up any more salt. In order to dissolve more salt, you have to add more water. The body works in precisely the same way. If we have a low supply of fluid, the blood will soon become saturated with.