Winter 2014 Update: Dr. Kristine Monroe’s most recent study (discussed briefly in the post below), which looks at the effects of grapefruit and its juice on healthy postmenopausal women, has just been published. She found that the whole fruit led to an increase in “the major form of circulating estrogen in postmenopausal women,” that grapefruit juices had no effect on that form of estrogen, but that they did lower another very potent form of estrogen. 

“The take-away message from this pilot study is that the process of hormone metabolism and absorption is complicated,” Monroe said, and more research is clearly needed.

The few studies on grapefruit—and whether it has an anti-cancer benefits or promotes cancer—are definitely at odds.

It’s clear that grapefruit juice contains natural substances that can inhibit an enzyme system (called CYP3A4) responsible for metabolizing certain meds. The result: an increase of medication circulating in the blood, with potentially toxic side effects.

But can grapefruit also increase your estrogen? University of Southern California researcher Dr. Kristine Monroe is trying to get to the bottom of that question.

“Studies have shown that the same CYP3A4 enzyme system…is also responsible for the partial metabolism of hormones,”  Monroe says.

When hormones are given orally,  grapefruit has been shown to lead to elevated blood levels of estrogens.  In fact, the US FDA has mandated that hormone products contain a warning about the juice possibly increasing estrogen.

In her 2007 study, Monroe found that postmenopausal women who consumed very modest amounts of grapefruit had elevated estrogen regardless of whether or not they were taking (or had been taking) hormones. Based on that study, risk of breast cancer (and presumably other estrogenic cancers) in postmenopausal women “is significantly increased—generally a 30 percent increase in risk among women who consume the equivalent of ½ grapefruit or more every other day,” she said.

But what to make of these conflicting studies?

A 2009 British study found no evidence of an association between grapefruit intake and risk of breast cancer.

The Harvard Nurses’ Health Study in 2008 also found no evidence.  In fact, it found  a protective effect in women with estrogen and progesterone negative tumors who had never used hormones.

Monroe is about to release the results of a follow-up study in which she looked at healthy postmenopausal women and the effects of 4 different grapefruit products –the whole fruit and 3 juice products. (Her earlier research tested the fruit only. ) While she cannot discuss the findings until after publication, “the present study found statistically significant effects of grapefruit intake on …estrogen levels.” The effects, however, were inconsistent among the products, with the juices actually decreasing a particularly potent form of estrogen.   

So studies for and studies against.  What to do?  Remember our risk analysis: Likelihood of the risk that grapefruit will increase your estrogen? Possible. Gravity of the risk?  Great.  Weigh that against what you’re giving up, and until we have more science, the answer is obvious: 

Sorry, Florida, but you can keep your grapefruit for now . We’ll stick to picking foods that are clearly anti-cancerous, not ones that are potentially harmful.