One thing that many bodybuilders and gym rats assume prima facie is that the natural (sans steroids) bodybuilding lifestyle is the ultimate and ideal in healthy living. There is definitely much truth to this as muscle is a great calorie-burner, improves appearance and self-image and many other important physiological factors. However, there is a dark side to bodybuilding that few discuss: animal protein and its impact on IGF-1.
The rule of thumb for bodybuilding is at least 1.0 g of protein per pound of weight. Thus, an 180 pound guy would need to consume about 180 g of protein per day. That's a lot of protein and - let's face it - that's probably anything but "natural". And, as I document in Muscle Madness link, eating meat, casein and soy can easily lead to cancer and heart disease (from the Saturated Fat)), inflammation and brain shrinkage.
Even worse, there is a substantial body of research that shows that animal protein leads to increased levels of IGF-1. IGF-1 is Insulin-like Growth Factor and is one of those things, like folate, in the human body that stimulates growth and tissue development. This may sound good, except that both IGF-1 and folic acid have numerous studies linking them with cancer. Yes, they stimulate growth of both good and bad cells and that can lead to an early and painful death if one is not careful.
One of the leading researchers in this area is T. Colin Campbell, author of the famous China Study, and he showed conclusively that casein, the main protein in milk, activates an enzyme complex along with increasing IGF-1 levels that can accelerate and lead to various cancers. 
NOTE: This is one of the dangers of Growth Hormone injections: they bypass the body's natural feedback mechanisms and directly increase IGF-1 levels beyond the patient's physiological norm. Many researchers are concerned that this will lead to increased risk for cancer.
So can one embrace the bodybuilding lifestyle in a healthy way without increasing their risk of cancer through artificially raised IGF-1 levels? Well, every person must research this for themselves and discuss with their doc, but I believe that there is a way around this issue:
1. Whey. Many studies have shown that casein, the primary milk protein, increases IGF-1 and inflammation, both of which are deadly. However, whey, the other milk protein, appears to be the white sheep of the family. One study, for example, specifically looked at whether it was casein or whey that caused increased IGF-1 levels and found that only casein was the culprit: whey left IGF-1 levels untouched.  Whey is also known for its immune-boosting powers, which should be an asset in fighting cancer as well. However, keep in mind that the typical whey is loaded with dangerous excitotoxins and I document that in my link called No Whey! The alternative is Undenatured or Biologically Active Whey..
2. Fruits, Vegetables and a Low Fat Diet. I have shown in my links on Cancer Protection and the Ornish Diet that these two are powerful protectors against cancer. Of course, there's no guarantees in life, but these definitely decrease the odds according to the research.
3. Exercise. It's no secret that exercise, especially with intensity, increases Growth Hormone levels. IGF-1 is a metabolite, i.e. a byproduct, of Growth Hormone in the liver. So then the more you exercise, the more IGF-1 and the more cancer risk, right? Wrong! Researchers have found that exercise increases the sensitivity of IGF-1 receptor sites and so athletes consistently have lower IGF-1 levels. 
Again, you must decide yourself whether consumption of animal protein is worth the risk. Jack LaLanne is an anecdotal example of doing this successfully: he ate abundant fruits and vegetables, lifted and exercised intensely all his life, and ate copius amounts of egg whites. However, I think that the above four factors can overcome these concerns.
1) Arch Pathol, 1968, 85:133 7, "The effect of dietary protein on carcinogenesis of aflatoxin".
2) Eur J Clin Nutr, 2009 Sep, 63(9):1076-83, Epub 2009 May 27, "Differential effects of casein versus whey on fasting plasma levels of insulin, IGF-1 and IGF-1/IGFBP-3: results from a randomized 7-day supplementation study in prepubertal boys"
3) "The Inter-Relationship Between Growth Hormone,