Intermittent fasting: Does it work for weight loss?
What is Intermittent Fasting?
Intermittent fasting (IF) is when a person delays the first meal of the day. This increases the natural fast that occurs during sleep. By doing so, people can in theory, lower the amount of calories consumed in the day. Intermittent fasting is not performed every day. This allows people some days where they consume their usual required calories.
Intermittent fasting (IF) differs from other calories restricted diets. Calories restricted diets are when calories are restricted every day, to promote weight loss. In scientific research they are referred to as continuous energy restriction (CER) diets.
A common IF diets include the 5:2 which allows 5 normal days and 2 restricted days of 500-700 calories. As well as alternate day fasting, which involves fasting every other day.
Why try intermittent fasting?
A person could try a fasting diet for a variety of reasons. Some may be backed by science, others maybe not.
- Weight loss
- Improve general health
- Appetite control
- Improve insulin levels
- Struggle following other types of energy restriction
- Easier to follow when very social
The main question, does IF help with weight loss? There is not one quick fix when it comes to achieving a healthy weight. However, there are many methods we can use to help us get to a healthy weight range, IF may be one of these.
Good news is that IF can help people achieve weight loss goals when done safely. Several systematic reviews in the past 5 years have found significant evidence to support this manner of eating for weight loss.
A 2016 article (1) published in the journal Ageing Research Reviews found all participants in a variety of studies lost significant amounts of weight. However, it’s important to note, that in the randomised controlled trials (RCTs) the article used, the IF groups and CER lost similar amounts of body weight.
A 2018 systematic review (2) published in the Journal of Translational Medicine found similar results, that IF was successful in weight loss but it was also similar to CER.
What does this mean? It means that IF could be as effective as CER, and a person who struggles with CER, may find intermittent fasting fits in well with their lifestyle, and maybe easier to maintain.
Is the weight loss different?
Are there any differences between the type of weight loss IF promotes, compared to prolonged calorie deficit? It cannot be conclusively said there is a difference, however one study found differences.
Harvie et al. 2013
Participants in the low carbohydrate IF arm of this randomised controlled trial (RCT) (3) had increased body fat loss in the low carbohydrate IF group compared to the CER group.
The RCT with was conducted on 115 overweight women, which does not make it easily generalisable to the population. The study also had a 23% dropout rate, perhaps due to the specific diets that had differing macro nutrient content.
The group who had a daily energy restriction followed a Mediterranean (high protein, med carbs, med fat) diet, while the IF groups, of which there were two, were randomised to a diet high protein and fat (15% carbs, 35% protein, 50% fat) on fast days, and Mediterranean diet on normal eating days.
Is this reflected in other literature?
Cioffi et al. 2018 systematic review could not find any differences in fat loss between IF and CER. This could mean that the Harvie at al. study is a false correlation, or that the specific macronutrient content they used generated this specific difference in increased fat loss between the different groups.
Are there other benefits?
Other benefits often cited for using an IF diet include:
- Stimulating brown adipose tissue in white adipose tissue
- Increased insulin sensitivity
- Lowers leptin
- Increases growth hormone, ghrelin, adiponectin levels
- Reduces inflammation and oxidative stress
It’s important to note that all of these benefits have been demonstrated in animal modals, the majority have not been confirmed in human trials.
What can be confirmed?
So far it appears there may only be an effect on insulin levels, Cioffi et al., however found this effect only in trials that used the 5:2 diet regimen. Other studies found that improvements in insulin levels and blood glucose are similar to improvements found in CER.
With current research, it’s only possible to suggest that IF is just as beneficial in short term weight loss as CER, and although specific forms of the diet (5:2) may improve insulin levels, most other claims are not supported.
What does it all mean?
So does this mean there’s no point to trying IF, if you’re struggling with reaching or maintaining a healthy weight range. Not at all, in fact if you are struggling with the tried and tested constant calorie counting every day, this may be the method of eating to try.
Important things to note, quality of diet is still incredibly important, on the days a person eats less and normally. Maintaining a diet high in vegetables, legumes, lean meats and other healthful foods will help set up healthy habits.
Normal days are not a free for all with food, this may make the fast days worthless and it can also lead into a dangerous restriction and binge cycle. Again by focussing on healthful vegetables, fruit and quality proteins this can be avoided.
Try to plan social engagements on days off fasting so that you can enjoy being social with family and friends. Lastly exercise is a key component in most of the trials looked at, and should be incorporated to form long term healthy habits.
1. Impact of intermittant fasting on health and disease processes. Mattson, M. P., et al. s.l. : Ageing Research Reviews, 2016, Vol. 39. 46-58.
2. Intermittent versus continuous energy restriction on weight loss and cardiometabolic outcomes: A systematic review and meta-analysis of randomized controlled trials. Cioffi, I., et al. s.l. : Journal of Translational Medicine, 2018, Vol. 16. 371.
3. The effect of intermittent energy and carbohydrate restriction v. daily energy restriction on weight loss and metabolic disease risk markers in overweight women. . Harvie, M., et al. 8, s.l. : British Journal of Nutrition, 2013, Vol. 110. 1534-1547.