James Miller

Caffeine raised adrenaline by 207% and cortisol by 30% in two double-blind trials. In people sitting completely at rest. In a 1978 double-blind crossover trial published in the New England Journal of Medicine, nine healthy young adults who had avoided coffee, tea and cola for three weeks received either 250 mg of caffeine or placebo. After caffeine: Plasma adrenaline rose 207%. Noradrenaline rose 75%. Blood pressure increased by around 14/10 mmHg. Your nervous system shifts toward fight or flight. In another double-blind, placebo-controlled study, 47 healthy young men received caffeine equivalent to roughly two to three cups of coffee while simply resting. One hour later, ACTH — the hormone that signals the adrenal glands to release cortisol — had increased by 33%. Cortisol had increased by 30%. And this was not a brief spike. ACTH remained significantly elevated from 30 minutes after ingestion through the final blood draw 3 hours later. Cortisol remained significantly elevated from 1 to 2 hours after ingestion. No exercise. No emergency. No stressful meeting. They were sitting at rest. The obvious objection: the 1978 subjects didn’t normally drink coffee. What about habitual drinkers? Don’t they build tolerance? A 1990 study in Psychosomatic Medicine tested exactly that. It compared 25 healthy men in two groups. Light consumers and habitual consumers. Light consumers averaged around 54 mg of caffeine a day, less than one cup of coffee. Habitual consumers averaged over 500 mg a day, roughly five cups or more. Each man received caffeine on one day and placebo on another, then completed a stressful task. Caffeine more than doubled the stress response. The adrenaline response was 233% of the placebo response. The cortisol response was 211%. The habitual drinkers responded no differently from the light drinkers. Roughly ten times the daily caffeine. No significant difference in the stress response. A larger 2005 trial looked at tolerance more closely. 96 young adults took 0, 300 or 600 mg of caffeine a day for five days, then received further doses while their cortisol was tracked across the day. After five days without caffeine, all caffeine doses raised cortisol across the whole day. After five days at either 300 or 600 mg a day, the cortisol response to the morning dose disappeared. But an afternoon dose still raised cortisol significantly from about 1 PM to 7 PM. Tolerance was partial. It did not switch the cortisol response completely off. The feeling you call “energy” is stimulation. Stephen Cherniske, a nutritional biochemist and medical writer, puts it this way in his 1998 book Caffeine Blues: “The fact is that caffeine never gave you energy. It stimulated your nervous system and adrenals, and that’s not energy, it’s stress. Likewise, the ‘energy’ you receive from caffeine is really just a loan from your adrenals and liver, and the interest is extremely high.” So caffeine activates the stress response. You feel awake. But feeling stimulated is not the same as having more energy.