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13 January 2016
Many people want to eat more healthily but find it difficult to change their
diet. So what happened when Michael Mosley altered not what he ate, but when he
ate?
We've known for some time that altering the time at which you eat can affect
your weight and metabolism. At least if you are a mouse.
Based on mice studies, it seems the secret to improving your health is to
restrict the time window within which you eat, and by doing so extend the
amount of time you go without food.
A few years ago Prof Satchidananda Panda, from the world-famous Salk Institute
in California, showed that mice fed on a high-fat diet, but only allowed to eat
within an eight-hour window, were healthier and slimmer than mice that were
given exactly the same food but allowed to eat it whenever they wanted.
In a more recent study the same researchers again subjected hundreds of mice to
different lengths of daily fasts, ranging from 12 to 15 hours.
Image copyright Science Photo Library
Image caption Mice put on more weight depending on when they were fed
Again they found that the mice that went for at least 12 hours without eating
remained healthier and slimmer than those who ate the same number of calories,
but spread out.
But how well would this work in humans? To find out, Trust Me I'm a Doctor
recruited 16 volunteers for a 10-week study run by Dr Jonathan Johnston at the
University of Surrey.
His team measured the volunteers' body fat, blood sugar levels, blood fat
(triglycerides) and cholesterol levels at the start of the study. They were
then randomly assigned to one of two groups, the blues or the reds.
Image caption Some of the volunteers (with Trust Me presenter Chris Van
Tulleken) in the diet experiment
Trust Me I'm A Doctor is broadcast on BBC Two at 20:00 GMT on Wednesday 13
January 2016, or you can catch up afterwards on iPlayer
Take the test: When should I eat my main meal?
The blues, who were the control group, were asked to carry on as normal. The
reds were asked to stick to their normal diet but move their breakfast 90
minutes later, and their dinner time 90 minutes earlier.
This meant that for three extra hours each day they went without food
(fasting). Everyone kept a food and sleep diary to ensure that they were eating
the same amount as normal.
So why would crunching the time within which you eat change anything? Well,
there are two possible mechanisms.
Firstly, there are now plenty of studies which have shown that going for longer
periods of time without eating - fasting - is beneficial.
It also seems that your body deals with calories better at certain times of
day. According to Johnston, one of the worst times to load up with sugar and
fat is late at night, when blood levels of these substances are already high.
To test this out I decided to carry out a rather unpleasant self-experiment.
Image copyright iStock
Image caption Best tackled in the morning?
After an overnight fast I had some bloods taken, then at 10:00 I had a classic
British fry-up - lots of bacon, eggs and sausage. I had more bloods taken
directly after the meal and every half hour for the next few hours. And yes, it
did hurt.
Twelve hours later, at 22:00, I had my second meal of the day. It was exactly
the same meal as I had had for breakfast. Again my bloods were taken regularly
over the next few hours before I was eventually allowed to crawl into bed.
The blood tests showed that after my morning meal my blood sugar level returned
to normal pretty quickly, while the levels of fat in my blood began to drop
after about three hours. In the evening, however, after exactly the same meal,
my blood sugar levels stayed high for much longer and the fat levels in my
blood were still rising four hours after I finished eating.
So Johnston is right - our bodies really don't like having to have to deal with
lots of food late at night. A midnight snack will have a worse impact on your
body than the same food eaten earlier in the day.
There's an old adage: "Breakfast like a king, lunch like a prince and dine like
a pauper," and it appears to be true. If you must have that fry-up, have it for
breakfast.
But what about the main experiment, reducing the time period within which our
volunteers were allowed to eat? Well, at the end of 10 weeks, we gathered all
the volunteers together and repeated the tests.
Image copyright iStock
What we found is that the group who had eaten breakfast later and dinner
earlier had, on average, lost more body fat and seen bigger falls in blood
sugar levels and cholesterol than the control group.
So it was very positive result and the first randomised trial of this sort
carried out in humans.
Sticking rigidly to a reduced eating window may, for many people, not be
entirely practical. But there does seem to be benefit from doing it when you
can - and it is certainly a good idea to avoid the midnight cheeseburger.