In 2014, the World Health Organisation (WHO) classified a massive 39% of the world’s adult population as overweight (which is a body mass index of between 25 and 29.9 kg/m2) and 13% as obese (BMI above 30kg/m2)(1). When you stop and think about that – over half of the world is at least overweight! It’s also common knowledge, that individual’s who are overweight are at risk from several diseases, namely cardiovascular disease (2, 3). However, simply decreasing as little as 5% of bodyweight can drastically reduce the risk of these diseases and improve long-term health (4, 5). Most people wish to lose excess fat, and often this is accomplished by improving their habitual diet and thus decreasing caloric intake, resulting in a utilization of stored energy (aka, that pesky fat). To the science guys, the continuous implementation of decreasing calories is known as continuous energy restriction (CER), however despite it’s success when employed, many people still struggle to maintain the decrease in calories consistently, or at least long-term (6). And when consistency is one of the key ingredients to fat loss, it’s no wonder many people end up back at square one.
Throughout the last decade, a new weight-loss strategy has been used by many dieters around the world (7). Intermittent Fasting, commonly known within the science community as Intermittent Energy Restriction (IER), is the process at which the dieter drastically restricts calories for a prolonged period of time, interspersed with regular eating. In recent years Dr Krista Varaday has investigated an IER protocol called Alternate Day Fasting (ADF) (7). The dieter is encouraged to eat regularly every other day – no calorie counting or restriction – then on the fasting days eat 600 calories (if you’re a man) or 500 calories (if you’re a woman). Varaday’s studies have demonstrated great results in terms of weight loss (7), and similar protocols have been advised for those that find ADF too difficult to sustain – such as the 5:2 diet, in which the dieter fasts for only 2 days a week and eats ad libitum (to satisfy appetite) on the other 5 days (8). As intermittent fasting and it’s different methods all aim to reduced calories, it is no surprise that those who experiment and undertake these protocols often lose weight. However, is it sustainable? After all, that’s what counts, right? We’ve all been a “Monday dieter” – the process of starting a diet on Monday, lasting until Thursday when your work colleague suggests an after-work quick drink that lasts until you’re sh*t-faced at 4am, elbows deep into 4 kebabs and a big-mac, muttering to yourself that you’ll start the diet again next Monday (then repeat for infinity). Don’t tell me I’m alone in that?! Simply Google search ‘Intermittent Fasting’, and you will be met with promises of weight-loss, better health benefits, increased growth hormone and even prolong life! But is this all hear ‘say? What does the research say? Let’s take a look…
Firstly, does IER decrease the chances of cardiovascular disease more than regular calorie restriction on a daily basis? Well, recently, researchers from the School of Pharmacy and Medical Sciences, University of South Australia, compiled a systematic review of IER and found that despite IER dieters losing just as much weight as those who exhibited CER, there was so significant difference between the decrease in cardiovascular disease (and relevant markers)(9). This isn’t to say that IER is less effective, simply, it didn’t matter whether you employed IER or CER, the benefits of decreasing cardiovascular disease were just as good as each other.
“We’ve all been a ‘Monday dieter’ – the process of starting a diet on Monday, lasting until Thursday when your work colleague suggests an after-work quick drink that lasts until you’re sh*t-faced at 4am, elbows deep into 4 kebabs and a Big-Mac, muttering to yourself that you’ll start the diet again next Monday (then repeat for infinity)”
The researchers also investigated studies that assessed the drop-out rate and success of both IER and CER diet strategies. They found that exhibiting EIR in studies of more than 6 months in duration had the same drop-out rate as similar studies with CER participants, which may suggest that applying intermittent fasting strategies is just as easy to stick to – or rather, more likely to quit – as regular calorie restricted diets. So again, no real difference there. But it’s evident that IER is a successful strategy at losing weight and producing equivalent reductions in BMI, waist and hip circumference (10).
What if restraining from eating lead to an increased appetite? Well, 4 studies indicated that IER increased appetite, 6 showed that IER decreased appetite and 2 had no significant effect (10). So slightly in favour or suppressing appetite, but more research is needed, with longer interventions and larger subject pool sizes.
What’s interesting about suppressing appetite is that ketogenic environments have been shown to suppress the desire to eat, therefor making any diet easier to stick to long term (11). Unfortunately, the mechanism for this is unknown and still being investigated. However, it is incredibly possible that implementing IER strategies with a long fasting phase may lead to a ketogenic state and thus suppress the need to snack like crazy. In a study on ADF, researches wanted to investigate if subjects would increase their caloric intake on non-fasting days (to make up for the missed calories)(12). The researchers hypothesized that subjects would increase their calories by 125% on the ‘feast’ days, but this wasn’t the case. In fact, the subjects only ate 95% of their maintenance caloric requirement, which was surprising despite being instructed to eat normally. It’s quite possible the subjects may have wanted to eat better because they knew they were participating in a study and their diet may be under scrutiny? Perhaps their stomachs had decreased in size? Highly unlikely, but more investigation is needed in the appetite suppressing benefits of IER.
Does IER affect mood and psychological states?
I had a quick search to see if any murders were committed as a results of intermittent fasting, but fortunately I couldn’t find anything. So that’s good. However, studies investigating IER vs CER in terms of mood changes found inconsistent results (13, 14). One study showed that participants in the IER group reported bad temper whereas the other study indicated no difference in mood, vigour self-reported tension, depression or anger. So despite my partner suggesting that not eating makes her grumpy, there’s no clear evidence that this is the case over multiple subjects (sorry boo).
Does IER reduce the adaptive responses to energy restriction?
Firstly, let’s clarify what adaptive responses are. When you decide to embark on your diet, cut your calories and start exercising more, your body tries to manage this – or better, put you back in to homeostasis. For instance, calorie restricted diets appear to show an increase in appetite, reduced physical activity and the energy cost of physical activity, reduced energy expenditure and promote the accumulation of adipose tissue (10). In a nutshell, cutting your calories and eating less causes your body to fight against the fat loss, decreasing your metabolic rate and stimulating the loss of lean tissue. That’s fantastic if you’re stranded on an island, but when you want to see your washboard abs and keep your lean muscle, not so good. It also appears that having a greater energy deficit increases these adaptive responses (15) – aka crash dieting makes fat loss even harder to accomplish. But on a good note, increasing calories may reverse these adaptive responses (16), especially when implementing exercise (17). But nevertheless, our body will still try to fight calorie deficits by lowering metabolic cost, and unfortunately there is no clear evidence that IER protocols can change this (10), not unless you stay in a calorie surplus or maintenance calorie intake for a longer duration (which defeats the initial calorie deficit from the fast day, halting fat loss). In other words, to lose fat, you need to be in an energy deficit, which also promotes adaptive responses trying to halt or delay fat loss, and therefor the only way of reversing this is to continually eat more so you’re not in a deficit… which defeats the point of the fast day and therefor no fat loss. I hope that makes sense!
What about hormones?
Well, there is no evidence that IER is any different than CER in regards to thyroid stimulating hormone or cortisol increase or decrease (18). In fact, there is no difference in testosterone, free androgen index, androstenedione, prolactin and globulin concentrations when comparing IER to a CER diet (10). Both IER and CER increase IGF-1 (19) and similar improvements in glucose homeostasis (10).
So is there any difference?
Well, apart from the possible decrease in the drive to eat with IER (which more than likely comes from being in a ketogenic state) intermittent fasting is no different to implementing a regular or continuous calorie deficit. Both fat loss strategies produced similar results when investigated for fat loss, reductions in waist or hip circumference, reductions in fat mass or fat free mass reduction, glucose homeostasis, changes in the endocrine system and overall difficulty in sticking to the diet (drop-out rate). This is not to say that intermittent fasting is bogus or a worse diet strategy. The truth is, it works for fat loss and thus provides the health benefits related to a reduction in fat mass. But any strategy that follows the two rules of eating fewer calories and being consistent with this calorie deficit, works. It’s all about preferential taste. You could implement intermittent fasting as another way of decreasing calories in a larger period, meaning that other days you will have more freedom to eat higher calorie dense foods. I (anecdotally) experimented with ADF a few years ago. I ate 600 calories on one day and then ate whatever I wanted on the next. In fact, I really went for it. I ate ‘bad’ foods, chips, drank beer. I had a great time! Then I ate 600 calories of high protein, nutrient-dense food the following day. Then I repeated it for 3 weeks. I lost no weight. Absolutely zero (according to the scales anyways). But rather than call this strategy bullshit, I was incredibly impressed that I could eat like I was locked in Willy Wonka’s chocolate factory and not gain any weight over a 3 week period! But I wouldn’t advise that to anyone. That was just my own little experiment.
Like most things, intermittent fasting requires more research in studies with longer durations and with larger participant pools in order to make a conclusive statement that fasting improves hormonal and metabolic markers over a long-term period. It has been proven countless times to be a successful strategy that you can use for fat loss, but don’t get in to the habit of telling people it’s giving you any further benefits than a regular caloric deficit diet would give… Not until more evidence comes out.
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