Carbohydrates are essential for your body to use as energy and are often overlooked when training. While it is our bodies most readily available source of energy it is not the only source. If we consume protein and fats with no carbohydrates you will find that you can end up feeling tired and lethargic, if your body hasn’t been trained for being a more efficient fat burning machine it will struggle to break down fats for energy (a great way of improving this is to increase your cardiovascular fitness (running/jogging/cycling/swimming etc). With continually reduced carbohydrates, your body will begin to use broken down stored fats for energy but this is far more difficult to do than just using carbohydrates. Overall calorie consumption must be monitored most carefully for weight management.
When we exercise we use the glycogen stored in our muscles and liver obtained from broken down, more complex carbohydrates. If we exercise for long enough we will naturally use more of these stores than we can replace and our bodies will need to find energy another energy source from elsewhere. For those that train with a goal of increasing strength, you will tend to find they consume far more carbohydrates. The reason for this is so that once they have performed their intermittent, incredibly high intensity lifts with massive amounts of muscle fibres contracting at once, their muscle glycogen stores will be depleted (most easily used macro for this type of exercise). What these athletes must ensure is that they replace everything lost so that they do not regress in their training so there is enough energy to replace absolutely everything they did in their workout and hopefully even more stores for next time. This is just one reason for putting on size when only an increase of strength is the goal.
If we train and want to put on lean muscle definition while shredding fat we must watch overall calorie consumption with a tendency towards increasing protein (1g of protein per lb of body weight is considered the industry standard, very rough ‘rule of thumb’). This will allow our bodies to use the glycogen stores as energy for the larger, more muscle stimulating lifts and when the time is right and our training changes to fat burning intensity, switch more to fat for energy (more likely to be extended periods of exercise at a lower intensity such a jogging/ running at 60% effort). Getting the balance of calories and carbohydrates correct here is key – consume too many and they will be stored as fat, too few and your body will eventually begin breaking down and using your hard earned muscle for energy.
Hopefully this has helped clear some things up.