walk in bath with door

It must be too good to be true. Can you really enjoy a long soak in a hot bath and burn calories? Surely not…

Exercise physiologist Steve Faulkner PhD and his team at Loughborough University explored just that. His team monitored the blood sugar levels and energy expenditure of 14 participants who spent an hour on a bike ride, and an hour in a ‘hot’ bath.

Much to the bath-lovers delight, they found one hour in the tub reduced peak blood sugar levels while causing their energy expenditure to spike significantly. Bathing was also shown to have a similar anti-inflammatory response on the body to exercise.

Not bad for something you can do while leisurely lounging in the comfort and privacy of your own bathroom. It sure beats the treadmill!

How does it work?

Whilst the study didn’t completely reveal the specific mechanism behind the results, the researchers hypothesise the actions could be the work of a family of proteins in the body that turn on to deal with external heat. In other words, the body has to work harder to balance its internal temperature — which could be accelerating calorie burn.

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These ‘Heat Shock Proteins’ are produced by your cells in response to stressful conditions, so they become elevated during both exercise and heating to certain temperatures, like when you’re taking a sauna or in this case, a hot bath.

The healthy proteins also appear to improve blood sugar control by helping the function of the hormone insulin. In fact, the study discovered participants’ blood sugar levels were about 10 per cent lower after a bath than they were after the bike ride.

This is significant for many people trying to lose weight or struggling with metabolic conditions such as Type 2 Diabetes. If your blood sugar is well controlled, you are less likely to experience a sugar crash which may cause you to overeat or reach for those sweet snacks.

Scientists refer to raising the body temperature in this way as ‘passive heating’. As well as having an impact on calories and blood sugar, the study also showed that this passive heating triggers the anti-inflammatory response, something that also happens when we exercise. Since inflammation is at the root cause of many illnesses, this is a significant finding.

The Loughborough University study wasn’t a one-off result either. As far back as 1999, studies have been undertaken regarding passive heating. Philip Hooper of McKee Medical Centre in Colorado used subjects that had Type II Diabetes. The diabetics in his study showed improvement in blood sugar levels and even a reduced dependence on insulin.

Following this in 2015, Finnish researchers suggested that saunas may reduce the risk of heart attack or stroke by decreasing inflammation and improving the health of blood vessels. Additionally, the following year it was claimed that regular hot baths may also lower blood pressure.


Too good to be true?

Well, yes and no. It’s no doubt an interesting discovery, but you shouldn’t completely swap your regular exercise sessions for only hot baths if you can help it. Let’s take a closer look at the details of Dr Faulkner’s study:


1 hour bath time vs 1 hour on a bike

Whilst the 140 calories the participants burned in an hour in the bath isn’t anything to sniff at, it’s worth noting that’s about the equivalent of a packet of crisps. There were in fact, much better results from cycling for the same length of time during the experiment – using up the equivalent of 630 calories.

What’s more, if you really wanted to ramp up the calorie burn, you can get there much faster by lifting weights, heading out for a run or going to a HIIT class. Having a bath is certainly more relaxing and can burn more calories than simply sitting down on the sofa, but it is still quite a slow way to go about losing weight.


Body composition and activity

Although the results were somewhat promising, it is important to stress that all the participants in the study were men. Some of the men were also ‘overweight’.

This is important because you are always burning calories, even when you’re sleeping, but the amount of calories each person burns doing a certain activity varies depending on a few factors, including their age, weight and body composition.

People who are larger or have more muscle burn more calories, even at rest. Furthermore, men usually have less body fat and more muscle than women of the same age and weight, which means in general, men burn more calories. Finally, age is also a factor. As you get older, the amount of muscle you have tends to decrease and fat often accounts for more of your weight, slowing down calorie burning.

So the younger, heavier participants may have achieved a higher calorie burn than that of their older, slimmer counterparts in the study. This means we can’t all expect the same results when we take that long, hot bath.

Will making the water hotter result in more calories burned?

So now you’re thinking, maybe if I increase the temperature, I’ll burn more calories, quicker. This makes sense in theory, as we now know the body burns calories by working hard to maintain an appropriate internal temperature. Dr Faulkner’s study ramped up the heat to 40 °C but even at this temperature, heat exhaustion or heat stroke can occur!

How about keeping the temperature the same but staying in for longer? Well, we all know the raisin-skin inducing results of staying in water too long but whiling away the time in hot water can also cause heat-related illnesses.

If you experience any symptoms of dizziness, nausea, fatigue, extreme sweating, rapid heartbeat, confusion, and elevated body temperature your body is telling you it’s time to get out, and perhaps even seek help.


Are there any other side effects?

It is now common knowledge that a relaxing soak in the tub helps promote deeper and better sleep. However, hot baths don’t necessarily make you tired. In fact, if you increase the temperature, you can actually emerge from your hot bath energized as it gets your blood pumping.

Therefore, if you’re planning to add a hot bath to help you lose weight, it might be worth taking it a couple of hours before bedtime or even in the morning to help you get the most benefit.


To bathe or not to bathe?

Whilst burning calories in the bath doesn’t quite compare to the calories you’ll expend in a vigorous workout, the passive heating method doesn’t have to be completely ruled out. Rather than having to choose between a hot bath or exercise, it seems the trick is to combine these methods.

Exercise provides many other vital health benefits than simply just burning calories that aren’t achieved passively. Moving your body helps to keep your bones, joints, and muscles strong, supports your mental health, improves lung capacity, supports your immune system, and benefits your overall wellbeing.

Nothing really replaces the multitude of advantages you will get with a regular exercise regime. But there’s no need to throw the baby out with the bathwater! Why not lounge in a hot bath after your workout and add a few extra bonus calories and health benefits to your daily counter.

Alternatively, if your health considerations for whatever reason make it difficult for you to engage in physical exercise, then passive heating could be a helpful recommendation and a better option than hours scrolling on your phone whilst lounging on the sofa.