Intermittent Fasting Benefits and Risks

So far, I have shared with you my own story, and details of the success that I achieved personally through a program of intermittent fasting. I hope that you have found my story interesting, perhaps even inspiring, and that you feel positive towards introducing a regime of intermittent fasting in your own life.

However, you may be wondering whether one person’s achievement can translate into success in other people – all with different bodies, lifestyles, and attitudes. And, you would be right to wonder. It is important before you embark on such a regime that you have confidence in the system, and that you believe it will work for you. To address this concern, I can offer two thoughts. Firstly, not only have I benefited through this regime myself, I have also helped hundreds of direct clients, and many thousands of readers of my blog, all achieve their best condition ever. This is a program that works for all types of body. Secondly, and just as importantly, the program of intermittent fasting that I describe in this book is supported by compelling, well-conducted, and published scientific evidence.

Intermittent fasting is not a fad or a craze dreamt up without basis in fact, but a regime that is founded on hard science. The purpose of the following chapter is to present and summarize the scientific literature that is available regarding the benefits of intermittent fasting, and to allow you to draw your own conclusions. I feel strongly that it is important for you to be satisfied with the science and convinced by the data before you begin your own regime.

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The studies discussed in the sections below have been identified by a comprehensive search of the literature. They are not studies that have been selected because they fit a particular thesis. The full reports of many of these studies are freely available online to any reader with an interest in exploring the subject further. If the full paper is not available free of charge, a summary (or “abstract”) of the paper almost certainly will be. The most convenient way to access the abstract (or in some cases, full paper) is through the PubMed interface to the Medline database, available at http://www.ncbi.nlm.nih.gov/sites/entrez.

Health benefits of intermittent fasting

A significant proportion of people who embark on a program of intermittent fasting undoubtedly do so in order to achieve a reduction in body weight. And, this is a perfectly reasonable goal in its own right. However, you will find as you progress with your regime that a number of other health benefits become apparent. For example, you may find that you have more energy, that you suffer fewer coughs and colds, and that you feel generally in a better state of health. There are a number of further advantages to this program which may not be immediately noticeable but which will improve your underlying health in general. For example, data show that intermittent fasting has positive effects in terms of cardiovascular health, neurological function, protection against disease, insulin sensitivity, and hormonal responses. The scientific evidence to support the benefits of intermittent fasting in these different areas is discussed over the following sections.

Body weight

A number of clinical research studies have evaluated the effects of different regimes of intermittent fasting on body weight.

In one recent study (2009) investigating a regime similar to the one that I follow (1), subjects were asked to observe a 12-14 hour fast every day. In addition, participants were required to perform prolonged, moderate-intensity aerobic exercise during this fasting period, on at least three to five occasions each week. No restrictions were placed on total daily calorie consumption. During the 12-week study, the average fat loss in the 27 participants was an impressive 7.4 kg, equivalent to one-quarter of the fat mass at baseline. Importantly, the rate of fat loss was at least as great in the second six weeks as in the first, suggesting that fat loss might have persisted for some time if the study had run over a longer period.

A slightly earlier study showed a similar benefit in terms of weight loss through a related regime of alternate-day fasting (2). In this study, 16 non-obese men and women who fasted every other day for 22 days lost on average a total of 2.5% (±0.5%) of their initial body weight and 4% (±1%) of their initial fat mass. These findings are even more impressive when you consider that this group of participants was not particularly overweight to begin with. Furthermore, improvements in fat oxidation were reported over the duration of the study. However, although this study demonstrates the benefits of intermittent fasting, the particular regime followed, alternate-day fasting, does not appear optimal. In particular, an increased hunger was reported on the first day, which did not decline throughout the remainder of the study. This observation suggests that a regime of alternate-day fasting may lead to problems of compliance.

A third study was performed to investigate whether an intermittent, very low calorie diet improves weight loss more than caloric restriction alone in diabetic patients or overweight individuals (3). This study found that subjects who followed a very low calorie diet for five consecutive days every five weeks lost more weight than those participants who received a standard reduced-calorie (1,500-1,800 kcal/day) diet continuously for the same duration. The intermittent group also revealed improvements in markers of diabetes, notably glycosylated hemoglobin (HbA1c) levels, compared with the constant calorie intake group.

One objection that is sometimes raised concerning intermittent fasting is whether the body is physically capable of withstanding the effects of fasting, or whether such a regime could actually cause damage to the systems of the body. For obvious reasons, very few studies investigating the effects of prolonged fasting have been conducted. However, one study in a group of eight hunger strikers who refused food for 43 days revealed a decrease in body fat of approximately 60% and a reduction in body mass index (BMI) of 18% (4). By the end of the starvation period, BMI (21.5 ±2.6 kg/m2) and body composition measurements were still within acceptable limits, suggesting that the body had been surprisingly efficient in conserving muscle mass. The researchers concluded that a regime of complete food withdrawal had a far greater impact on body fat than on muscle mass, and that patients were not morphologically malnourished after 43 days of fasting. From this extreme example we can be confident that fasting for a relatively short duration every day offers no threat to physiological safety.

From the results of the studies discussed above, it becomes apparent that intermittent fasting offers significant benefits in terms of weight reduction and fat loss and that a regime involving of daily periods of fasting appears superior to alternate-day fasting in terms of reduced sensations of hunger and consequent compliance.

Health markers

As well as a clear, and expected, benefit in terms of body weight, intermittent fasting also offers advantages in relation to some of the underlying physiological processes that keep us healthy without our being aware of it. Although these benefits may not be so readily apparent as weight loss, they can be measured by following certain health “markers” that provide an insight into the underlying processes. The effects of intermittent fasting on lipid profiles and inflammatory parameters, two examples of health markers, will be discussed in detail below.

Lipid profile

“Hyperlipidemia” is a condition in which excessive quantities of fatty substances known as “lipids” are present in the blood. Lipids, including cholesterol, triglycerides, cholesterol esters, and phospholipids, are transported in the blood as part of large molecules called “lipoproteins”. The blood concentration of the different lipoproteins is collectively described as the “lipid profile”.

The presence of hyperlipidemia places you at greater risk of “atherosclerosis”, a condition in which fatty deposits accumulate on the inside of your blood vessels. There is overwhelming evidence to demonstrate that atherosclerosis increases your risk of heart disease, stroke, high blood pressure (“hypertension”) and other problems.

Hyperlipidemia can be controlled to a large degree through diet and exercise. The greatest effects result from reducing intake of saturated and trans fats; increasing intake of polyunsaturated and monounsaturated fats; fortifying foods with plant stanols or sterols; and adopting a Mediterranean, low-carbohydrate, or low-fat diet (5). However, there is also growing evidence to suggest that your lipid profile can be affected not only by what you eat, but also when you eat.

Evidence that a program of intermittent fasting can improve lipid profiles has been found in animal studies (6). For example, in one study of 144 mice, triglyceride levels were found to decrease significantly in mice that were fed hunger-resistant food on alternate days compared with those mice who were allowed to eat ad libitum (7). This is an important observation as it demonstrates the effect of an eating schedule on lipid levels. However, while I am sure that such a finding is of major interest to the academic community, I am equally confident that you would be more convinced by a demonstration of a similar benefit in humans.

Interestingly, a number of studies conducted in humans also show a benefit of intermittent fasting in terms of lipid profiles. Several of these studies have been conducted among Muslims observing the period of Ramadan (8). As we saw during Chapter 1, Ramadan is a time of religious observance during which Muslims abstain from eating and drinking from sunrise until sunset. The fasting period during Ramadan is approximately 13-16 hours, depending on the time at which participants stop eating. Although drinking is prohibited during the fasting periods of Ramadan (an approach which is certainly not advocated by proponents of intermittent fasting), Ramadan fasting may be considered a good model for the approach discussed in this book. In particular, the length and frequency of the fasting periods during Ramadan is very close to the regime I follow myself.

A comprehensive review of all studies cataloged in Medline [1] , three international congresses on health and Ramadan, and several cases from local journals (8), concludes overall that Ramadan-style fasting results in an improvement in lipid profiles that could be beneficial for the cardiovascular system. In particular, the review finds that levels of “good” cholesterol, known as “high-density lipoprotein” (or “HDL”) increase while levels of “bad” cholesterol, known as “low-density lipoprotein” (or “LDL”) decrease as a result of fasting during Ramadan.

To further investigate the benefits of Ramadan-style fasting on lipid profiles, I will briefly consider three studies conducted in healthy observers of Ramadan. As mentioned in Chapter 1, these studies are observational in nature rather than strictly controlled, as the researchers do not intervene in the dietary practices and not all of the studies involve a comparator group. However, there is still considerable useful information to be gained from studying Ramadan in this way.

The first of these studies involved 40 healthy male and female observers of normal weight who fasted during Ramadan and another 28 healthy volunteers who did not fast (but who were age- and BMI-matched) (9). This study found that the ratio of total cholesterol (TC) to HDL cholesterol was decreased during and after Ramadan in both men and women who fasted, while no such changes were observed in the non-fasting group. In the second of the Ramadan studies (10), conducted in 50 healthy subjects, the ratio of LDL cholesterol (“bad” cholesterol) to HDL cholesterol (“good” cholesterol) ratio was significantly decreased during the month of Ramadan (p<0.05). Finally, in a third study investigating the effects of Ramadan-style fasting in healthy observers (11), HDL levels were significantly elevated during Ramadan (p<0.001) and 20 days after Ramadan (p<0.05). The findings of these three studies suggest that a 13-16-hour period of fasting every day for a prolonged period causes beneficial changes in lipid profiles which predispose to an improved cardiovascular risk profile.

Another interesting study of Ramadan-type fasting was conducted in 64 religious observers who had previously been diagnosed with diabetes or hyperlipidemia (12). This is an important area in relation to intermittent fasting because individuals with nutritional disorders of this type are normally required to comply strictly with advice on the timing and composition of food. Indeed, it could be anticipated that prolonged daylight fasting during the month of Ramadan may even produce undesirable biochemical consequences in these individuals. In the hyperlipidemic subjects without diabetes, intermittent fasting had no detrimental effect on triglyceride, TC and LDL levels, contrary to some expectations. In diabetic patients, TC levels increased somewhat following Ramadan, but not by a significant amount. It would be interesting to investigate the effects of intermittent fasting in diabetic patients further, but certainly in patients with hyperlipidemia, the findings of this study suggest that intermittent fasting may improve lipid profile and therefore favorably influence risk of coronary heart disease.

One final study to be considered in relation to lipid profiles was conducted in patients with asthma (13). The regime followed in this study was an 8-week program of alternate day calorie restriction, in which overweight subjects could eat ad libitum on alternate days while consuming less than 20% of their normal calorie intake on the intervening days. Although this scheme differs from the regime that I follow personally, it nevertheless provides us with a great deal of useful information. This study will be discussed again at greater length later in this chapter, particularly in relation to the observed benefits in terms of asthma symptoms. However, among the many interesting findings reported, the researchers found that those subjects who adhered to the diet not only lost an average of 8% of their initial body weight, but also presented with decreased levels of serum cholesterol and triglycerides at the end of the study.

The various studies discussed above would seem to indicate strongly that a regime of intermittent fasting achieves a beneficial effect in terms of lipid profiles, and resulting reduction in risk of consequences of atherosclerosis, including heart disease, stroke, hypertension and other conditions. This seems to be a very powerful argument for the introduction of a regime of intermittent fasting, in addition to the more obvious benefits of weight loss.

Inflammatory markers

Most people associate inflammation with a condition that affects the joints and connective tissues of the body. Perhaps rheumatoid arthritis comes to mind, or other diseases of that type. However, recent studies have linked inflammation of the arteries to heart attacks and strokes. Indeed, in perhaps up to 50% of cases of heart attack and stroke, patients do not present with the classic risk factors such as hypertension or raised lipids. Research is beginning to suggest that in these cases the underlying cause may be inflammation of the arteries (14). Inflammation of the arteries can be measured using inflammatory markers, including soluble tumor necrosis factor (TNF-α), C-reactive protein, and interleukin-6.

Once again, a number of studies have investigated the potential benefit of intermittent fasting on inflammatory markers. In a study described in the previous section, the ratio of total cholesterol to HDL cholesterol was found to decrease in 40 healthy male and female observers of Ramadan (9). However, this study also measured the effects of Ramadan fasting on serum interleukin-6, and C-reactive protein levels and found that both were significantly lower (p < 0.001) during Ramadan in the fasting subjects of both genders when compared to basal values (one week before Ramadan).