The South Beach diet program can be followed by persons with intense physical activity only in special circumstances.

Here is the body background during exercise:

Muscles need energy. All of the energy necessary for muscular contractions is provided by a nucleotide — adenosine triphosphate (ATP).

The dominant ATP production system is glycolysis, that produces ATP for strenuous activities.

Glycolysis involves the breakdown of carbohydrate (glucose or glycogen) within muscle cells. It is a process that takes place in the absence of oxygen. That is why for very intense activities that require more than 60% of your VO2max, more carbohydrates are needed. For exercise that occurs at or below 50% VO2max, fat is a great fuel. The body has enough stored fat to fuel activity for several days as long as there is sufficient oxygen to allow fat metabolism to occur.

Training induces several cardiovascular adaptations that dramatically improve the ability to reach higher levels of exercise intensity over longer periods of time by increasing the capacity of oxygen intake and use (VO2max).

So, the easiest way to approach the South Beach Diet in circumstances of intense physical activity would be either to:

• begin directly with phase 2, keeping more carbs in the diet, but not expecting spectacular results at once
• to go for the first phase if:
  • either you have a solid training that increased your VO2max in such a measure that the exercises you are doing do not surpass 50% of your VO2max, so you don't need a great amount of carbs
  • you are willing to reduce intensity of exercise so as to keep under 50% VO2max (so that the body can burn only fat without needing too much glucose)

So, naturally, the first phase of the diet would not be advisable for periods of competition or whenever straining physical activity is required. In these cases, before an event that requires strenuous exercise(such as a long run) carbo-loading is advisable, with low glycemic index carbs such as oatmeal and the lower glycemic pastas, this creating a low infusion of sugar during the event. After the event and when training vigorously for consecutive days, high glycemic foods are optimal to replace glycogen stores that have been depleted during South Beach diet exercise.

In any case, the level of good carbs from vegetables and all allowed sources during South Beach Diet Phase 1 must be kept at a maintenance level so as to ensure a minimum of energy resource for the brain so that it wouldn't have to take it from muscles.

For the following phases, the key thing is the amount and timing of eating “good” carbohydrates (from foods that are rich in fiber such as vegetables, fruits and whole grains).

Useful hints

• Adjust the carbs intake based on the intensity of your activity and focus on eating balanced carbs, proteins and fats. For example, on days you don't exercise, eat less carbs and vary the types.
• The best time to indulge carb cravings is right after you exercise, without abusing of course.( for example, if you want to cheat and have some sweets or an ice-cream the best time would be right after a hard workout).
• Eat about six meals a day and make sure to drop carbohydrates around 5 o'clock giving your body time to burn them off before going to bed.
• Do not exercise vigorously immediately after a meal, because your blood is at work absorbing nutrients from your food and will not be freely available to flow to your muscles.
• Make sure you always keep hydrated!