Let’s review some basic nutritional biochemistry: You can convert fats and proteins to carbs via gluconeogenesis, but that is inefficient and produces toxic byproducts. Getting out of carb/protein/fat balance is more than just calorie trade-offs; it forces your body to shift over to breaking down fats and proteins to sythesize needed glucose via gluconeogenesis for glycosis and aerobic respiration; while burning up fats is desirable (to a certain extent), using needed proteins for glycosis isn’t. The reactions are inefficient and produce waste products that have to be scavenged. During lipolysis (in which unbound fatty acids are released from fat cells) ketones are produced; these are processed out by the liver, but when carbohydrate starvation is occuring they’re produced in excess which leads to ketosis. This can reduce pH levels in the body as well as stressing the liver and kidneys.
In addition, with carbohydrates you get a number of compounds that contribute to the process of glycosis that may not be present in sufficient quantities of high protein or high fat foods, leading to incomplete digestion. Taken to an extreme, this results in fermentation (or anaerobic respiration) and the production or lactic acid as well as ketones from side reactions. This is neither desirable nor maintainable. In any case, you would not want to take any diet to the extent that your blood glucose level falls out of the appropriate range.
Converting from protein>glucose>fat is highly inefficient; a high protein diet without sufficient fats or starved of easy fuel carbs will result in some degree of nutritional starvation (hence, the applicability to high protein reducing diets like Atkins), and it’s quite possible to die from having too lean or carb-poor a diet, regardless of how much protein you consume. You’re certainly not going to be able to maintain an aerobically active lifestyle without a majority share of carbohydrates. Humans are not historically primary carnivores like the large cats, but scavenger/gatherers who are opportunistic hunters similar to ursines. This can be seen from the length and construction of the human digestic tract which is much longer than that of pure carnivores. We do require sources of protein to fuel our abnormally oversized brains, and in prehistory the only way to regularly and reliably obtain the requisite proteins (and energy rich lipids) was from dense animal source muscle protein, but with a few isolated populations it has never comprised the bulk of any natural diet (and those that do also have a very high fresh saturated fat percentage which typically includes a fair amount of carbs and a lot of vitamins).
To make the analogy to construction, proteins are like bricks, fats are the mortar that holds them together, and carbohydrates are the cast you use to pay the bricklayers. You can try paying the bricklayer in bricks and mortar, but it’s going to cost you a lot more and he’s only going to take a certain amount before he quits. Pumping in more lean proteins is a game of diminishing returns; the guy eating two pounds of red meat every day in addition to a henhouse full of eggs is probably excreting a significant amount of that protein back out.
Epimetheus makes a very good point; most of these guys are not trained nutritional experts and have essentially no knowledge of nutritional biochemistry or human physiology beyond what they’ve read in muscle rags. They know what “works” in terms of building muscle mass, but that doesn’t mean that it’s the most effecient or healthy in the long term, and the magazines themselves have every incentive to sell the notion of high protein diets in order to sell protein supplements.
Whether or not these actors use anabolic steroids is unknown, but it’s clear that there’s little reason for them to do so; as the cited article makes clear, it’s leanness, not muscle bulk, which gives the kind of definition that makes actors look “buff” and muscular. If you want to look like Lou Ferrigno in mere months then you’re probably going to be popping steriods like Pez, but if you just want to look like Daniel Craig, you need to burn up the fat percentage to single digit levels while focusing on some definition rather than bulk muscle growth.
robardin, reducing carbohydrates (in moderation) is advisable if you are doing strictly anaerobic strength training and trying to reduce body fat percentage, particularly on off-days. However, this is more about reducing fat than increasing muscle mass, and has to be carefully managed in order to keep from reducing muscle growth. I don’t have a specific cite for you offhand, but this is basic human physiology, regardless of what any bodybuilder tells you about eating nothing but pure protein shakes and a banana a day.