Does fasting result in weight loss?

A study has found that people who restrict their eating to between 10 am and 6 pm and fast for the other 16 hours of the day lose 3% of their body weight.

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The study which was published in the journal: Nutrition and Healthy Aging took 23 obese people who had to meet the following criteria:

  • BMI between 30-45 (this means obese)
  • Aged 25-65 years
  • Not going through the menopause
  • Sedentary to lightly active
  • Stable weight over the last 3 months
  • Non-diabetic
  • No history of heart disease or stroke
  • Non-smoker
  • Not a shift worker
  • Not taking medication for weight loss or to reduce lipid or glucose levels

The people in the study were instructed to only eat in the 8 hour window, but they could eat whatever they wanted and didn’t need to calorie count.  Outside of these hours they were allowed to drink water, black tea, coffee and diet soda.  The researchers took measurements at the beginning and end of the 12 week trial to assess their:

  • Body weight
  • Blood pressure
  • Heart rate
  • Total cholesterol, LDL (bad cholesterol), HDL (good cholesterol) and triglyceride levels
  • Fasting glucose
  • Fasting insulin

These results were compared to 23 individuals who has already taken part in a trial several years ago, between 2011-2015.  They were the control group and had been asked to maintain their weight and not to change their levels of eating or physical activity.

The people in the 8 hour eating group noted down when they started and stopped eating each day.  On average it was found that the participants had been adherent to the strict eating window on six out of seven days a week.

By the end of the trial six people in the fasting group had dropped out, leaving just 17 people who had completed the 12 week study.  Although it should be noted that none of the dropouts said they left due to the diet.

So what happened to these 17 people compared to those in the control group?

  • They ate 300 less calories a day
  • They lost 3% of their body weight
  • Their BMI reduced
  • Their systolic blood pressure dropped by 7 mmHg

Sounds great did anything not improve?

Well some things stayed the same when compared to the control group, including:

  • Macronutrients, cholesterol and fibre intake
  • The amount of steps walked a day
  • Fat mass, lean mass and visceral mass
  • Diastolic blood pressure
  • Heart rate
  • Total cholesterol, LDL, HDL, triglycerides, glucose and insulin

But still weight loss right? Does this mean we should all start fasting… No, not so fast [pun intended]…

There are negatives in this study…

  • Only 17 people followed this eating regime for duration of the 12 week study.
  • The drop out rate was quite high with 1 in 4 who started the diet giving it up.
  • We only know that the participants were adhering to this eating window from their self-reported records, which is not an accurate assessment.
  • The study only looked at healthy obese individuals, therefore it doesn’t represent what would happen if non-obese people took on this eating regime or how it would impact on a obese person who for example had diabetes.
  • The best design for this type of experiment is a randomised control study, where people are randomly allocated to an eating regime.  This was not one of those trials, instead it compared the people who were fasting to those who were in a different study several years beforehand.  In this time the general population’s knowledge of weight control may have shifted and the availability of food and seasons may have been different.
  • The study was short as it only took place over 12 weeks, so we don’t know what would happen in the longer term.

What about those sugar free drinks could they have helped or hindered this weight loss?

People were allowed to drink caffeine in the form of black tea and coffee outside of the fasting hours which can affect the body clock.  The principle of fasting is based on how the daily body clock regulates metabolism.  So while the calories in these drinks are low, the impact on the body clock and therefore metabolism can be significant.  I.e. another reason to cut down on your caffeine intake if you want to try this eating regime out.

Is the 10am- 6pm window the best time to eat if I want to fast for 16 hours a day?

This is a good window for fitting in three meals with a typical 9-5 job, as you can have a late breakfast, early lunch and early evening meal.  However other studies have found that eating larger meals early in the day can produce better weight loss than eating your main meal in the evening.  So more studies are needed to look at the effect of shifting this eating window to earlier in the day .