Benign prostatic hyperplasia, or BPH, is a non-cancerous enlargement of the prostate that typically presents itself with uncomfortable and abnormal urinary function. Men over the age of 50, and men with a family history of BPH, are most at risk. That being said, we don't really know how to prevent it.
From a nutrition standpoint, lycopene (an antioxidant in tomatoes) and increased fruit intake have both been linked to decreased risk of developing BPH. Although the studies that support these diet changes are certainly not conclusive.
Other studies have linked the intake of total calories, protein and polyunsaturated fats to risk of BPH.
Interestingly, a recent clinical trial studying the effects of selenium and/or Vitamin E was stopped early because no benefit was found. The group of men receiving Vitamin E supplementation actually had higher rates of prostate cancer.
As always, the recommendation is to maintain a healthy body weight, avoid excess consumption of fats and stay physically active.