The DiabetesHealth magazine featured an article on the study done by South African University pharmacologist, Dr. Peter Owira, who found that rats with T2 had dangerously high level of lactic acid when fed with grapefruit juice together with metformin. Lactic acidosis is a condition in which lactic acid level became elevated to a dangerous and potentially life-threatening level. Symptoms of lactic acidosis includes rapid breathing, excessive sweating, cold and clammy skin, sweet-smelling breath, belly pain, nausea or vomiting , and coma. It was speculated that if this could happen in diabetic rats, it might happen to T2 diabetics too.
Before everyone goes out and burns their grapefruits in a big fire out back, realize that there are some limits on this study. It observed elevated lactic acid in rats, not humans. Lactic acid is produced naturally by your body and in fact exercise can produce elevated lactic acid in your body. Lactic acid is actually cleared quickly and efficiently by a normally operating body. This article (http://care.diabetesjournals.org/content/27/7/1791.full) actually seems to indicate that lactic acidosis risks from metformin are also perhaps smaller than has been suggested.
I would certainly caution against drinking a gallon of grapefruit juice a day (for blood sugar reasons as well), but I don’t think one should either worry about your glass of grapefruit juice in the morning or your exercise in the afternoon (unless of course you are a diabetic rat).