If you're a serious bodybuilder who wants to gain and maintain size, you should consume a variety of fruits and vegetables. While we often push protein because it's the most critical macronutrient category for bodybuilders, regardless of the type of diet, we also emphasize the value of mixed meals based around whole high-protein foods. Powders are great and can make it easier and more convenient to consume large amounts of protein (without extra fat and carbs), and thus achieve your maximal potential, but they are not essential for developing a massive and monstrously strong physique. Remember, food forms the basis of your nutrition plan and supplements enhance a balanced diet.
Fruits and veggies are typically promoted because of their content of fiber, phytochemicals, vitamins and minerals, as well as their direct and indirect influence on metabolism and overall health. The benefits are many and potentially quite significant, especially in the long run. Fiber has significant "gut building" (as in the lining of the intestines, not your waistline) effects that, among other things, help process and utilize protein more efficiently. One very important but little-publicized reason for bodybuilders to hit fruits and veggies hard is the potential anticatabolic effect of these foods--their ability to help maintain muscle and bone.
PROTEIN AND METABOLIC ACIDOSIS
When your diet is unbalanced, you stand to lose muscle and bone through metabolic acidosis, a situation in which the body retains more acid than it excretes. When that happens, the body strives to correct the acidosis in two main ways. First, glutamine is cannibalized from muscle; this generates bicarbonate, a major acid buffer, and increases urinary nitrogen excretion. (This is another important reason to supplement with L-glutamine or glutamine peptide.) Second, calcium is released from bone to help buffer and eliminate excess acidity. It doesn't take a genius to realize that avoiding acidosis is a good idea if you want to hang on to--and build--muscle and bone mass.
The metabolism of animal-based proteins leads to the production of phosphoric and sulfuric acids. Although food proteins differ greatly in their potential acid load and, therefore, in their ability to generate acids, a diet high in animal-source foods--and especially one low in fruits and vegetables--can cause chronic low-grade metabolic acidosis, even in healthy young men.
Whether and to what extent this occurs in bodybuilders is not known, but it's certainly conceivable that many bodybuilders suffer from chronic low-grade acidosis. Consuming 300-500 grams (g) of animal-source protein per day would raise levels of blood sulfuric acid (by breaking down sulfur-containing amino acids, such as cysteine and methionine) and phosphoric acid (by breaking down phosphorylated amino acids, such as phosphoserine and phosphothreonine). This could easily lead to low-grade acidosis. Diets that are low in carbohydrates, and especially those that are concurrently low in calories, also contribute to acute acidosis and negative nitrogen balance.
For these reasons, bodybuilders probably have a higher-than-normal risk of low-grade acidosis.
FRUITS AND VEGGIES CAN BE ANTICATABOLIC
To regulate the potential catabolic effects of an unbalanced high-protein diet, you can consume fruits and veggies. They contain organic compounds (e.g., citrate in citrus fruits, malate in apples), the metabolism of which yields acid-neutralizing bicarbonate ions. The addition of fruits and vegetables, especially those high in potassium, to a high-protein diet reduces the output of acidic urine, ammonium and net acid excretion, as well as decreases the amount of calcium lost through urine. Bone loss may be halted and bone building may actually occur, not to mention improved capacities for skeletal muscle contraction, relaxation and growth.
The U.S. government wisely suggests a minimum of five servings a day of fruits and vegetables, but we'd prefer to see you consume eight to 10, depending on how much protein you're slamming. If you follow our guidelines for fiber intake--10-15 g per 1,000 calories consumed (working up to that amount over a period of four to six weeks)--your fruit and veggie intake should be adequate.