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Caffeine also stimulates certain portions of the autonomic nervous system.Caffeine is a bitter, white crystalline purine, a methylxanthine alkaloid, and is chemically related to the adenine and guanine bases of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).Some people experience insomnia or sleep disruption if they consume caffeine, especially during the evening hours, but others show little disturbance.Evidence of a risk during pregnancy is equivocal; some authorities recommend that pregnant women limit consumption to the equivalent of two cups of coffee per day or less.There are several known mechanisms of action to explain the effects of caffeine.The most prominent is that it reversibly blocks the action of adenosine on its receptor and consequently prevents the onset of drowsiness induced by adenosine.Based on average body weights of children, this translates to the following age-based intake limits: Health Canada has not developed advice for adolescents because of insufficient data.However, they suggest that daily caffeine intake for this age group be no more than 2.5 mg/kg body weight.
Thus it requires roughly 50–100 ordinary cups of coffee to reach a lethal dose.
A 2011 risk analysis review found that caffeine consumption during pregnancy does not appear to increase the risk of congenital malformations, miscarriage or growth retardation even when consumed in moderate to high amounts.
There is some evidence that the hormonal changes during pregnancy slow the metabolic clearance of caffeine from the system, causing a given dose to have longer-lasting effects (as long as 15 hours in the third trimester).
However pure powdered caffeine, which is available as a dietary supplement, can be lethal in tablespoon-sized amounts.
For children age 12 and under, Health Canada recommends a maximum daily caffeine intake of no more than 2.5 milligrams per kilogram of body weight.