Intermittent Fasting for Weight Loss: Fasting Myths Debunked

 

Intermittent fasting (also known as IF) is becoming increasingly popular among people who are looking to lose weight and fat, and even among those who are trying to build muscle and optimize their body composition.

Proponents of this meal timing approach to dieting are convinced that it is superior to other forms of energy restriction, with a number of benefits being attributed to the “fasting” element, including:

• more weight loss,
• higher Resting Metabolic Rate (RMR),
• increased muscle retention, and
• increased insulin sensitivity.

Is there any truth to the above claims, however, and is there good scientific evidence to support the weight loss benefits attributed to intermittent fasting?
Read on find out!

Intermittent Fasting Defined

Before proceeding any further, let’s quickly look at what exactly intermittent fasting is and what it entails.

In simple terms, intermittent fasting is an eating pattern which involves alternating periods of little or no energy intake (i.e. caloric restriction) with intervening periods of normal food intake, on a recurring basis (1).

Different sub-classes of intermittent fasting include the following:

• Alternate day fasting (ADF), which involves a 24-hour fasting or very low calorie (~25% of daily energy requirements) period, alternated with a 24-hour ad libitum (i.e. at one’s pleasure) eating period.
• Whole day fasting (WDF), which usually involves 1-2 days of fasting or very low calorie (~25% of daily energy requirements) periods per week, with the remaining days of the week eating at maintenance.
• Time-restricted feeding (TRF), which involves a fasting period of 16–20 hours and a feeding period of 4–8 hours daily, and includes Ramadan style fasting as well as the popular Lean Gains approach (2).

Takeaway point: Intermittent fasting (IF) is an eating pattern which involves alternating periods of little or no energy intake with intervening periods of normal food intake, and includes alternate day fasting (ADF), whole day fasting (WDF) and time restricted feeding (TRF).

 

 

Intermittent Fasting vs Caloric Restriction

As you may have noticed from the definition of intermittent fasting above as well as from the descriptions of its different sub-classes, intermittent fasting tends to go hand in hand with caloric restriction.

In ADF, for example, although subjects are allowed to eat ad libitum (as much as they want) on their feeding days, they usually tend to not eat enough to compensate for the caloric deficit they create on fasting days. This also tends to happen with TRF, where subjects restrict their feeding window to a few hours per day (2).

Similarly, since WDF entails 1-2 days per week of fasting or very low-calorie dieting, with the remaining days of the week eating at maintenance, it also results in weekly caloric restriction and, consequently, in a weekly energy deficit (2).

Many people miss this important point and think about the two (IF and caloric restriction) as being fundamentally different or even antithetical to each other. This means that much of the research that the proponents of IF use, in order to support their claims, doesn’t control for the confounding (and very important) variable of caloric intake.

The above is why the research that is cited in this article controls for caloric intake – so that we can identify the effects of intermittent fasting itself rather than just the effects of the associated caloric restriction.

Takeaway point: Intermittent fasting is, essentially, a form of caloric restriction, with its main difference to “normal” caloric restriction being that the fasting periods are longer in duration and further apart from each other. It is, therefore, important to make sure that the research we read on IF controls for the confounding variable of caloric restriction (or, at least, to take it into account when interpreting research).

Common Weight Loss Effects of IF

With the above said, let’s examine a few common weight loss and body composition benefits that are usually attributed to IF.

Intermittent Fasting and Weight Loss

In recent years, research examining the effects of intermittent fasting on weight loss in humans has been growing steadily. Unfortunately, however, most of the available studies we have to date suffer from the following limitations:

• They have no control group to compare results to: doing something is, usually, better than doing nothing.
• The control group doesn’t receive any “treatment” – i.e. they just eat as usual: it makes sense that a group of subjects doing any sort of dieting will lose more weight than the group of subjects who keep eating as usual.
• The control group receives “treatment” but caloric intake isn’t matched between groups: we can’t attribute weight loss effects to one variable (fasting) when a confounding variable (caloric intake) isn’t controlled for.
• The control group receives “treatment” and caloric intake is matched between groups, but protein intake is not: since protein is considerably more thermogenic and satiating than carbs and fats (3), the group eating significantly more protein will have higher energy expenditure and will naturally tend to eat fewer calories while often underreporting their food intake.

When excluding studies that suffer from the above limitations, we are left with a relatively small number of studies which compare the effects of intermittent fasting to continuous energy restriction on body weight, and which show that, overall, the two diet types result in identical outcomes in terms of body weight and body fat reduction (4, 5, 6, 7).

Takeaway point: most of the research on intermittent fasting and weight loss suffers from a number of limitations, including a lack of a control group, comparing intermittent fasting to a “no treatment” group, and not controlling for caloric or protein intakes. Research that doesn’t suffer from the above limitations suggests that intermittent fasting and continuous energy restriction result in identical weight and fat loss.

 

 

Intermittent Fasting and Metabolic Rate

Metabolic rate also called resting metabolic rate (RMR) and resting energy expenditure (REE), usually makes up around 60-75% of our total daily energy expenditure (8). This means that it can easily determine overall energy balance and, consequently, whether an individual will lose or gain fat.

Metabolic rate is known to be reduced with caloric restriction, mainly because of reductions in:

• fat mass,
• fat free mass,
• circulating leptin and thyroid hormones, and
• sympathetic nerve activity (9).

It is often argued that intermittent fasting results in a lower reduction in REE relative to continuous caloric restriction when adjusting for decreases in body mass, while some even believe that metabolic rate increases in response to fasting. Is there, however, any truth to this? Well, kind of.

Overall, the available evidence suggests that intermittent fasting results in the same adaptive responses as continuous energy restriction, at least on feeding days.

Some studies, for example, have shown increases in metabolic rate during the first few days of fasting, with the average increase in energy expenditure being around 5.5% (10, 11, 12). Other studies, however, have found both decreased and unaltered REE as measured by indirect calorimetry during fasting (13, 14, 15, 16, 17, 18).

On the whole, studies which have measured REE on feeding days of intermittent fasting have mostly shown reductions in REE similar to those that happen with “normal” dieting in, obese, overweight and normal-weight subjects (6, 19, 20, 21, 22), with one exception being a recent trial where REE decreased with continuous caloric restriction but not with alternate day fasting (23).

Overall, the available evidence suggests that intermittent fasting results in the same adaptive responses as continuous energy restriction, at least on feeding days.

Takeaway point: while some research has found an increase in REE with IF when measured on fasting days, at least in the short term, most research that has measured REE on feeding days has reported reductions in REE similar to those that happen with continuous caloric restriction.

Intermittent Fasting and Muscle Retention

Ideally, weight loss diets should cause the desired rate of weight loss while minimizing fat free mass loss in order to maintain physical function and to prevent reductions in metabolic rate, thereby preventing future weight regain (24).

In general, caloric restriction results in a reduction in both fat mass and fat free mass. However, the percentage of fat free mass lost seems to depend on a number of factors, including:

• initial body fat levels, with higher body fat levels resulting in lower fat free mass losses,
• the degree of caloric deficit imposed, with larger deficits generally resulting in bigger fat free mass losses,
• the extent and type of exercise performed, with resistance training resulting in better fat free mass retention than aerobic exercise and even better retention compared to no exercise at all, and
• the protein intake of the diet, with higher intakes resulting in better retention of fat free mass (25).

Proponents of intermittent fasting argue that less fat free mass is lost with this type of dieting approach, since the increased levels of growth hormone that are associated with the fasting state cause the body to reduce protein metabolism in favor of fatty acid metabolism.

Although it’s true that fasting causes an increase in growth hormone secretion (26), what does the available scientific evidence show with regards to the effects of intermittent fasting compared with a continuous caloric restriction on fat free mass retention?

Well, when the variables listed above (body fat levels, degree of caloric deficit, exercise, and protein intake) are controlled for, research suggests that there are no significant differences in terms of fat free mass loss between groups.

For example, this study by Harvie et al found no statistically significant differences in terms of fat free mass loss between groups (4). Moreover, this trial by Hill et al (5) as well as this 2013 trial by Harvie et al (27) also reported equal levels of fat free mass loss between an intermittent and a continuous caloric restriction group when caloric and protein intakes were matched between groups.

Takeaway point: the amount of fat free mass that is lost during dieting seems to depend on the subjects initial body fat levels, the degree of energy deficit imposed, the extent and type of exercise performed, and the protein intake of the diet. Studies that control for the above variables generally show no statistically significant differences in fat free mass loss between intermittent fasting and continuous energy restriction.

 

 

Intermittent Fasting and Insulin Resistance

Insulin is a peptide hormone produced in the pancreas, the functions of which include:

• increasing the absorption of glucose into the muscles and fat tissue,
• inhibiting muscle protein breakdown,
• increasing the synthesis and absorption of lipids in fat tissue, and
• reducing the breakdown of lipids and increasing glycogen synthesis in the liver (28).

Insulin resistance is a pathological condition in which cells fail to respond normally to insulin, with overweight and obesity being major risk factors for the development of both peripheral and hepatic insulin resistance (28).

Proponents of intermittent fasting argue that it results in improvements in insulin resistance independent of changes in body weight. What does the research suggest, however? Well, unsurprisingly, studies on both overweight and obese individuals report variable results.

One study by Ash et al, for example, reported similar improvements in glycaemic control in 51 men with type 2 diabetes after 12 weeks of intermittent (four days per week) or continuous energy restriction (6).

A 2011 study by Harvie et al which measured hepatic insulin sensitivity in overweight or obese subjects reported a 25% greater reduction in insulin resistance compared to the daily energy restriction group when measured on the morning after five normal feeding days.

Overall, the limited available research has reported variable effects of intermittent fasting on peripheral and hepatic insulin sensitivity, which may be gender-specific.

It also reported a further 25% reduction in insulin resistance compared with daily energy restriction on the morning after the two energy-restricted days. Importantly, these differences in insulin sensitivity happened despite similar reductions in body fat between the groups (4).

Three other studies that have assessed the effects of 2–3 weeks of whole day fasting and which were designed to ensure that there was no overall energy deficit or weight loss have also reported variable results.

Specifically, the first study by Halberg et al reported improvements in insulin-mediated whole-body glucose uptake and insulin-induced inhibition of adipose tissue lipolysis when measured after two normal feeding days (29), while these results could not be replicated by the Soeters et al study (22).

Interestingly, the third study by Heilbronn et al reported a significant reduction in insulin response and improved glucose uptake and insulin sensitivity in male subjects, whilst female subjects experienced impaired glucose uptake and apparent skeletal muscle insulin resistance (30).

Overall, the limited available research has reported variable effects of intermittent fasting on peripheral and hepatic insulin sensitivity, which may be gender-specific.

Takeaway point: the results of studies which compare energy-matched intermittent fasting to continuous energy restriction or which were designed to ensure that there was no overall energy deficit or weight loss have reported variable results with regards to improvements in insulin sensitivity.

Conclusions

Based on the available well-designed trials, we can draw the following conclusions:

• Intermittent fasting and continuous energy restriction result in identical weight and fat loss when energy and protein intakes are matched between groups.
• In most studies, when measured on feeding days, the magnitude of metabolic rate reduction is similar with intermittent fasting and with continuous caloric restriction.
• When controlling for initial body fat levels, the degree of energy deficit, the extent and type of exercise performed, and the protein intake of the diet, there are no statistically significant differences in fat free mass loss with intermittent fasting or with continuous energy restriction.
• In studies that compared energy-matched intermittent fasting to continuous energy restriction or which were designed to ensure that there was no overall energy deficit or weight loss, variable results with regards to improvements in insulin sensitivity have been reported.

What is also clearly evident is that there is a need for more high-quality research comparing long-term outcomes of intermittent fasting and continuous energy restriction in order to establish any real benefits of the former with regards to controlling body weight and improving metabolic health in humans.

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