What is a grapefruit?
The grapefruit is a sub-tropical citrus tree grown for its fruit which was originally named the "forbidden fruit" of Barbados. (wiki)
Grapefruit w/drug interactions
Grapefruit can have a number of interactions with drugs, often increasing the effective potency of compounds. Grapefruit contains naringin, bergamottin and dihydroxybergamottin, which inhibit the cytochrome P450 isoform CYP3A4 in the intestine. It is via inhibition of this enzyme that grapefruit increases the effects of buspirone (Buspar), carbamazepine, several statin drugs (such as simvastatin), terfenadine, felodipine, nifedipine, verapamil, estradiol, tacrolimus, dextromethorphan (significant only at recreational doses), benzodiazepines, and ciclosporin. The effect of grapefruit juice with regard to drug absorption was originally discovered in 1989. However, the effect became well-publicized after being responsible for a number of deaths due to overdosing on medication. (wiki)
Grapefruit on Nutrition
Grapefruit is an excellent source of many nutrients and phytochemicals, for a healthy diet. Grapefruit is a good source of vitamin C, pectin fiber, and the pink and red hues contain the beneficial antioxidant lycopene. Studies have shown grapefruit helps lower cholesterol and there is evidence that the seeds have low levels of antioxidant properties. Grapefruit forms a core part of the "grapefruit diet", the theory being that the fruit's low glycemic index is able to help the body's metabolism burn fat.
Grapefruit seed extract has been claimed to be a strong antimicrobial with proven activity against bacteria and fungi. However, studies have shown the efficacy of grapefruit seed extract as an antimicrobial is not demonstrated. Although GSE is promoted as a highly effective plant-based preservative by some natural personal care manufacturers, studies indicate the universal antimicrobial activity associated with GSE preparations is merely due to contamination with synthetic preservatives.
A 2007 study found a correlation between eating a quarter of grapefruit daily and a 30% increase in risk for breast cancer in post-menopausal women. The study points to the inhibition of CYP3A4 enzyme by grapefruit, which metabolizes estrogen. (wiki)
Things in Grapefruit that i will focus on
Naringin is the major flavonoid glycoside in grapefruit and gives grapefruit juice its bitter taste. It is metabolized to the flavanone naringenin in humans. Both naringenin and hesperetin, which are the aglycones of naringin and hesperidin, occur naturally in citrus fruits.
Narinigin exerts a variety of pharmacological effects such as antioxidant activity, blood lipid-lowering, anticarcinogenic activity, and inhibition of selected cytochrome P450 enzymes including CYP3A4 and CYP1A2, which may result in several drug interactions in-vitro. (wiki)
Naringin is a citrus flavanoid commonly found in grapefruit, and is responsible for grapefruit's distinctive bitter taste. Most of us have heard how it's important to be careful consuming grapefruit juice along with certain medications -- studies reveal that naringininterferes with some digestive enzymes which can affect the metabolismand breakdown of these drugs. This can in effect lead to an inadvertantlyhigher dose of the drug, which may pose the threat of adverse reactions in some cases.
However, naringin displays many other interesting properties, some of which make it suitable for inclusion into the ingredients list of many popular fat burners. (Akira finding)
For one, there is some evidence that the effects and levels of caffeine can be extended when consumed with naringin. Since virtually 99% of thefat burners on the market contain caffeine (often derived from natural sourceslike Kola nut, Yerba mate, Green tea, and so on), and since caffeine doesexhibit thermogenic (fat burning) properties, naringin is an obvious "no brainer" way to improve and extend its effects. (Akira finding)
Naringin has also been shown to exhibit cholesterol-lowering effects, which is always a good thing for dieters. It's also an aldose reductase inhibitor,which means it may also have a role to play battling retinal disease indiabetics. While naringin may not be the most "sexy" ingredient in your favorite fat burner,it is a worthwhile one. (Akira finding)
Bergamottin is a natural furanocoumarin found principally in grapefruit juice. It is also found in the oil of bergamot, from which it was first isolated and from which its name is derived. To a lesser extent, bergamottin is also present in the essential oils of other citrus fruits. Along with the chemically related compound 6’,7’-dihydroxybergamottin, it is believed to be responsible for the grapefruit juice effect in which the consumption of the juice affects the metabolism of a variety of pharmaceutical drugs. (wiki)
Chemically, bergamottin and dihydroxybergamottin are linear furanocoumarins functionalized with side chains derived from geraniol. They are inhibitors of some isoforms of the cytochrome P450 enzyme, particularly CYP3A4.  This prevents oxidative metabolism of certain drugs by the enzyme, resulting in an elevated concentration of drug in the bloodstream. (wiki)
Normally, the grapefruit juice effect is considered to be a negative interaction, and patients are often warned not to consume grapefruit or its juice when taking medication. However, some current research is focused on the potential benefits of cytochrome P450 inhibition. Bergamottin, dihydroxybergamottin, or synthetic analogs may be developed as drugs that are targeted to increase the oral bioavailability of other drugs. Drugs that may have limited use because they are metabolized by CYP3A4 may become viable medications when taken with a CYP3A4 inhibitor because the dose required to achieve a necessary concentration in the blood would be lowered. (wiki)
WHAT THIS ALL MEANS
Naringin appears to interfere with the activity of several enzymes that are responsible for the breakdown of certain nutrients in the intestines, along with many types of drugs, resulting in higher levels of those substances in the blood. As such, naringin is used to enhance the effectiveness and increase the half-life of several supplements and related substances, such as caffeine.
Most studies have shown that a dose of 25 milligrams of naringin is enough to increase the bioavailability and half-life of certain nutrients and/or drugs consumed with it.
The main source of naringin is pure grapefruit juice. However, many types of grapefruit juice are either blended with other juices, or use grapefruits that have low naringin levels, to help remove bitterness and improve taste. Naringin supplements are also available, and are sold in pill, liquid and capsule form.
What can happen if I take too much naringin? Are there any interactions I should be aware of? What precautions should I take?
Because naringin can alter the metabolism of certain drugs so that they remain in the bloodstream longer, taking naringin supplements can result in higher-than-expected levels of those drugs in the blood, which may cause a variety of unwanted side-effects. As a result, patients should not take any drugs with naringin or grapefruit juice without first consulting a licensed health care provider. In addition, the effects of taking naringin and/or drinking grapefruit juice are cumulative; the more naringin that is ingested, the greater its interaction with certain drugs and other nutrients.
Among the drugs known to be affected by naringin are calcium channel blockers; estrogen supplements; sedatives; high blood pressure medications; and cholesterol-lowering medications. As always, make sure to consult with a licensed health care provider before taking naringin or any other herbal remedy or dietary supplement.
"Don't get too carried away with this. It can lead to dangerous, toxic levels of some drugs or possibly supplements (one example I think someone mentioned might be red yeast rice). I suggest you look into which ones have a bad interaction with grapefruit on your own if you are indeed taking any medication (bitterplacebo)."
Jonny21 says it great "Grapefruit juice inhibits certain enzymes that metabolize chemicals therefore allowing more chemical to enter blood.
Prescribed dose of Lipitor is 10mg. Lipitor begins to get metabolized by cyp p450 3a isoenzyme in intestinal wall. Lets say for arguments sake that only 5mg makes it into bloodstream. Now, you drink some grapefruit juice which inhibits cyp p450 3a isoenzyme, and then take 10mg lipitor. Again for arguments sake, lets say the entire 10g makes it into the bloodstream. This can pose problems since it will allow for higher blood levels than expected, especially for blood pressure meds.
The same enzyme metabolizes Testosterone so theoretically taking grapefruit juice prior to oral testosterone would increase chances of Test making into the bloodstream. That is if it makes it past the stomach."
The potential for this type of drug interaction is enormous. Dr. David Bailey, stated in the September 3rd, 1998 issue of the Medical Tribune:
"Sixty percent of drugs that are (commonly) prescribed are metabolized to some extent by this enzyme, CYP3A4 (Cytochrome P450 3A4). Many drugs undergo first-pass metabolism by CYP3A4. Hence, the inhibition of this enzyme in the gut causes plasma concentrations of these drugs to increase markedly after oral administration. The result can be significant. For example, taking one tablet of lovastatin (Mevacor) with a glass of grapefruit juice is the same as taking 12 to 15 tablets with a glass of water. Certain antihistamines, benzodiazepines, cyclosporin, caffeine, calcium antagonists and cisapride are among other drugs with which grapefruit interacts. So this interaction is very important and the list (of drugs) is going to continue to grow".
I've read that the effects can be seen up to 72 hours after an 8oz glass of grapefruit juice. But that's a general statement for which I couldn't find any specific data on what thresh hold for the inhibition was used to say that the effects wouldn't be seen anymore.
up to 72hours from one serving. translation. only need one serving per 2 days or every other day for something which can possibly provide such huge benefits.