Shaun Ward MSc ANutr
What Is Intermittent Fasting?
Intermittent fasting is a type of eating protocol based around short ‘eating windows’ where food may be consumed, followed by much longer periods without consuming food – known as fasting.
Unlike other traditional diets, intermittent fasting places no emphasis on food quality or quantity, but rather focuses on eating frequency. No restrictions are placed on what can be eaten, or how much, but only when meals are to take place. For this reason, many people do not view it as a diet, but more of a structured pattern of eating.
The minimum length of a fast on any method is 16 hours, as less than this would incur eating habits too similar to normal schedules that it would not be labelled as a form of intermittent fasting. The 16:8 method is currently the most popular form of intermittent fasting, as most users tend to find that just by skipping breakfast you can achieve this method.
Advocates of the diet view it as a more natural form of eating compared to today’s conventional breakfast, lunch and dinner; not to mention the many snacks in-between.
From an evolutionary perspective, food availability was comparatively scarce, and people would have extended periods without any food but would eat extremely large portions when it became available. Intermittent fasting attempts to mimic this routine.
The Different Methods Of Intermittent Fasting
There are many different types of intermittent fasting, with variances in the length of eating and fasting windows - we've broken down the main types below:
The 16/8 Method
As previously mentioned, the 16/8 method is by far the most popular as it's easily implementable and not particularly difficult to stick to over a long term period. It involves a 16-hour fasting window, followed by an 8-hour eating window (these times for eating can be selected by the user dependent on their schedule).
There is no set daily calorie target, however it goes without saying that you should not be "overeating". It is recommended that those fasting should be consuming their caloric maintenance number - using a total energy expenditure calculator should help you establish what that amount should be.
The 5:2 Method
This method is arguably the second most popular method (as it's a fully-fledged diet plan in and of itself). It involves 5 days of regular eating capped at your daily caloric maintenance, followed by 2 days of fasting where users are allowed no more than 500kcal per day.
While many people sing the 5:2 diet's praises, there's a degree of skepticism on whether or not it allows followers to experience the full health benefits of intermittent fasting; it's a great way to steadily lose weight, but if you're aim is autophagy and general health improvements, you'll likely be better off following a 16/8 method.
If you hadn't already guessed by the name, alternate-day fasting involves 1 day of unrestricted eating followed by 1 day of fasting - certain programs allow the follower and small amount of calories on fasting days, while others insist the day should be a water-only fast.
The Warrior Diet
It may have a completely different name, but in actuality the warrior diet is simply a modified 16/8 method. Rather than fasting for 16 hours, this program extends the fast to 20 hours. Followers are given a 4-hour eating window in the evening.
There is no daily calorie target for the warrior diet method, however as the eating window is so short, most people will find they usually end up eating under their maintenance number, which is why it's recommended for those who want to use intermittent fasting specifically for the purposes of losing weight.
This plan simply involves the follower completing a 24-hour water fasts (i.e. zero calories consumed), once or twice per week, with the strict rule that the fasts should not be consecutive.
On the remaining 5-6 "non-fasting days", followers are instructed to limit their calorie intake to no more than 2500 kcal per day (or no more than 2000 kcal per day for women).
This method of fasting is primarily focused on helping the follower lose body fat, rather than helping them experience the full health benefits that true intermittent fasting can offer.
Is Fasting Safe?
or the vast majority of healthy adults, adopting this any of the methods will be completely safe, and may even be beneficial to your health over the long term, as long as you ensure that you're giving your body all of the vitamins, minerals and nutrients it requires to remain completely functional.
Fasting Isn't Advisably In Certain Circumstances
Despite the advocates singing its praises, here are still certain types of people who should consider avoiding any sort of fasting entirely, these include:
- Pregnant or breastfeeding women.
- Those in ill-health.
- Those who are underweight.
- Those who are under the age of 18.
- The elderly.
The Potential Benefits of Fasting
Aside from the claims it's useful for burning body fat, one of the biggest appeals of intermittent fasting is the general health benefits the program has. We've compiled and broken down the most common benefits below to help you gain a better understanding of just what you could potentially experience from sustaining such an eating regime:
Improved Glucose Control & Insulin Sensitivity
Intermittent fasting is associated with improved fasting glucose, glucose control, plasma insulin, and insulin sensitivity.
The diets effect on improving glucose metabolism by lowering insulin levels is claimed to have an anti-diabetic effect, and could potentially be a future treatment for diabetic patients.
The improvement in glucose metabolism is partly due to its ability to switch the substrate that is preferentially used for energy. In other words, the body becomes less reliant on carbohydrate, and more likely to use fatty acids as a fuel source. This is evidenced by a decline in blood glucose concentrations, and a significant increase in whole-body lipolysis and fat oxidation, stemming from reduced plasma insulin concentrations.
Increased fat oxidation may also be partly due to an increased number of mitochondrial fatty-acid transporters .
Further, the diets avoidance of repeated eating periods may promote improvements in insulin sensitivity via the reduced frequency of fat accumulation in the blood, which is one of the factors that causes insulin resistance; cells unable to effectively use insulin.
It Can Increase Longevity
Interestingly, one of the main reasons why it is thought to improve longevity is by transiently increasing stress on the body, which subsequently improves the protective mechanisms to stressors within cells.
To make a relevant comparison, it is similar to taking a vaccine whereby an individual improves their immunity to a particular disease by allowing a weakened form of the disease-causing microorganism into their body.
One of these protective adaptations is the increased expression of enzymes that contribute to cellular regulation, which protect cells from the damaging effects of free radicals and reactive oxygen species.
Other cellular adaptations involve advanced DNA repair and autophagy - a regulated mechanism of a cell that removes the unnecessary components.
All these factors may contribute greatly to enhancing the immune system, reducing inflammation, and promoting longevity.
Is It An Effective Weight Loss Method?
As the typical intermittent fasting program reduces the total number of hours available for eating and, thereby, likely reduce overall energy intake, it can be viewed as a good method to lose weight.
However, as most intermittent fasting protocols indirectly reduce food intake, it is hard to assess the isolated metabolic effects of this style of eating on weight loss.
Systematic reviews of it conclude that it is effective for short-term weight loss in all populations, with more long-term trials needed to assess dietary adherence and maintenance of weight loss .
This is a crucial point considering it is ultimately the degree of dietary adherence and sustainability, rather than the type of dietary strategy, that can successfully predict weight-loss outcomes.
Some data shows that an intermittent fasting protocol may produce higher dietary compliance, although this is certainly up for debate seen as fasting approaches tend to increase feelings of persistent hunger and decrease fullness .
Typical weight reductions from intermittent fasting are:
- 5% body weight loss after 4 weeks 
- 5% body weight loss after 12 weeks 
- 3-7% body weight loss after 3-12 weeks 
- 9lb’s body weight loss after 3-12 months 
At present, it has not been confirmed whether the effects of intermittent fasting are solely due to the calorie-restricted nature of the diet, and its effect on weight loss, or if the principle of fasting does in fact produce isolated benefits to metabolic pathways and adaptations.
The majority of publications report that the overall metabolic benefits of fasting regimens are not superior to those of continuous energy restriction .
However, counter evidence suggests that the mechanism for greater fat loss on intermittent fasting cannot only be explained by changes in the quantity or quality of diet, and that the difference in meal distribution must be having an influence on results.
The main advocated mechanism by which this may occur is by an increase in resting (or basal) energy expenditure. This increase in energy expenditure is hypothesized to be due to two reasons; 1) Elevated plasma adiponectin (protein hormone) that indirectly stimulates mitochondrial expression and content, and 2) Heightened thermogenic (heat production) response from an increased adrenaline release caused by the stressors from fasting.
Short-term fasting has been shown to directly increase resting energy expenditure by 3-14% , with some studies using isocaloric fasting diets (same energy intake between groups) concluding that intermittent fasting results in greater weight loss compared to continuous dieting . However, the majority of the scientific literature opposes this position.
Are The Health Benefits Only Because It Can Help You Lose Weight?
As most intermittent fasting studies often co-exist with weight loss, it is hard to factor how much of a result the weight loss is having upon improving biomarkers of health.
Seen as a reduction of energy intake from any dietary method can in theory produce the same health benefits as are seen in studies on intermittent fasting, it is near impossible to differentiate what factors are responsible for improving health and why.
This is increasingly difficult to assess when ~75% of intermittent fasting trials observe a statistically significant weight reduction .
It is noted that some factors such as insulin sensitivity have improved from intermittent fasting in the absence of weight-loss, although studies with these results are often criticized for their poor methodologies.
Future research is needed where participants are controlled to maintain weight, so an analysis on health biomarkers independent from weight loss can be conducted.
Is Intermittent Fasting Good For Athletes & Bodybuilders?
Intermittent fasting has limited studies analyzing its effect on athletic populations, specifically regarding performance and optimal body composition.
From the data so far, intermittent fasting seems to reduce body weight yet not adversely affect lean mass retention or muscular improvements in resistance-training athletes.
Some research has highlighted a concern for the diets ability to maintain lean body mass, but the majority of these studies did not utilize exercise interventions and therefore should be interpreted with caution.
In general, an intermittent fasting regimen is appropriate for athletes during maintenance phases of training in which the goal is to maintain muscle mass while reducing fat mass.
The main factor against intermittent fasting for athletes is that the limited eating window will not allow an athlete to maximize their anabolic responses, specifically regarding muscle protein synthesis.
This is because increases in muscle protein synthesis from a single meal generally only last ~3 hours before returning to baseline, despite a continued availability of circulating amino acids and sustained signaling of anabolic pathways – this has been named the ‘muscle full’ effect.
In other words, there is a maximum anabolic response per individual meal, or protein feeding, which is achieved by ~30-40 grams of high-quality protein.
For this reason, it is advised that athletes consume a protein source (~30 grams) every 3-4 hours in order to continually keep muscle protein synthesis, and anabolism, as high as possible throughout a 24-hour period.
Based on this, intermittent fasting is limited in maximizing anabolic responses, and although it can still lead to body composition and performance adaptations, provided daily protein intake is sufficient, it is by no means an optimal protocol for athletes.
However, although intermittent fasting may not be ‘optimal’, as long as the overall nutrient content of the fasting diet remains similar then the negative influence of nutrient timing may be negligible.
Intermittent Fasting & Its Impact On Growth Hormone Levels
Growth hormone is a peptide that stimulates growth and cell reproduction regeneration.
Extraordinary claims have been made for intermittent fasting regarding its effect on increasing growth hormone levels, with some sources claiming up to a 5000% increase.
Claims like this are generally brought upon by the fitness community, with growth hormone levels being linked with an increase in lean body mass and muscular hypertrophy.
Data seems to indicate that there is a fast-induced enhancement of growth hormone release, caused by an initiation of sporadic growth hormone bursts and persistent activation of the growth hormone signal . The increased growth hormone bursts and signaling are likely due to increased IGF-1 expression from fasting.
However, the benefits of increased growth hormone are yet to be determined. At this point, it is known that growth hormone does not significantly affect the anabolic status of muscle tissue, but may significantly enhance connective tissue (tissue that connects other tissues and organs).
This voids most claims by the fitness industry that discuss fasting’s benefits to protein synthesis and anabolism.
However, the increased growth hormone may also have an important role in longevity and cell regeneration, and future research should look to clarify this relationship.
However, it is speculated that the increase in IGF-1 levels from intermittent fasting may have a negative effect on cancer growth, as this hormone has demonstrated to increase tumor growth and progression rates.
Intermittent fasting can be done using many different methods, all of which essentially impose a diet that controls the followers' eating frequency rather than limiting certain food-types or daily caloric intake (for the most part).
There's solid evidence to suggest that this type of fasting diet can certainly help with weight loss. As it doesn't really require the dieter to remove foods they enjoy eating from their daily meal plans, it may be a great choice for those who have previously failed other diets that do impose restrictions to certain food choices.
It also has a whole host of positive health benefits too - we've seen that it can help improve cholesterol levels, lower inflammation, help support glucose control, and can even promote longevity!
If you're an athlete or bodybuilder, we wouldn't personally recommend it over a normal eating pattern, however it can be utilized effectively as it doesn't limit the amount of calories or macronutrients you can intake on a daily basis.