You hear 40/30/30 tossed around here a lot and it simply is a term for a diet that is 40% carbohydrates, 30% protein, 30% fat, which is a nutrient ratio made popular by the Zone diet a while back.

A low-carb diet will have less carbs, depending on its phase, with some extreme examples being as low as 20% carbohydrates.

Conversely, athletic diets can run as high as 65% carbohydrates.

The reason for this variation, in general, is that carbs are for energy and cannot be stored. Simply put, the more exercise you do the more carbs you need to eat. If you don't have ample carbs in your system you will run out of "glycogen" or blood sugar and you will also run out of energy--an term called "bonking".

We tend to steer people trying to lose weight towards a clean (no or little junk food) diet that is 40/30/30. We've found it to be effective for weight loss and also for those with a somewhat sedentary lifestyle. Active people need more carbs, a percentage that changes directly in response to how much exercsie they do.

We occasionally recommend low-carb diets, generally as a short-term "kick start" type of diet for people who need to lose some weight. Done right, these can be effective over the short term. A healthy person will transition to more well-rounded dietary needs any time between a few days and a few months. We consider a 30/40/30 diet, with 40 being protein intake, about as high in protein as a low-carb diet should go. Beyond this you are putting your body under too much stress.

As for low-fat, it's importatnt to note the dietary fat is essential for well being and eating a diet of 30% fat does not mean that your body composition will be 30% fat. Fat consumption should never fall under 20% of you daily caloric requirements, most of which should come from unsaturated fat. You need very little saturated fat and no trans fat.