What happens when you take too much caffeine

It is there naturally in your morning coffee and cup of tea or added to your favourite energy drink and many popular soft drinks. For many of us, a slug of caffeine can give our bodies and brains a much needed boost if we are feeling a little sluggish.

But recently, a brand of lemonade on sale in the US was discontinued amid accusations its caffeine content was dangerously high, despite reportedly being within the country's recommended daily intake of caffeine for adults. 

It has raised questions about just how much caffeine is too much, and does it matter where we get it from?

While there's a vast number of studies showing that some caffeinated drinks have health benefits, there's still some uncertainty around how we should be consuming it.

How caffeine affects the body

There are many crucial functions at play in our bodies all the time, including heart rate, blood flow and sleep-wake cycles. Many of these affected by adenosine – a chemical that occurs naturally in your body, and whose job it is to make us tired at the end of the day.

"Adenosine is one of the naturally produced substances in the body to cause a quieting of activity in various organs that are under stress or in need of lowering energy demand," 

says Kenneth Jacobson, chief of the molecular recognition section at the National Institute of Diabetes and Digestive and Kidney Diseases in the US.

Adenosine receptors are found on the outer surface of many cells in the body, he says, and they react to variable levels of adenosine near the cell to send a command inside the cell to lower its level of activity. This promotes sleep in the heart, kidneys, immune system and other tissues.

When we consume caffeine, it's quickly absorbed into our bloodstream, where it out-competes adenosine by preventing it from connecting to these receptors and doing its job. This is why consuming caffeine can make us feel more awake and alert.

Caffeine can also boost levels of other neurotransmitters like dopamine and adrenaline, which can make you feel more stimulated, says Damian Bailey, professor of physiology at the University of South Wales in the UK.

"The brain is like a big muscle," Bailey says. "It needs to perform things, and dopamine, adrenaline and caffeine all stimulate it."

Health benefits of caffeine

There's a huge body of evidence around caffeine's effects on our health – particularly coffee, since this is one of the biggest contributors of caffeine in the diets of most populations.

An umbrella review analysing more than 200 meta-analyses in 2017 found that drinking three to four cups of coffee a day was more often linked to health benefits rather than harm,

and that studies finding harmful associations could be explained by the higher proportion of coffee drinkers that also smoke.