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 .

Low calorie sugar substitutes cause hunger later on

A study has compared the effect of drinking a beverage of sucrose against three low calorie sugar substitutes and found those eating the substitute versions ended up overcompensating their intake to eat more calories at their next meal.

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The study took 30 men, of average BMI (21.7) and average body fat (17.6%).  The day before the study they would undertake no vigorous exercise, eat a standard evening meal and fast for 10 hours.  They would eat a standard breakfast and then have a drink mid morning which they had to consume within 15 minutes.  The drink contained either:

  • 0.44 g of artificial sugar aspartame
  • 0.63 g of natural monk fruit, a plant native to china
  • 0.33 g of stevia, a  natural sweetener extracted from plant leaves
  • or 65 g of sucrose (the amount normally found in commercially available sweetened drinks)

The drinks were flavoured with strawberry and dyed with a pink food colouring so they looked and tasted similar.

The participants would then rate their appetite and mood every 15 minutes until the first hour.  They would then receive lunch of fried rice and could eat as little or as much as they wanted.  The amount they ate was monitored and participants were asked to keep a food diary to assess what they ate for the rest of the day.  This was then added up to work out their daily intake of energy.

Blood was also taken to look at what happened to their blood glucose and insulin levels for 3 hours after the beverage intake.

It was a randomised crossover study, meaning all participants would come in for 4 separate days and try each of the sugar beverages over the study.  There were at least 5 non test days in-between these test days.  The participants were blinded so they didn’t know which drink they were having and so were researchers who assessed the outcomes and performed the analysis.

Overall the stevia and monk fruit were rated as slightly less sweet and more bitter than the sucrose and aspartame.

The low calorie sugars led to overcompensation of eating later on 

Those eating the non-sucrose sugars (aspartame, monk fruit and stevia) had less calorie intake from the beverage but made up for it by eating more lunch.  Over the entire day of food intake there was no difference between the amount of energy intake between the four drinks.  So while the low calorie sugars saved energy intake from the beverage it led to an increased appetite and overcompensation of food intake later in the day.


The sucrose caused the blood glucose and insulin to spike soon after consumption, this was more stable in the other sugars initially but they caused a sharper rise in the glucose and insulin response following lunch.  With no overall difference in glucose or insulin over the three hours between the four sugars.

The benefits of this research was that it was the first study to investigate the effect of monk fruit on energy intake, glycemic and insulin responses.

But it does have limitations, with only 30 participates, it wasn’t a large study and it was also a very short study looking at just one day per beverage, this doesn’t show you what would happen to energy intake and weight over weeks to months of intaking sugar substitutes.

While the study suggests that those who have a low calorie sugar substitute will eat more.  If someone was to drink a low calorie beverage instead of a high calorie beverage and then ensured they stuck to a strict intake following this, it could promote weight loss.

The study also relied on participants recording what they ate and drank for the rest of their day in a food diary, this may not have been accurately recorded.  To attempt to overcome this the researchers asked all participates to take photographs of what they consumed, it was reviewed by trained researchers and just one nutritionalist then entered the dietary data for consistency.