Coffee is well recognised as having a positive impact on long-term health.
Drinking the equivalent of three to four cups of instant coffee a day reduces the risk of many health conditions including heart disease, type 2 diabetes and some cancers.
Most people gain small amounts of weight each year as they age. But can coffee help prevent this gradual weight gain?
A group of researchers examined whether drinking an extra cup of coffee a day – or adding sugar, cream or a non-dairy alternative – resulted in more or less weight gain than those who didn’t adjust their intake.
Their research (currently a pre-proof, which means it has been peer reviewed but is yet to undergo the final formatting and copyediting) found a modest link between coffee and gaining less weight than expected.
People who drank an extra cup of coffee a day gained 0.12 kg less weight than expected over four years. Adding sugar resulted in a fraction more (0.09 kg) weight gain than expected over four years.
How was the study conducted? What did it find?
Researchers combined data from three large studies from the United States: two Nurses’ Health Studies from 1986 to 2010, and from 1991 to 2015, and a Health Professional Follow-up study from 1991 to 2014.
The Nurses’ Health Studies are two of the largest cohort studies, with more 230,000 participants, and investigates chronic disease risks for women. The Health Professional Follow-up study involves more than 50,000 male health professionals and investigates the relationship between diet and health outcomes.
Participants in all three studies completed a baseline questionnaire, and another questionnaire every four years to assess their food and drink intake. Using the combined datasets, researchers analysed changes in coffee intake and changes in the participants’ self-reported weight at four-year intervals.
The average four year weight-gains for the nurses’ studies were 1.2kg and 1.7kg, while participants in the health professionals study gained an average of 0.8kg.
The researchers found that increasing unsweetened caffeinated or decaffeinated coffee intake by one cup a day was associated with a weight gain that was 0.12 kg less than expected over four years.
Adding creamer (milk) or a non-dairy alternative did not significantly affect this weight change.
However, adding sugar (one teaspoon) to coffee was associated with a weight gain that was 0.09 kg more than expected over four years.
These associations were stronger in participants who were younger and had a higher body mass index at the beginning of the studies.
What are the pros and cons of the study?
This study is unique in two ways. It had a very large sample size and followed participants for many years. This adds confidence that the associations were real and can likely be applied to other populations.
However, there are three reasons to be cautious.
First, the findings represent an association, not causation. This means the study does not prove that coffee intake is the true reason for the weight change. Rather, it shows the two changes were observed together over time.
Second, the findings around weight were very modest. The average four-year weight gain averted, based on one cup of coffee, was 0.12 kilograms, which is about 30 grams per year. This amount may not be a meaningful change for most people looking to manage weight.
Finally, this analysis did not consider the variability in the amount of caffeine in coffee (which we know can be high), it just assumed a standard amount of caffeine per cup.
How could coffee help with weight management?
Caffeine is a natural stimulant which has been shown to temporarily reduce appetite and increase alertness. This may help to feel less hungry for a short period, potentially leading to reduced energy intake.
Some people consume coffee before exercise as a stimulant to improve their workout performance – if a workout is more effective, more energy may be expended. However, the benefit is largely thought to be short-lived, rather than long-term.
Caffeine has also been shown to speed up our metabolism, causing more energy to be burned while resting. However, this effect is relatively small and is not a suitable substitute for regular physical activity and a healthy diet.
Finally, coffee has a mild diuretic effect, which can lead to temporary water weight loss. This is water loss, not fat loss, and the weight is quickly regained when you re-hydrate.
Is it worth trying coffee for weight loss?
Losing weight can be influenced by various factors, so don’t get too enthusiastic about the coffee-weight link highlighted in this new study, or increase your coffee intake to unreasonable levels.
Most adults can safely consume around 400mg of caffeine a day. That’s the equivalent of two espressos or four cups of instant coffee or eight cups of tea.
If you are pregnant or breastfeeding, it is important to talk to your doctor before increasing your caffeine intake, because caffeine can be passed through to your growing baby.
If you need individualised weight guidance, talk to your GP or visit an accredited practising dietitian.
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Written by Lauren Ball, Emily Burch/The Conversation.