This is Dr. Daniel Weiss from Your Diabetes Endocrine Nutrition
Group. My topic today is grapefruit. Many of us enjoy eating this delicious
treat or drinking its juice. But warnings abound about grapefruit. Your
doctor or pharmacist may have cautioned you about grapefruit. And many
popular magazines warn not to drink grapefruit juice if you take certain
prescription drugs. Adding to the worry, a recent news report described an
increased risk of breast cancer in women eating grapefruit. What should you
It seems to me that the media, likes to grab our attention by scaring
people. Unfortunately mass media, especially television, offer a poor source
of scientific information. Now I’d like to talk about the latest scare you
A recent study, published in the British Journal of Cancer, reported on
postmenopausal women living in Hawaii and the Los Angeles area. In the
1990’s these women filled out a food frequency questionnaire, which asked
them how often and how much they ate of a variety of different foods. There
was, of course, a question asked about how much and how often the person
ate grapefruit. Importantly, women in this study were not asked about
The researchers continued to check on these women over time, in fact,
for an average of 7 years. Of the 50,000 or so women at the start who filled
out the questionnaire, 1657 women developed breast cancer. These
researchers found that those women who indicated on their questionnaire
that they ate grapefruit daily were more likely to have been among those
who developed breast cancer as compared to those who never ate grapefruit.
Now, this kind of study design like so many published reports in
medical journals or reports in the news cannot prove anything. It does not
provide good evidence of cause and effect, but simply describes an
For example, an association has been found between rates of ice
cream consumption and murder in certain cities. Clearly eating ice cream
does not cause murder, nor does murder make people eat ice cream. Another
factor is more likely to explain this association. It may be, in this case, that
both ice cream consumption and murder are increased in hot weather.
In contrast, a randomized controlled study can give evidence of cause
and effect. If the grapefruit study were done as a randomized controlled trial,
it would go something like this: find a large group of women, then using the
flip of a coin, assign each one of them, to either eat grapefruit or never eat
any grapefruit. Keep all other factors about these women the same. Then
some years later, determine if there is more breast cancer in one group as
This study is simply too impractical and unrealistic. So researchers
turn to an association type of study and try to draw conclusions from that.
That’s why I believe that when you hear reports like this reporting
associations, you should be very skeptical. Wait for more evidence before
coming to any conclusion. Wait before you worry.
And if you were a regular reader of the British Journal of Cancer you
didn’t have to wait long. You didn’t hear this one on the news though.
Two months after the first report, another report came out, this time
from the Nurses’ Health Study or NHS for short. The NHS researchers
gathered information on more than 73,000 postmenopausal women over
about 8 years. Of these women, 3570 developed breast cancer. The NHS
researchers factored in grapefruit juice intake as well as grapefruit and they
found absolutely no connection or association between breast cancer and grapefruit or grapefruit juice intake. As you might expect, other studies
have suggested benefit from grapefruit including a reduced risk of death
Grapefruit and interaction with prescription medications
Okay, so much for that. Now how about the interaction of grapefruit with
your prescription medications? First you should understand that the body
handles foods and medications in very complicated ways. Before food gets
into the bloodstream, before it is absorbed, it goes through the stomach and
then the small intestines. The intestines is the area of the digestive tract
where food enters the bloodstream. Blood from the digestive tract passes
through the liver before traveling to the rest of the body.
The liver and the kidney are the main organs that metabolize, change,
breakdown, or eliminate the chemicals that are in our medications or the
Our body is made up of chemicals in careful balance; food and
medications also are made up of chemical molecules which affect that
It turns out that besides the liver and the kidneys, the intestines are
also a place where medications are metabolized or changed. Grapefruit juice
contains chemical substances which can reduce the amount of the intestinal
enzyme called CYP3A4 which breaks down many medications. With less of
this enzyme, the levels of certain drugs in the blood can be higher, because
less of the drug is broken down in the intestines before it can be absorbed.
On the other hand, recent evidence shows that grapefruit juice can
also reduce absorption into the blood stream of some drugs by reducing the
action of a transporter protein called OATP1A2. This reduced absorption
would make levels of that drug lower.
So in theory, one chemical in grapefruit can raise blood levels of some
medicines and a different chemical in grapefruit can lower blood levels of
other medicines. Therefore, the grapefruit effect on blood levels of drugs, if
But the key point here is that although changes in blood levels of certain
drugs can occur with grapefruit consumption, reports of grapefruit actually
causing problems or symptoms are almost non-existent. This whole issue, in
my opinion, is overblown. Or in medical jargon, this issue is clinically
For example, of all the medications for high cholesterol, called statins,
only 2 list a caution for grapefruit juice in their official package insert. And
those, simvastatin and lovastatin, caution about drinking 1 quart or more per
day if you are taking that particular drug.
Now there must be some people who drink this much grapefruit juice
and also take simvastatin or lovastatin. But there are no published reports of
people running into a problem doing this. Clinical problems from grapefruit
An extraordinary number of foods, probably all foods, not just
grapefruit, affect our body chemistry and how we handle medications. All of
this is very poorly understood at this point. The field of metabolomics is new
and attempts to explore these issues.
In conclusion, my recommendations are to be cautious regarding grapefruit
• If you are on chemotherapy for cancer OR
• If you are on drug therapy for HIV infection OR
So be sure to consult your doctor if you have a question. For most people,
My goal in these pod casts is to bring you important new scientific findings
about nutrition, diabetes and other medical issues. I want to remind you that
scientific knowledge is always changing. What is known today may be seen
from a different perspective or may be changed tomorrow. I will share with
you my understanding of current research related to day to day decisions
Monroe KR, et. al. British Journal of Cancer 2007; 97: 440-445.
Kim EH, et. al. British Journal of Cancer 2008; 98: 240-241.
Mink PJ, et. al. American Journal of Clinical Nutrition 2007; 85: 895-909.
Li P, et. al. Drug Metabolism and Disposition; 2007; 35: 1023-1031.
Glaeser H, et. al. Clinical Pharmacology and Therapeutics 2007; 81:
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