Caffeine, alcohol, food and cortisol: what changes your levels and what it means for testing

Caffeine raises cortisol, but less in people who are used to it. After five days without caffeine, a dose of caffeine produced a robust rise in cortisol; after several days of taking 300 to 600 mg a day, the rise was smaller but did not disappear.¹ Alcohol is linked with higher cortisol too, and months of heavy drinking show up in hair.²³ A protein-rich meal raises cortisol for a while,⁴ and three weeks of strict dieting raised total cortisol output.⁵ Most of these are short-term effects, which matter most when you are having a blood or saliva test. A hair test, which averages months, is not changed by a short-lived rise in cortisol at the time the sample is cut.⁶

Caffeine: a real rise, partly blunted by habit

In a carefully controlled study, people went without caffeine for five days and were then given caffeine. Their cortisol rose robustly across the waking hours. After a period of taking 300 to 600 mg of caffeine a day, the same challenge still raised cortisol, but by less: regular use built up partial tolerance, not complete tolerance.¹

In practice, if you drink coffee every day, each cup probably nudges your cortisol up less than it would for someone who rarely drinks it. Whether years of regular coffee drinking change long-term cortisol is a different question. It does not appear to have been studied with hair tests, so there is no good basis for claims either way. If coffee affects your sleep, cutting back later in the day may help indirectly, because a short night raises cortisol the following evening.⁷ See cortisol and sleep.

Alcohol: short-term rises and long-term patterns

In the Whitehall II study of 2,693 men and 977 women, men's salivary cortisol was 3 per cent higher for each extra unit of alcohol they drank a week. Women who drank heavily had a larger cortisol rise after waking (14.15 against 8.69 nmol/L).²

Hair shows what sustained heavy drinking does. People with alcohol dependence, sampled during withdrawal after a period of active drinking, had three to four times the hair cortisol of people with alcohol dependence who had stopped drinking, or of a control group. Those who had stopped did not differ from the controls,³ which suggests long-term cortisol can come back down after drinking stops. MedlinePlus also lists alcohol use disorder among the conditions linked with cortisol that is high some of the time.⁸

Food: protein-rich meals and crash diets

Eating itself moves cortisol. In a laboratory study, a protein-rich midday meal reliably raised salivary cortisol, while a low-protein meal did not.⁴ This is a normal response and nothing to worry about, but it means a meal shortly before a saliva sample could nudge the result.

Longer-term eating patterns matter too. In a study of 121 women, three weeks on 1,200 kcal a day increased total cortisol output.⁵ Strict calorie cutting seems to act as a stress in its own right; see cortisol and weight. There do not appear to be robust trials of diet with hair cortisol as the outcome, so claims that particular foods or "cortisol diets" lower long-term cortisol are not backed by good evidence.

Preparing for a blood or saliva test

Blood and saliva tests capture cortisol at the moment of the sample, so anything that moves it in the hours before can move the result. The most important rule is to follow the instructions for your test exactly. MedlinePlus advises following all the instructions your provider gives, resting beforehand because stress and exercise can raise cortisol, and telling your provider about all your medicines, including skin creams, without stopping any of them on your own.⁸

• Saliva samples. Do not eat, drink, brush or floss your teeth for 30 minutes before collecting saliva.⁸ One NHS laboratory gives the same 30-minute rule for eating and tooth brushing, to avoid contaminating the sample with blood.⁹
• Late-night saliva tests. These are used to look for Cushing's syndrome. Liquorice, chewing tobacco, smoking on the day of collection and stress just before collection can all raise the result, and steroid-containing lotions or gels can contaminate the collection device.¹⁰
• Morning blood tests. Blood cortisol peaks at about 8.30 am,¹¹ and NICE advises an 8 to 9 am sample when adrenal insufficiency is suspected.¹² Having blood taken can itself raise cortisol, in about 30 to 47 per cent of people.¹³
• Caffeine and alcohol. Given the research above, it makes sense to keep both out of the hours before a blood or saliva sample unless your instructions say otherwise.

For what each test involves, see the guides to the saliva cortisol test and the blood cortisol test.

Why food and drink do not affect a hair test

Hair works on a different timescale. It records cortisol as it grows, about 1 cm a month, so the 3 cm nearest the scalp reflect roughly the last three months.¹⁴ A short-lived rise in cortisol at the time the sample is cut, whether from coffee, a meal or a stressful morning, does not change the result, and hair cortisol does not vary with the time of day.⁶ There is nothing to avoid eating or drinking before you cut a hair sample.

What hair does reflect is your habits over months: sustained heavy drinking, for example, showed up as hair cortisol three to four times higher.³ Because hair is far more consistent from test to test, correlating at r = 0.84 from one month to the next against 0.38 to 0.61 from week to week for saliva,¹⁵ it is the most accurate way to measure long-term cortisol from a single sample. Hair has its own influences instead. In one large study, people who dyed their hair had 12.5 per cent lower hair cortisol, and use of systemic steroid medicines, such as tablets, was linked with higher values.¹⁶ See testing dyed or treated hair and hair vs saliva.

Do you need to change what you eat and drink?

For long-term cortisol, the clearest message is about alcohol: higher intake goes with higher cortisol, and heavy drinking with much higher long-term levels that appear to come down after stopping.²³ Crash dieting can raise cortisol,⁵ so steady changes are the gentler route. For caffeine, the short-term rise is real but partly blunted in regular drinkers,¹ and there is no long-term hair evidence either way. For the wider picture of what lowers cortisol, see how to lower cortisol.