Os efeitos do café no ser humano não podem ser comparados ao da cafeína isolada.
A substância mais pesquisada e conhecida do café é a cafeína, descoberta na Alemanha em 1820 pelo químico Ferdinand Runge. Deste então a ciência médica dedicou uma atenção quase que obsessiva e exclusiva à cafeína e muito pouca aos demais compostos bioativos do café, como a niacina, sais minerais e os ácidos clorogênicos/quinídeos, dentre centenas de outros, a maioria voláteis. A grande maioria dos artigos médico-científicos avaliam os efeitos da cafeína sobre o organismo humano ou sobre tecidos isolados, algo que não pode ser extrapolado para o café. Caso a ciência fosse realmente rigorosa, poderíamos dizer que ainda são necessários mais estudos com o café (café torrado e moído, café solúvel, café descafeinado, etc), particularmente conhecendo o teor final de seus componentes, pois a maioria deles, ao contrário da cafeína, que é termoestável, são termolábeis. Por isto nem todos os cafés possuem a mesma composição e assim os mesmos efeitos sobre o organismo humano.
A SUMMARY OF THE PHYSIOLOGICAL EFFECTS OF CAFFEINE
Soon after drinking a cup of coffee, or tea or cola, caffeine is distributed throughout the body. As it is similar to substances normally present in the tissues, caffeine could affect all the systems of the body: nervous, cardiovascular, respiratory and so on. However, caffeine does not accumulate in the body, so its effects are short-lived and transitory.
Whether or not caffeine's effects are physiologically important (or even noticeable) depends on a number of factors. Every individual reacts differently to caffeine. For example, caffeine may stay in the body of pregnant women for up to 3 times as long as is usual in adults, whereas smokers eliminate caffeine twice as quickly as non-smokers. This may help to explain why women often feel more sensitive to coffee in the latter stages of pregnancy, or why heavy smokers are usually heavy coffee drinkers as well. Some of the effects of caffeine, such as those on the heart and blood vessels, are contradictory and have no net effect - others may only be noticeable when regular consumers suddenly cut out caffeine. The body can become habituated to caffeine so that regular users are less sensitive to the stimulant effects than others. In fact, people tend to regulate their coffee consumption according to their experience - as many cups of coffee in the morning as they find give a pleasant, stimulating effect - perhaps none a few hours before going to bed.
Of all the physiological effects of coffee, the best known is that it is a stimulant to the nervous system. One or two cups of coffee can make one feel more awake, alert and able to concentrate. Caffeine has been shown to counteract fatigue and restore flagging performance. However, in sensitive individuals, caffeine may delay the onset of sleep, decrease sleep time and even lower the subjective quality of the sleep. Caffeine has various effects on mood, ranging from pleasant stimulation and mood elevation to anxiety, nervousness and irritability, but these are transient and dose-related.
Other physiological effects of caffeine, in the short term, include increases in blood pressure, plasma catecholamines, plasma renin and serum free fatty acids; the production of urine and of gastric acid are also increased. Regular consumption in normal individuals rapidly leads to tolerance and has no adverse effects.
The vast bulk of scientific and epidemiological evidence points to the conclusion that normal, regular consumption of coffee and caffeine containing beverages is not associated with heart or cardiovascular diseases, damage to the foetus, benign breast disease or cancer of any kind. Some people with irregular heartbeat syndromes may choose to drink decaffeinated coffee since caffeine has been known to precipitate arrythmias or ventricular premature beats, as do alcohol, exercise, stress and many drugs.
Standard reference
Evaluation of Caffeine Safety, a scientific status summary by the Institute of Food Technologists' Expert Panel on Food Safety and Nutrition, 1987. Food Technology, Institute of Food Technologists, Chicago, 41(6):105-113.June 1987
The amount of caffeine in a cup of coffee can vary greatly, depending on its origin or the composition of the blend, the method of brewing and the strength of the brew. Instant, or soluble, coffee generally contains less caffeine than roast and ground coffee, but may be consumed in greater volume. Robusta coffees have about twice as much caffeine as arabicas. A 'cup' is usually understood to contain 150 ml (5 oz in the United States) but an espresso may be as small as 40 ml.
The U.S. Food and Drug Administration gives the following ranges for caffeine contents:
(mg per 5 oz cup)
|
| range | average |
| Roast and ground | 60-180 40-170 | 115 80 |
| Instant coffee | 30-120 | 65 |
Consumer surveys
An interesting survey of caffeine contents in cups of coffee was conducted in Canada (Stavric et al, reference below). Whether the coffee was prepared by housewives at home, for sale in commercial outlets, or by individuals at work, the mean caffeine content was about 80 mg per cup (about 350 mcg per ml). The means for roast and ground coffee, both drip and percolator brewed, were below 85 mg; for instant coffee 71 mg. However, the range of measurements was so wide that, for most individuals, it would be inaccurate to use these figures as a basis for calculating the exact amount of caffeine consumed in a day. The size of the cup/serving varied from 25 ml (Greek coffee) to 330 ml at home and from 130 ml to 280 ml in the out-of-home situation.
DECAFFEINATED COFFEE
Whatever method of decaffeination is used, the decaffeinated green coffee must contain less than 0.1 % caffeine (dry weight basis) to comply with EC regulations. This corresponds to about 3mg caffeine in a cup of decaffeinated coffee.
OTHER BEVERAGES
Tea contains more caffeine than coffee weight for weight, but less weight is used, in general, to brew a cup of tea. In the Canadian study referred to above, both the type of tea used and the steeping time affected the caffeine concentration of samples prepared in the laboratory as follows:
(mean, mcg per ml)
|
| 2 minutes | 5 minutes |
| Tea-bag | 238 | 402 |
| Loose tea | 189 | 295 |
The average caffeine concentration of samples of tea prepared at home was lower, at 159 mcg per ml, but with a wide variation.
The caffeine content of a cup of tea is usually less than 60 mg, but a strong cup of tea may contain more caffeine than a weak cup of regular coffee.
Cocoa and chocolate drinks contribute 4-Smg caffeine per cup to the diet, dark chocolate and cooking chocolate 20-26mg per ounce (0.7-0.9mg per gram). Many soft drinks, including colas and "peppers", contain caffeine, which as well as being present in cola nuts is often added as a flavour ingredient. A 12-ounce serving may contain 30-60mg caffeine. The major brands of cola on sale in the UK contain about 120mg caffeine per litre.
DRUGS
Caffeine is present in many prescription and non-prescription (over-the-counter) drugs, including some taken for headache, pain relief, appetite control, staying awake, colds, asthma and fluid retention. The caffeine contents of drugs varies from 7mg to 200mg per tablet.
LEVELS OF CAFFEINE CONSUMPTION
Caffeine is generally consumed in amounts less than 300mg per day, roughly equivalent to:
3-4 cups of roast and ground coffee
5 cups of instant coffee
5 cups of tea
6 servings of some colas or
10 tablets of some painkillers
It has been suggested that the British consume more caffeine on average than Americans, but there are no large scale studies to support the observation. The nine, normal subjects recruited by Dr M.S. Bruce and his colleagues, as habitual caffeine users, for a study in London (reference below) were found to consume on average 428mg caffeine a day, with a range from 230mg to 670mg.
Customary caffeine consumption has been classified as follows:
Low caffeine users: less than 200mg per day
Moderate caffeine users: 200-400mg per day
High caffeine users: more than 400mg per day
References
Bruce M.S. et al. British Journal of Clinical Pharmacology,22: 81-87. 1986.
Lecos C. The latest caffeine scorecard. FDA Consumer, March 1984.
Stavric B. et al. Variability in caffeine consumption from coffee and tea: possible significance for epidemiological studies. Fd Chem Toxic 26(2):111-118. 1988.
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