The flesh and juice of grapefruit offer many benefits if you have high blood pressure. However, grapefruit also has many food-drug interactions of which you need to be wary as some of these involve some blood pressure drugs and statins that are often prescribed to people with hypertension, whether or not their cholesterol is raised. I’ve provided an overview of the pros and cons of eating grapefruit and drinking its juice, below, but if you are on any medication do check the Patient Information Leaflet inside your packs to see if any grapefruit interactions are listed.
Why grapefruit is good for blood pressure
As a member of the citrus family, grapefruit is a good source of vitamin C, with a single fruit supplying at least half your daily requirement.
Grapefruit contain citrus bioflavonoids (eg limonene, hesperidin, tangeritin and naringenin) which also have powerful antioxidant and anti-inflammatory actions. Together, these antioxidants help to protect against hardening and furring up of the arteries – a key contributor to the age-related increase in blood pressure that occurs in many people.
As a source of potassium, grapefruit also helps to flush sodium through the kidneys to reduce fluid retention and lower blood pressure. Drinking 500ml daily of juice from the Sweetie (a cross between grapefruit and pummelo), for example, significantly lowered blood pressure in people with hypertension from an average of 142/89 mmHg down to 136/81 mmHg within five weeks.
Why grapefruit is good for cholesterol
Grapefruit has more pith and membranes than most citrus fruit. These are rich in pectin, a soluble fibre that reduces cholesterol absorption. The high level of bitter naringenin present also has cholesterol-lowering properties.
Drinking grapefruit juice can lower your circulating LDL-cholesterol as much as taking a plant sterol supplement or using sterol-fortified spreads. Red grapefruit juice has the most powerful effect, and can lower ‘bad’ LDL-cholesterol by as much as 15% compared with around 7% for juice from blond grapefruit.
Grapefruit drug interactions
Grapefruit juice interacts with the absorption and metabolism of numerous drugs. The interaction was discovered by accident when researchers were looking at how drinking alcohol affected the absorption of a particular blood pressure medication called felodipine (a calcium channel blocker). Grapefruit juice was used as a mixer to disguise the alcohol taste and, while the alcohol itself did not have a significant effect, the grapefruit juice greatly increased absorption and the resulting level of the drug within the circulation.
How exactly does grapefruit affect medicines?
Researchers have found that grapefruit alters the way drugs are handled in the body in several different ways. Some grapefruit components (eg naringen, furanocoumarins) block the production of an enzyme (CYP3A4) within the intestinal wall. This enzyme usually inactivates some drugs before they are absorbed, so switching off its release allows a greater amount of the affected drugs to pass into the circulation. This enzyme blockade is irreversible and lasts for at least 24 hours, so that drinking commercial grapefruit juice regularly, or eating fresh fruit segments, can cause certain drug levels to rise enough to cause overdose symptoms. Grapefruit also appears to affect the way the transport of certain drugs across the intestinal wall, and how they are broken down in the liver.
Which drugs does grapefruit interact with?
The list of drugs with which grapefruit is known to interact is lengthening all the time, and includes some drugs used to treat high blood pressure, such as some:
- calcium channel blockers (eg felodipine, nicardipine, nifedipine, verapamil)
- beta-blockers (eg carvedilol, used to treat congestive heart failure)
- antidiabetes drugs (eg repaglinide)
- anti-arrhythmia drugs (eg amiodarone, quinidine, disopyramide)
- antiviral drugs
- drugs used to treat erectile dysfunction (eg sildenafil, tadalafil, vardenafil)
- migraine drugs
- statins used to treat a raised cholesterol level.
Are you taking a statin?
Statins are widely prescribed for people with high blood pressure as these may reduce the long-term risk of a heart attack of stroke – independent of your cholesterol levels.
Taking one particular statin, lovastatin, with a glass of grapefruit juice was found to produce the same blood levels of the drug as when taking 12 tablets with water! A similar 12-fold increase was found for simvastatin, too.
The grapefruit-statin interaction may not be so pronounced for other statins, however. One study found that grapefruit juice had no effect on pravastatin absorption, for example.
How long does the grapefruit effect last?
Researchers investigated the duration of the grapefruit juice interaction by asking ten, healthy volunteers to take 40 mg simvastatin with water, or with high dose grapefruit juice (200 ml, three times a day for three days) and then one, three and seven days after they’d drunk the high-dose grapefruit juice.
The results showed that taking simvastatin with high-dose grapefruit juice increased its absorption and blood concentration by over 12-fold compared with water.
When the simvastatin was taken 24 hours after the last glass of grapefruit juice, its blood levels were increased by 2.4 fold.
When simvastatin was taken three days after the last grapefruit juice, blood levels of the statin still rose by 1.5 fold compared to no juice.
This suggests the grapefruit effect lasts for more than three days. After a seven-day gap between simvastatin and grapefruit juice, no significant rise above normal was seen.
Does increased drug absorption matter?
Higher than normal blood levels of any drug increase the risk of developing side effects and can cause potentially harmful problems associated with overdose. These effects will be listed in the patient information leaflet that accompanies your medication.
It’s possible that the interaction might prove beneficial in some cases, although this is controversial. For example, a recent study found that drinking a daily glass of grapefruit juice increased blood levels of the statins, simvastatin and lovastatin, by 260% if taken at the same time and by 90% if taken 12 hours apart.
When the researchers assessed the resulting benefits in a 60-year old man with a raised LDL-cholesterol, his predicted risk of heart disease was significantly reduced by the combination of a statin plus grapefruit juice. In other words, the interaction was deemed beneficial due to the increased cholesterol-lowering effect of the higher statin levels and, presumably, the grapefruit antioxidants. The researchers even suggested that the risk of statin-related muscle side effects was minimal compared to the greater effect in preventing heart disease, and that grapefruit juice should not be contraindicated in people who are taking statins.
This is a controversial view. It is important to check the drug information sheet provided with any medication you are taking, and to follow any instruction provided for avoiding grapefruit and its juice.
The severity of grapefruit-drug interactions varies between individuals and may be more extensive in older people. There is a strong genetic link, as the interaction partly depends on the particular gene you’ve inherited that codes for the production of the CYP3A4 enzyme. Until a simple test becomes available to predict how you will respond, best advice is to follow the instructions that come with your medication.
If your blood pressure is raised, self-monitoring is key to maintaining good control.
Click here for advice on choosing a blood pressure monitor to use at home.
Image credits: ploedfisch, katia vasileva, jhy, pressmaster / shutterstock; united_states_government/flickr;