Niki Bezzant: How much coffee should you drink?

Coffee and caffeine have their ups - and downs.

One of the questions I'm always asked when speaking to corporate groups is about coffee — is it good or bad for you?

New Zealand was once a nation of tea drinkers, but coffee is now the fuel that runs us.

Coffee and caffeine have been widely studied. And the answer to the question seems to be, well, both.

First, the good.

Caffeine has been shown to reduce sleepiness and increase alertness, as anyone who's ever used a long black to power up after a sleepless night knows.

Caffeine from coffee can also, it seems, improve athletic performance and endurance. In a review published in 2015 researchers found that between 3 and 7mg of caffeine per kilo of body weight increased endurance performance by about 24 per cent.

The caffeine in a cup of coffee can vary from 80mg to 150mg, depending on the variety and how it's roasted and brewed. Espresso has more caffeine than instant coffee, and cold-brew coffee can pack an even heftier punch.

Perhaps because of this, caffeine has been on the banned list for athletes in the past — although it's not currently despite being acknowledged as a performance enhancer.

For non-athlete humans, a moderate amount of caffeine can have positive effects on our alertness, concentration and working memory; have a coffee at least 15 minutes ahead of your big presentation and chances are you'll get a boost.

These effects work better when you don't habitually drink much coffee; heavy drinkers build up tolerance and won't feel it as much.

Coffee may also have some other benefits in disease prevention: it's linked with reduced risk for type 2 diabetes, Alzheimer's, Parkinson's disease and liver cancer.

Coffee, like tea, also contains antioxidants, which are beneficial to health.

The downside is that caffeine has a long half-life — about six hours, depending on the individual.

That means that six hours after your flat white, half of its caffeine is still in your system, and could stay there for even longer.

Not a problem at 2pm; potentially an issue at 10pm, because caffeine also interferes with the duration and quality of your sleep.

You might fall asleep, but have less of the deep, restorative sleep you need to wake up feeling rested.

Too much caffeine can have other side-effects. These include nervousness, heartburn, constipation and diarrhoea.

Longer-term effects include impaired judgment, emotional fatigue, mood swings, depression and anxiety.

There are no official guidelines on how much coffee is okay. It's partly because we tend to process caffeine at different rates; one person's mellow buzz turns another into a jittery wreck.

I'm a two-coffee-a-week person; I know someone who happily drinks five or six a day. Pregnant women are advised to limit it to about one a day. For all of us it's probably best to lay off the coffee in the late afternoon and evening, and if sleep is an issue, try cutting back.

• Niki Bezzant is editor-at-large for Healthy Food Guide. www.healthyfood.co.nz