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To caffeinate, or decaffeinate?

Updated: Apr 16, 2022

Whether you love it or you hate it, coffee is one of the world’s most consumed beverages; with around two billion cups of the brown stuff consumed every day. Everyone has an opinion about coffee; but what is the consensus on drinking coffee? In this blog post I will unravel some of the evidence regarding this popular hot beverage.

10/10 for coffee art; Starbucks take note. (Photo credit: Josie Porter)

The good


Hopefully by now you are aware that coffee contains a chemical called caffeine. Caffeine is contained in many household products; including painkillers such as paracetamol and ibuprofen. Coffee is the most concentrated source of dietary caffeine. Caffeine can induce alertness through activation of the central and peripheral nervous system. Caffeine acts by binding to adenosine receptors in the brain in place of adenosine (a chemical responsible for increasing tiredness in high levels); preventing the feeling of tiredness.

But it dehydrates you!

As I mentioned previously, caffeine acts on adenosine receptors to stimulate alertness. Caffeine acts in this way to reduce sodium reabsorption in the kidneys. This can lead to high levels of circulating sodium in your blood which can result in excessive loss of water otherwise known as dehydration. When consumed in large doses (4 filter or 3 double espresso coffees), caffeine can elicit this diuretic effect. Diuretics increase the amount of urine your body produces to increase how much sodium and fluid lost from the body; which can lead to dehydration.

However, moderate caffeine intake will unlikely cause a diuretic effect and will actually contribute towards your total daily fluid intake. There is also some evidence that tolerance to caffeine’s diuretic effect builds up in regular caffeine consumers. So if you are a religious coffee drinker like myself then you can be reassured that your morning cup of coffee will not negatively impact your hydration. If you are cutting down on coffee and not replacing it with something else, then this may actually reduce total fluid intake and therefore hydration status.


Experimental studies have associated caffeine consumption to increased performance time in endurance exercise and enhanced strength and power in short-term high intensity exercise. Although the effect of caffeine on strength and power is less clear. The amount of caffeine needed to induce ergogenic benefits may depend on the sport. More recent evidence suggests that for prolonged exercise (>60 minute duration), even low-moderate amounts of caffeine can induce ergogenic benefits (taken prior, during or post activity). Caffeine is quickly absorbed from the gastrointestinal tract into blood circulation, in a range of studies typically maximum plasma concentration is found 30-90 minutes after consumption (with some variances due to factors such as smoking, food intake, infections, pregnancy, gastric emptying time). Therefore, if you are clever you can time your coffee intake prior to exercise to reap some of the ergogenic benefits.

Beware though, as caffeine can remain in your system for a minimum of 2-4 hours – if you are performing exercise in the evening drinking coffee this late may increase the amount of time it takes you to get to sleep and decrease total sleeping time.


Coffee contains a compound called polyphenols. Polyphenols are a type of phytonutrient (found in plants) that act as dietary antioxidants. Epidemiological evidence suggests that polyphenols are protective of the development of diseases such as cardiovascular disease and type 2 diabetes. Polyphenols can decrease blood pressure, the risk of heart attack and stroke. There is also some evidence from observational studies that a diet rich in polyphenols may be neuroprotective in the development dementia. However these studies look at total polyphenol intake as opposed coffee specifically. Polyphenols are also found in berries, leafy greens, citrus fruits, red wine, tea, olive oil, as well as other fruits and vegetables. Larger studies with longer follow-up periods are warranted to further support the findings from smaller, case-controlled and longitudinal studies.

Certain polyphenols can inhibit the absorption of plant-based sources of iron (non-team). Hence, coffee is thought to be major inhibiting factors of iron absorption. Experimental studies have found that consuming coffee with meals decreases non-haem iron absorption but does not lead to iron deficiency and/or low iron stores in healthy adults. It appears that total dietary iron intake is a more important factor then bioavailability in determining iron status. Studies now demonstrate that bioavailability of non-haem iron adapts to dietary patterns. Individuals eating a high phytate/fibre diet adapt by increasing iron absorption and being more efficient in how iron is used. However, for those with iron-deficiency anaemia the evidence is not clear; so I would recommend avoiding drinking coffee and other polyphenol containing beverages with your meals as precaution.

The bad

Irritable bowel syndrome

Many individuals complain of having irritable bowel syndrome (IBS); whether diagnosed or not. Observational studies have found that coffee is associated with IBS symptoms such as reflux, dyspepsia, abdominal pain and loose stools. Due to the low osmolality (low dilution) of black coffee it is rapidly absorbed in the gut which can potentially lead to loose stools. However, drinking it with milk or alongside a meal may help manage the symptoms of loose stools. Although caffeine has not conclusively been found to affect IBS, you may wish to try decaffeinated alternatives or switch your coffee for an alternative decaffeinated drink such as peppermint tea to try to manage your symptoms.

Side effects

Somewhere in the world there is a conversation whereby two people are in disagreement on how coffee effects them. That is because there is significant variation in how each person is effected by caffeine from products such as coffee. Differences in individual responses to caffeine may be as a results of tolerance, withdrawal and/or genetics. For some, drinking coffee can cause anxiety, jitteriness, nervousness, gastrointestinal upset, tachycardia and insomnia. Like with any build-up of tolerance there can be withdrawal like symptoms such as irritation, headaches, fatigue, drowsiness, low concentration and mood when not consumed.

Caffeine is metabolised by the liver; the same location that medications are detoxified. Due to the caffeine in coffee, the metabolism of certain drugs broken down by the same enzymes as caffeine can be effected or alter the effect of other drugs. Therefore, you may want to consult your GP if you are taking regular medications and you are concerned about them being by caffeine.

The evidence overall is supportive of moderate coffee consumption for healthy adults. It is important to note that some studies included were observational (lifestyle factors may influence findings), some participants may not have accurately recalled diet/coffee intake or had small sample sizes - therefore do not show causation. But coffee can definitely be part of a healthful lifestyle.

Caffeine content can vary greatly; different types of coffee, different coffee outlets and different preparations all make a difference. Approximately 100mg contained in a cup of instant, 140mg in a cup of filter and 200mg in a double espresso. Consumption of up to 400mg of caffeine a day in healthy adults is not associated with adverse effects; with no more than 200mg if you are pregnant.

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