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VI. HEART DISEASE, CANCER AND CAFFEINE
Unlike many other foods or recreational drugs (like alcohol or nicotine), there is little evidence implicating caffeine to cancer or cardiovascular disease -- although heart disturbances can occur on high doses or in sensitive individuals. Administered acutely, moderate doses of caffeine can increase blood pressure and decrease heart rate. For most people, tolerance develops to these effects within a few days and there is no alteration in cardiovascular effects for habitual caffeine users.
In a study of one hundred healthy males, 3 cups of coffee daily over 8 weeks led to a small increase in both LDL & HDL cholesterol, which would not be expected to increase the risk of coronary artery disease [JOURNAL OF THE AMERICAN MEDICAL ASSOCIATION 267(6):811-815 (1992)]. The cholesterol-raising factors have been identified as the lipids cafestol & kahweol, which can be removed by paper filtration and are not present in instant coffee [ANNUAL REVIEW OF NUTRITION 17:305-324 (1997)]. ......
In the period between the 1950s and 1970s many believed that caffeine could be a serious cause of cancer in humans because of studies in plants showing chromosome breaks, inhibition of mitosis and formation of chromatin bridges after high-dose caffeine treatment [MUTATION RESEARCH 26:53-71 (1974)]. A 1994 review, however, concluded that caffeine at the doses experienced in coffee-drinking is not mutagenic in humans, although heterocyclic amines from roasting and alkylating carbonyls in coffee are somewhat mutagenic [MUTATION RESEARCH 317:145-162 (1994)]. Nonetheless, more recent evidence does show a capacity for caffeine to worsen the mutagenicity of ionizing radiation and other carcinogenic agents through interference with cell cycle control [MUTATION RESEARCH 532:85-102 (2003)].
Green tea is reputed to help prevent cancer due to its polyphenols.The polyphenol most highly concentrated in coffee is chlorogenic acid (5-10%), but a coffee polyphenol present in lesser concentrations (caffeic acid) is associated with anti-oxidant activity. Caffeine, like the xanthine-derivative uric acid, is an antioxidant that can scavenge the hydroxyl radical which can be produced from the hydrogen peroxide in coffee & tea undergoing a Fenton reaction with metal ions [FOOD AND CHEMICAL TOXICOLOGY 29(1):1-6 (1991)].
A review of studies on animals & humans found no conclusive evidence for any increased incidence of cancer in any organ due to coffee [WORLD REVIEW OF NUTRITION AND DIETETICS 79:185-221 (1996)]. In fact, a meta-analysis of epidemiological studies concluded that persons consuming four or more cups of coffee per day have a 24% lower risk of colon cancer than those who rarely or never drink coffee [AMERICAN JOURNAL OF EPIDEMIOLOGY 147(11):1043-1052 (1998)]. Nonetheless, such an association might be due to the higher socio-economic status of coffee-drinkers rather than to the coffee. ......
Caffeine administered acutely increases diuresis (urination), but this effect is not seen for habitual users who consume the equivalent of a few cups of coffee per day. Nor do such users typically suffer much sleep disturbance -- especially when most caffeine is consumed in the morning. Tolerance to these effects is believed to be due mainly to the adaptive increase (upregulation) in number of adenosine receptors. Although tolerance is usually seen for elevation of blood glucose by caffeine, there may be little tolerance of this effect for people who are obese or who are suffering from maturity-onset diabetes.
Caffeine can significantly increase the secretion of gastric acid & pepsin. But coffee -- even decaf -- has an even stronger effect. Apparently there is a non-caffeine component to coffee which has a significant effect on gastric secretion.
Caffeine can increase plasma levels of free fatty acids, cortisol and epinephrine -- effects thought to improve athletic performance. But effects on the brain may be more important for improving endurance -- such as increased dopamine signalling in the basal ganglia and reduced serotonin signalling (insofar as serotonin mediates fatigue). Elevated epinephrine & free fatty acids due to caffeine may cause insulin resistance, but it remains undetermined whether tolerance can develop to this effect [DIABETES CARE 25(2):304-369 & 399-400 (2002)]. Elevation of free fatty acids (which can be twice as high as normal) due to caffeine is not simply due to elevation of epinephrine [JOURNAL OF APPLIED PHYSIOLOGY 80(3):999-1005 (1996)]. Obese women increase thermogenesis in response to caffeine to a lesser extent than do lean women [AMERICAN JOURNAL OF PHYSIOLOGY 269(4 Pt 1):E671-E678 (1995)].
Caffeine regularly increases energy metabolism throughout the brain while decreasing cerebral blood flow -- and there is no tolerance for these effects. Vasoconstriction due to 250 milligrams of caffeine can decrease central blood flow by 20-30%, which is why caffeine has been used to treat migraine headache. Because blood glucose is usually more than ample for cerebral metabolism the combination of increased metabolism & decreased blood flow would be more likely to induce hypoxia than ischemia. But if caffeine increases oxygen intake by bronchodilation or increases sensitivity to carbon dioxide in the medulla, then there may be compensation. (Both hypoxia and caffeine elevate plasma adenosine.)
Green tea three times per day can increase total basal metabolism by 4%, but this effect is more due to epigallocatechin gallate than caffeine [AMERICAN JOURNAL OF CLINICAL NUTRITION; Dullo,AG; 70;1040-1045 (1999)].
Of specific interest to females:
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Caffeine can increase both the prevalence and severity of PreMenstrual Syndrome (PMS). Among women with severe PMS a single daily cup of caffeine beverage increased the chance of PMS by one third, whereas 8-10 daily cups increased the prevalence seven times [AMERICAN JOURNAL OF PUBLIC HEALTH; Rossignol,AM; 80(9):1106-1110 (1990)].
The consumption of fewer than four cups of coffee daily during pregnancy is not deemed to endanger the child [FOOD AND CHEMICAL TOXICOLOGY 40:1271-1310 (2002)]. For a 60 kilogram (132 pound) pregnant woman, an increased risk of spontaneous abortion is only seen for coffee consumption in excess of 6 cups per day (equivalent to 12 cups of tea) [NEW ENGLAND JOURNAL OF MEDICINE; Klebanoff,MA; 341(22):1639-1644 (1999)].
