I found it really interesting researching intermittent fasting diets for the February edition of NHD Magazine (PDF available at the end of this post). It was fascinating to explore the actual evidence behind this increasingly popular trend, especially as this has changed my initial opinion of these diets.
“Intermittent fasting” involves alternating cycles of eating and fasting without specifying which types of foods can be eaten on non-fasting days. On a fasting day a person’s intake is often limited to non-calorific fluids such as water, tea, coffee and diet drinks or it may allow a very restricted amount of daily calories. These diets have a strong media presence and have been linked with many celebrities such as Beyoncé, Chris Martin, Ben Affleck and Jennifer Lopez.
Different types of fasting diets:
- The 5:2 diet – limits calorie intake on two days per week to 500 kcal per day for women and 600 kcal per day for men, and advises that the usual recommended daily calorie intake on the five non-fasting days (i.e. roughly 2000 kcal for women and 2400 kcal for men).
- The 6:1 diet – usual dietary intake for six days per week and a 24 hour fast on one day per week.
- The Eat-Stop-Eat diet – a variation of the 6:1 diet which can include two 24 hour fasts per week.
- The 16:8 diet – fasting for 16 hours per day by consuming all meals within an eight hour window.
Common Arguments for Fasting Diets:
- Our ancestors would have had periods of fasting depending on food availability.
- Some people prefer an ‘all or nothing’ approach when trying to restrict calories for weight loss compared to a ‘moderation’ approach.
- Promoters of intermittent fasting report a host of long term health benefits such as: increased longevity, improved metabolic health, improved weight loss and a reduction in diseases such as heart disease and type 2 Diabetes.
Common Arguments against Fasting Diets:
- May lead to tiredness, headaches, lack of concentration and poor mood.
- May be dangerous if unsupervised by a medical professional depending on the person’s age, medical history and lifestyle.
- Not a very balanced approach, potential to interfere with metabolic rate.
- An overall lack of evidence and no significant differences in outcomes found between more moderate daily restriction and this extreme fasting approach.
But what does the evidence say?
Intermittent Fasting and Weight Management
Data from randomised control trials up to one year in length have found that there is no difference in weight management outcomes from either eating regularly or intermittent fasting1.
For example, a recent systematic review reported that although intermittent fasting resulted in significant weight loss (0.2-0.8kg per week), there was no difference in the results when this method was compared to continuous energy restriction when the weekly calorie restriction was similar between groups. Therefore this study concluded that intermittent fasting “may be an effective alternative strategy for health practitioners to promote weight loss for selected overweight and obese people”2. This is supported by a systematic review by Clifton et al. (2016)3 which found that intermittent fasting and continuous energy restriction produce similar results in terms of weight loss and improvements in cholesterol, blood glucose and insulin levels.
In terms of specific outcomes Hankey et al. (2015)4 identified a mean weight loss of 2.6%– 8.9% (after 1- 6 months respectively) as well as improvements in blood pressure and insulin levels as a result of intermittent fasting interventions. Interestingly this systematic review also found that these trials had an average 20% drop out level; which is relatively low compared to other weight loss interventions which can range from 10% – 80%4,5.
However, all of the above studies highlighted that further research is needed and that the longer term effects of this method remain unclear2-4. There is also some evidence which has reported that rather than true weight loss including a loss of fat, fasting can result in a higher loss of intracellular fluid6.
Intermittent Fasting and Aging
A recent systematic review of human studies found that intermittent fasting may improve inflammatory cytokines profile, such as adiponectin:leptin ratio and changes in insulin-like growth factors, which may have a positive effect on aging7. Mattson et al. (2016)8 also reported that intermittent fasting is involved in the “activation of adaptive cellular stress response signalling pathways that enhance mitochondrial health, DNA repair and autophagy”. However, overall large scale research in this area appears limited9.
The Effect of Meal Patterns
An important issue when weighing up the pros and cons of intermittent fasting is whether our meal patterns affect our health; does it make a difference if we eat regularly throughout the day or not?
There is a lack of evidence related to meal frequency and subsequent appetite regulation and calorie intake; however based on small short term randomised control trials no significant difference in appetite regulation has been identified when those who eat frequently were compared to those with a more irregular meal pattern10.
There is some limited evidence from randomised control trials that having fewer meals in the day may increase the metabolic cost of digesting food, as the thermic effect of food (i.e. the energy the body uses above the resting metabolic rate to digest and store food) may be lower when meals are eaten more regularly over the course of a day rather than in one go; although there is not enough evidence to make any practical recommendations based on this11. Yet, it is interesting to note that there is also fair level of evidence that skipping breakfast is associated with having a higher BMI and being overweight or obese; although no direct cause and effect relationship can be assumed from this12. Skipping breakfast for 2 weeks has also been found to raise LDL cholesterol and produce insulin resistance in a small study by Farshchi et al. (2013)13.
The body of evidence in relation to cholesterol level and meal pattern has found that eating more regularly can modestly decrease total and LDL cholesterol levels among adults with normal cholesterol levels to begin with; however the evidence in those with high cholesterol is more limited and doesn’t display this relationship. A limited level of evidence based on data in non-diabetic subjects has also found that a more regular meal pattern improves blood glucose and insulin stability14.
Potential Risks of Fasting
Although some earlier studies conducted in rodents found that fasting can increase the amount of liver enzymes which metabolise toxins, subsequent investigation found that this was due to liver shrinkage rather than an improvement in enzyme levels and concerns with fasting were identified such as an increased susceptibility to toxin damage from reduced antioxidant levels present in the liver14-16. A review of the evidence related to fasting from PEN (2013)17 concluded that:
“Close examination of the evidence regarding the health effects of fasting does not support the denial of eating as a health-promoting activity. In fact, fasting may actually be deleterious to health by increasing insulin resistance while decreasing important liver detoxifying enzyme functions, muscle mass, and body nutrients, and one’s ability to work and exercise”.
This report also highlights further issues related to fasting such as: a reduction in immune function and an increase in non-dehydration related headaches.
Similarly, a systematic review by Horne et al. (2015)9 of the potential health benefits or health risks of intermittent fasting found that although a few randomized controlled trials and observational studies have identified benefits from fasting “substantial further research in humans is needed before the use of fasting as a health intervention can be recommended”.
In terms of weight loss and cardiovascular health current research indicates that intermittent fasting and continuous calorie restriction seem to produce similar outcomes, so there may be an argument for considering intermittent fasting as a weight loss method, as there is no one universal solution when it comes to weight management. However if this was to be considered as an option it would be important to highlight the potential risks related to fasting and low calorie intakes, as well as the overall limited evidence base and uncertainty about the longer term outcomes of intermittent fasting.
It is interesting to see potential benefit of intermittent fasting relation to healthy aging; however more research is also needed in this area.
Similarly there is a scarcity of evidence when it comes to the effect of meal patterns on health; but it is interesting that although eating breakfast is associated with a healthier weight and a regular meal pattern may encourage better cholesterol and glycaemic levels, at present there does not seem to be enough evidence to suggest that a regular meal pattern improves appetite regulation or the metabolic cost of digesting food.
Overall, there is quite a lot of contrasting findings in relation to intermittent fasting but the one common message is that more evidence is needed to examine the effects of this method; in particular longer term human studies. However current research indicates that intermittent fasting can be an effective method to promote weight loss, so this may be worth considering for some individuals while weighing up the pros and cons of intermittent fasting as discussed in this article.
- PEN (2015) Available via: https://www.pennutrition.com/KnowledgePathway.aspx?kpid=15329&pqcatid=146&pqid=8776
- Davis et al. (2015) “Intermittent energy restriction and weight loss: a systematic review” (http://www.nature.com/ejcn/journal/v70/n3/full/ejcn2015195a.html)
- Clifton et al. (2016) “Weight-Loss Outcomes: A Systematic Review and Meta-Analysis of Intermittent Energy Restriction trials Lasting a Minimum of 6 Months”
- Hankey et al. (2015) “A Systematic Review of the Literature on Intermittent Fasting for Weight Management”
- Jiandani et al. (2016) “Predictors of early attrition and successful weight loss in patients attending an obesity management program” (https://bmcobes.biomedcentral.com/articles/10.1186/s40608-016-0098-0)
- Siervo et al. (2010) “Use of the cellular model of body composition to describe changes in body water compartments after total fasting, very low calorie diet and low calorie diet in obese men” (http://www.ncbi.nlm.nih.gov/pubmed/2014 2822)
- Lettieri-Barbato et al. (2016) “Effects of dietary restriction on adipose mass and biomarkers of healthy aging in human” (http://www.aging-us.com/article/YmhYbE6ipoL6ykpfz/text
- Mattson et al. (2016) “Impact of intermittent fasting on health and disease processes” (https://www.ncbi.nlm.nih.gov/pubmed/27810402)
- Horne et al. (2015) “Health effects of intermittent fasting: hormesis or harm? A systematic review” (https://www.ncbi.nlm.nih.gov/pubmed/26135345)
- Kulovitz et al. (2014) “Potential role of meal frequency as a strategy for weight loss and health in overweight or obese adults”
- Tai et al. (1991) “Meal size and frequency: effect on the thermic effect of food”
- Horikawa et al. (2011) “Skipping breakfast and prevalence of overweight and obesity in Asian and Pacific regions: a meta-analysis” (https://www.ncbi.nlm.nih.gov/pubmed/21925535)
- Farshchi et al. (2005) “Deleterious effects of omitting breakfast on insulin sensitivity and fasting lipid profiles in healthy lean women”
- Jones et al. (1999) “Meal frequency influences circulating hormone levels but not lipogenesis rates in humans.”
- Hong et al. (1987) “The induction of a specific form of cytochrome P-450 (P-450j) by fasting” (http://www.ncbi.nlm.nih.gov/pubmed/3827 895)
- Fry et al. (1999) “Depression of glutathione content, elevation of CYP2E1-dependent activation, and the principal determinant of the fasting-mediated enhancement of 1,3- dichloro-2-propanol hepatotoxicity in the rat” (http://www.ncbi.nlm.nih.gov/pubmed/1041 8953)
- PEN (2013) “Is Fasting Healthy?” (http://www.pennutrition.com/docviewer.aspx?id=10895)