‘Exercising before breakfast lets you lose extra weight’, the Daily Mail reports, perhaps prompting readers to drop their sausage sarnies and pop out for a jog.
However, the Mail’s sweeping headline is actually based on a very small study of just 10 overweight men.
In the study, researchers compared the effects of a single session of exercise performed either before or after breakfast, and how this affected the metabolism (the chemical reactions that provide the body with energy) of fats and carbohydrates afterwards. The researchers also carried out a ‘control’ experiment in which the men did no exercise at all. The 10 men each carried out all three of the experiments to see which one caused the greatest breakdown of fat and carbohydrate and the greatest overall energy expenditure.
The researchers found that fat and carbohydrate breakdown (as measured by blood tests) and overall energy expenditure were greater in the exercise-before and exercise-after breakfast conditions when compared to the no exercise condition – which hardly seems surprising. However, they also found that fat breakdown relative to carbohydrate breakdown was greater, and overall energy expenditure was more, when men exercised before breakfast, compared to when they exercised after breakfast.
Although these results sound promising, they should also be viewed with caution due to the extremely small sample size which could mean that the differences are purely down to chance. Also, despite the headlines, the findings tell us nothing about weight loss, which has not been examined – just the metabolism of fat and carbohydrate in the blood.
Importantly, in order to give meaningful results, it will be necessary to study people under normal living conditions – outside of the experimental laboratory setting.
Where did the story come from?
The study was carried out by researchers from the University of Glasgow and the Universiti Kebangsaan, Malaysia. Sources of funding were not reported and the authors report no conflicts of interest.
The study was published in the peer-reviewed British Journal of Nutrition.
The headline in the Daily Mail is misleading and readers may be given the impression that exercising before breakfast results in long-term weight loss, which is not the case. Once past the headline, the story is reported appropriately, although the findings are slightly exaggerated.
What kind of research was this?
This was an experimental study, based in a laboratory, which looked at the effects of a single session of exercise performed either before or after breakfast on fat balance and metabolism over an 8.5 hour period. As a comparison, a third experiment looked at the effects of performing no exercise at all.
An experimental study is any study in which the conditions are under the direct control of the researcher. This usually involves giving a group of people an intervention that would not have occurred naturally. Experiments are often used to test the effects of a treatment in people and usually involve comparison with a group who do not get the treatment (controls).
A more useful study design would have been a randomised controlled trial (RCT), which is the best type of design to determine whether a particular treatment is effective. Such a trial would ideally look at a much larger sample of people than the 10 included in this study, and be conducted over an extended period of time to look at more meaningful, longer-term effects of the exercise approaches, such as weight change and other health outcomes.
What did the research involve?
The researchers recruited 10 men who had an overweight body mass index (BMI) above 25kg/m2. All of the men were recruited through advertisements and reported low levels of physical activity – less than one hour a week of moderate-to-vigorous activity. The men were all non-smokers with no known history of cardiovascular disease or diabetes and were not consuming any type of specialised diet, or taking medications to interfere with their metabolism and appetite.
None of the men were considered to be ‘restrained eaters' (measured using two different eating behaviour questionnaires).
Each man completed three different 8.5 hour experiments set in the university laboratory with a one- to two-week rest period in between each experiment. The order in which the men did each experiment was allocated randomly. The three experiments were:
- exercise before a breakfast meal
- exercise after a breakfast meal
- control condition (no exercise)
For the exercise before breakfast experiment, exercise began at 9am and participants completed 60 minutes of treadmill walking at an intensity of 50% of their maximal oxygen uptake. They walked at an average speed of 5.5km/h on a gradient of 4.3%.
Breakfast was given 30 minutes after exercise completion and participants underwent a further seven hours of observation.
For the exercise after breakfast experiment, participants rested for one hour, from 9am to 10am, received the standardised breakfast at 10.30am and carried out the same exercise as described above from 11am to 12pm. In the control group, participants did not undertake any exercise and remained rested from 9am to noon. They received the standardised breakfast at 10.30am.
A buffet lunch was provided to all participants 3.5 hours after breakfast where participants were told to eat until they were comfortably full.
Consumption of this lunch was measured – and none of the participants were aware of this (if they had been aware they were being monitored they may have eaten more or less than usual).
The researchers took regular blood samples over the 8.5 hour period and used laboratory methods to look at energy expenditure and fat and carbohydrate break-down. They also took into account the energy intake of the buffet lunch.
Before the experiments, participants were asked to weigh and record their daily food and drink intake and were asked to replicate this diet in the two days preceding each of the three experiments. The men were also asked to refrain from alcohol and planned exercise and to maintain their usual day-to-day activities during the experimental period.
What were the basic results?
The 10 men had an average age of 28.1 years and an average BMI of 29kg/m2 (which would be considered as being overweight, but not clinically obese). The researchers found that:
- Fat and carbohydrate balances were significantly lower in the exercise before breakfast and exercise after breakfast experiments, compared to the control experiment (that is, fat and carbohydrate breakdown was greater and overall energy expenditure was more in both of the experimental conditions).
- In the exercise before breakfast experiment, compared to the exercise after breakfast expermient, fat breakdown was greater than carbohydrate and overall energy expenditure was significantly more.
- There were no significant differences in energy intake of the buffet lunch between the experiments.
How did the researchers interpret the results?
The researchers conclude that there may be an advantage for body fat regulation and fat breakdown in exercising before breakfast compared with after breakfast.
Dr Gill, one of the researchers, is quoted as saying, ‘the biggest difference is between doing nothing and doing something’. He also said that, ‘if you are going to do something, then there is a slight advantage in doing it in a fasted state. But if you find you keel over because you can’t do exercise before you have your slice of toast, then do it afterwards. You are still going to get a huge benefit’.
Overall, this study provides limited evidence on the timing of exercise either before or after the breakfast meal and its effect on fat loss. The authors do note that a ‘degree of caution is advised’ in interpreting these ‘short-term findings’ and that further study is needed to determine whether the results from the present short-term laboratory study extend over the long-term.
There are some important limitations to this study, some of which are noted by the authors:
The size of the study was very small, with only 10 participants, all of which were men with an average age of 28 years. This means that the findings may not be effectively generalised to groups outside of those included in this study, including women and older men. Larger studies that include men and women from a range of ages and ethnicities are required to be able to draw firm conclusions. The results here could be purely due to chance.
Artificial experimental setting
The setting of this study was in a laboratory, where the opportunity for spontaneous activity by the participants was limited (for example, breakfast was set at the rather late hour of 10.30am). The authors note that such spontaneous activity under ‘free-living’ conditions may offset the differences observed in the fat balances between experiments and that longer term studies are required to look at this possibility.
Intervention may not reflect real daily activity
The exercise in this study was limited to treadmill walking, which the participants all rated as ‘light’ exercise. This may not reflect the participants’ actual day-to-day physical activity levels or the intensity of activity they would usually perform. Further study is required to determine whether these findings are similar under ‘normal’ living conditions, outside of the laboratory setting.
Considerably more reliable and meaningful results could be drawn from a randomised controlled trial, which randomised a sample of people to different exercise programmes and then followed them over a longer period of time to look at weight change and other health effects.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
Daily Mail, 29 October 2012
Links to the science
British Journal of Nutrition. Published online October 26 2012