Often coffee is said to raise blood pressure. Studies show that the effect is very slight and temporary, or none at all. Neither is it dehydrating the body. It is as hydrating as water, and not more diuretic.
The most interesting part is that coffee can be beneficial. The caffeine increases alertness and in moderate amounts it can improve fine motor control. It boosts learning abilities and powers of reasoning, and it improves performance and endurance during prolonged, exhausting exercise.
Adenosine is a compound facilitating sleep, and that happens when it binds to its specific receptors in the central nervous system. Caffeine acts as an adenosine-receptor antagonist, which means that it binds to those receptors without triggering the symptoms of sleep; while at the same time being in the way for adenosine, inhibiting its effects. That makes you more alert and fights off sleepiness.
Many independent studies have shown that coffee reduces the risk of getting Parkinson's disease, often as much as 60-80%! The more coffee, the better protection. The same applies to Alzheimer's disease! The best protection against dementia seems to be to drink coffee in midlife.
Dopamine is a neurotransmitter, a compound crucial for life itself. When present amount drops, you get certain typical symptoms; and if it drops further, you die. Parkinson's disease is characterised by that dopamine-producing brain cells stop working, so the level of this important substance is falling. Caffeine stimulates dopamine production, and it is possible that it somehow protects the cells producing it. But the mechanism is not totally understood at this point. It is also puzzling that this connection between coffee and reduced frequency of Parkinson's disease, has been found only for males.
Coffee is not only caffeine, however; it contains a large number of compounds, some of them as yet poorly studied or understood; but that is rapidly improving, and new results are arriving almost weekly.
It contains a large number of natural polyphenolic antioxidants. The brewing process and the roasting add another set of healthy substances. According to a study by Nestle Research Center (published in Journal of Agricultural and Food Chemistry), coffee has far more antioxidants than green tea, cocoa, herbal teas or red wine. Polyphenolic antioxidants might be more important than those amongst the vitamins, and coffee is one of the richest sources known.
Several studies have shown that coffee can prevent diabetes mellitus (adult onset diabetes, or type 2). Six or more cups per day were required to get a good result. What it is in coffee that causes this, is still a matter of discussion. (The results of these studies are controversial.) Certainly it is not the caffeine, which has previously been shown slightly to increase the risk for diabetes.
A compound found in coffee (and only there), methylpyridinium, has shown to reduce the risk of colon cancer. It stimulates blood enzymes assumed to protect against this disease. Interestingly, methylpyridinium is formed during roasting of the beans. Darker roasts produce higher levels of methylpyridinium.
Coffee protects the liver. Those who drink more than one cup of coffee per day are less likely to develop liver cancer. Several studies have proved that coffee also reduces the risk of liver cirrhosis, by as much as 80%, and prevents gallstones. In addition to that, it positively affects the microflora of the intestines.
A few studies suggest that coffee can protect against damages caused by abuse of tobacco or alcohol. For the latter, that is strongly linked to the liver-protecting effect.
Certain circles maintain that coffee enemas stimulate detoxification of the liver. There are some results supporting such a view.
Lately, it has been indicated that coffee might prevent sun-induced skin cancer. But these studies are still insufficient, so one should be careful with conclusions.
Other studies have found that coffee protects against several forms of cancer, and strengthens the heart, reducing the risk of heart disease. There are, however, people with a genetically based slow breakdown of caffeine. This can increase the risk of heart disease. If you belong to this category, you should keep consumption low, perhaps not more than 1-2 cups per day.
Coffee increases male fertility by increasing the moving speed of the sperm. Female fertility, however, seems to drop by it.
Italian research suggests that a compound in coffee, trigonelline, can prevent dental decay. It is antibacterial. Other results link tannins in coffee to reduced caries risk.
On the negative side, coffee can cause sleeping disorders in some people. Moreover, by its acidity it can worsen an already present ulcer and there is one study that indicates a possible connection between a high intake of coffee and cancer of the bladder. The latter is insufficiently studied though.
The acidity also adds to the metabolic acidity, thus it can worsen osteoporosis. People with a pH problem should be careful. Extra minerals might be needed.
There is a widespread misconception that the cholesterol level is raised by coffee drinking. This is true for boiled coffee, as it is traditionally made in Scandinavia, and it is caused by the release of high levels of diterpenes, cafestol and kahweol. This does not happen with brewed coffee, if a paper filter is used.
Coffee interferes with iron absorption, and can cause iron anemia. This, however, is not dangerous for most people, since iron is a very dangerous element, strongly linked to cancer. Even a slight regular overdose can, in the long run, be deadly. Surplus iron cannot be removed from the body by anything else than bleeding: phlebotomy, or bloodletting. A little too little is better than a little too much. Here coffee might even contribute to preventing excessive levels of iron, and thus cancer and liver disease.
The general conclusion here is that although scientific studies can sometimes be misleading, the evidence so far is overwhelmingly in favour of coffee. Its positive effects by far outnumber the negative ones. But it should be brewed, as strong as possible and as darkly roasted as possible. Black of course, without sugar. And use it with sound judgement! Everything is harmful if the amount is too high (even water and oxygen); excess is never good. Drinking 50 cups a day, like Voltaire did, is hardly beneficial. In any case your stomach will certainly set the limit with strong protests if you overdo it. If any problem arises from your intake of coffee, you probably exceed your own individual tolerance.
As a security measure, pregnant women should avoid excessive consumption. Its effect on pregnancy or foetus is unknown, although some results indicate an increased risk for stillbirth and miscarriage.
Headache, nervousness, and other symptoms often described as a result of coffee, might be the opposite - a result of lack of coffee. Caffeine is an addictive drug. When you do not get as much as you are used to, you get withdrawal symptoms. They can be quite strong, although there is a considerable individual variation in sensitivity. It can be one or more of headache, dizziness, anxiety, nausea, even fever and fits of shivering. If you avoid coffee then, everything will be back to normal in a few days.
Please note that caffeine is a pharmacologically active drug that might interact with certain medicines. It is known to strengthen the effect of certain pain-killers, most notably aspirin, and of ergot-based compounds. If you are on medication, ask your doctor about possible conflicts with caffeine.
My Series about Coffee:
2. Coffee: Plants & the Function of Caffeine
4. Do You Drink Much Coffee? I Bet You Can't Beat These Enthusiasts!
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The only negative effect is the acidity and osteoporosis. Which can counteract that perhaps?