The consumption of caffeine begins at an early age for many people. Caffeine is a natural ingredient in tea and coffee, and it is used as an additive in many baked goods, frozen dairy products, sweets, gelatins, puddings, and soft drinks. The quantities of caffeine in some commonly used items are summarized in the following table.

Based on these values, the National Institute of Nutrition in Canada estimated that Canadians consume approximately 450 mg of caffeine per day. Children also consume large quantities of caffeine through soft drinks and sweets, and this is a matter of some concern. Adults absorb 99% of the caffeine they consume and reach peak blood levels within 15-45 minutes. Caffeine is found in breast milk and can cross the placenta, thus influencing the unborn child. In newborn infants, the rate of elimination of caffeine is much slower than in adults, and the half-life is 82 hours. In pre-term infants, the half-life ranges from 62-102 hours.ⁱⁱ Some races also experience slower elimination rates than others. For example, Asians have much slower rate of elimination than Europeans. Pregnancy and the use of oral contraceptives also substantially increase the clearance rate.

The effects of excessive caffeine intake, which in some individuals may be manifested at levels as low as 500 mg/day, include insomnia, headaches, anxiety, irritability, and depression. When consumed on an empty stomach, caffeine can produce tremors, and at consumption rates of 1g (which is not unusual for some people) can produce symptoms such as fever, agitation, trembling, rapid breathing and heart rate, cardiac palpitations, diuresis, nausea, and anorexia. Still higher intakes (5-100 g or 50-100 cups of coffee) have caused tachycardia, convulsions, respiratory and heart failure, and coma and death due to shock.ⁱⁱⁱ People who stop drinking beverages containing caffeine may experience equally unpleasant withdrawal symptoms; the most common of which are muscle tension, nervousness, irritability, and headaches. Caffeine also affects urinary calcium excretion, particularly in postmenopausal women.^iv Rats fed instant coffee for three to four weeks also showed increased calcium loss via the urine and feces.^v In one study carried out on a group of women (age 50-84), it was found that the consumption of more than two units of caffeinated beverages (one unit equaling one cup of coffee or two cups of tea) increased the risk of hip fracture by 69%.^vi Metabolic studies have shown that the kidneys and intestinal system are also directly affected by caffeine.

Of even greater concern than these immediate symptoms are the long-term dangers associated with caffeine, which can occur at lower levels and may be more subtle and difficult to detect. In studies on animals, caffeine was shown to affect the nervous system and influence such behaviours as learning, memory, motor performance, sensory function, and emotional reactivity.^vii,viii These findings have prompted the Federation of American Societies for Experimental Biology (FASEB) to voice their concerns about behavioural effects of caffeine, and effects on the development of the nervous system in children who consume large amounts of cola-type beverages.

Studies where caffeine is administered to pregnant mice indicate that caffeine has toxic effects on the unborn offspring and can possibly produce birth defects. Some of the birth defects noted after the administration of caffeine were cleft palate, digital defects, muscular disorders, facial deformities, absence of eyes, and exencephaly. In rats, the situation is similar, and incomplete ossification in the offspring was also reported.

As these studies suffered from lack of certain controls and low sample numbers, the FDA undertook two new studies to resolve the issue of the teratogenic effects of caffeine. These studies revealed that high doses of caffeine result in death and resorption of embryos, significant reductions in fetal weight, and skeletal abnormalities such as reduced pubis size, reduced dorsal arch, and missing hind digits. In fact irreversible birth defects were noted at levels as low as 80 mg/kg and other defects at levels as low as 6 mg/kg.^ix It is still uncertainty whether caffeine increases the risk of birth defects in humans, and it is premature to make such claims. Nevertheless, the studies on animals indicate that there are reasons for concern.

i Review from the National Institute of Nutrition in Canada. "Caffeine: A perspective on current concerns." Nutrition Today (1987): 36-38.

ii ibid.

iii ibid.

iv L.K. Massey and S.J. Whiting, "Caffeine, urinary calcium, calcium metabolism and bone," Journal of Nutrition 123 (1993):1611-1614.

v J.K. Jeh and J.F. Aloia, "Differential effect of caffeine administration on calcium and vitamin D metabolism in young and adult rats," Journal of Bone and Mineral Research 1 (1986): 1251-258.

vi P. Kiel et al. "Caffeine and the risk of hip fracture: the Framingham study," American Journal Epidemology 132 (1990):675-684.

vii S.L. Nightingale and W.G. Flamm, "Caffeine and health. Current status," Nutrition Update 1 (1983):3-19.

viii K.R. Kirsh, M.G. Pinzone, and J.H. Forde, "Spontaneous locomotor activity changes evoked by caffeine in mice." Federation Proceedings 33 (1974):466.

ix S.L. Nightingale and W.G. Flamm, "Caffeine and health. Current status," Nutrition Update 1 (1983):3-19.