Should I be intermittent fasting?
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What Is Intermittent Fasting?
Intermittent fasting (IF) refers to recurring periods with little or no food intake. (1). A really common example is the 5:2. On fasting days, a person’s intake may be limited to non-calorific fluids such as water, tea, coffee and diet drinks, or it may allow a very restricted amount of daily calories from food (2). So in short: yes, intermittent fasting is another diet. Celebrities such as Beyoncé and Jennifer Lopez swear by this “simple weight loss method”, and the alleged benefits are alluring raging from increased longevity to improved metabolic health, weight loss and reductions in heart disease and Type 2 diabetes. But what does the research say? Noteworthy, the majority of these claims are either untested or under-tested in humans (3).
Is Intermittent Fasting the same as time restricted feeding (TRF)?
IF and TRF are similar but have some important differences. Whilst IF refers to recurring periods with little or no food intake, TRF pertains to consistent daily shorter eating intervals e.g. eating within an 8 hour interval sometimes known as the 16:8. Within TRF you can separate out into early TRF e.g. 8am-4pm or mid-later TRF e.g. 12pm-8pm.
Can Intermittent Fasting Improve Health and Longevity?
In laboratory rats, intermittent fasting has been found to improve outcomes in experimental models for a range of age-related disorders including diabetes, cardiovascular disease and neurological disorders such as Alzheimer’s disease and Parkinson’s disease (4). However, as these findings came from studies carried out on rats their relevance to humans is very limited and we have to be very careful when extrapolating research findings from rats to humans.
A systematic review of human studies found that intermittent fasting may improve adiponectin:leptin ratio and changes in insulin-like growth factors, which may have a positive effect on ageing (5). However, large scale research in this area is very limited and more research will needed to be conducted before we can make any conclusions.
Is intermittent fasting good for weight loss?
In 2020, a 12-week randomised clinical trial looked at the effect of time-restricted eating on weight loss and metabolic health in 116 adults that were “ove**eight” or “ob**se” (1). Participants were randomised to either a consistent meal timing group (3 structured meals per day) or a time-restricted eating group (eating what/ how/ as much as they wanted from 12:00 pm until 8:00 pm and fasting from 8:00 pm until 12:00 pm the following day).
They found that there was no greater weight loss in the time-restricted eating group compared with the consistent meal timing group (1.17% and 0.75% respectively). They also found no statistically significant benefit from intermittent fasting for health outcomes either (1). As fat mass, fasting insulin, glucose and blood lipids, did not significantly change between the two groups (1).
The only distinction for fasting was in fact, a negative side effect as participants following the time-restricted diet lost significantly more lean mass compared with participants eating regular meals (1). The average weight loss in the time-restricted eating group was 1.70kg, with 1.10kg coming from lean mass loss (approximately 65% of the weight lost). The loss of lean mass in this study far exceeds the normal range of 20% - 30%. Suggesting that intermittent fasting can cause muscle loss. This is worrying as appendicular lean mass (ALM) is correlated with nutritional and physical status, and a reduced ALM can lead to weakness, disability and, impaired quality of life (1).
Other studies have found that there is no difference in weight management outcomes from either eating regularly or intermittent fasting (2). Some studies have found that intermittent fasting has resulted in weight loss, such as Hankey et al in 2015, which found that intermittent fasting resulted in a mean weight loss of 2.6%-8.9% for the participants (after one to six months respectively) (6). However, the long-term effects of intermittent fasting on weight-loss were not studied which is so often case for studies on weight-loss and intermittent fasting. To date, we have no long term studies that show individuals can lose weight and keep it off long-term and 95% of all dieters will regain their lost weight in 1-5 years (7).
Are there any negative effects of intermittent fasting?
A review of the evidence related to fasting from PEN (2013) 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’ (2). This report also highlights further issues related to fasting, such as a reduction in immune function and an increase in non-dehydration-related headaches (2).
A study conducted in 2020 with 110 participants (27 males, 83 females between 13-86 yrs) fasting from between 4-10 days found cortisol was 27% higher, fasting glucose was 10% higher, triglycerides were 8% higher, LDL cholesterol and pulse rate were also 1% higher post the fasting period as compared to before. Benefits were noted as a reduction in blood pressure and weight loss (10). A meta-analysis and systematic review conducted in 2016 looking at 13 studies found severe calorie restriction e.g. in fasting results in elevated cortisol which may have negative effects overtime (12).
A less recent study on 34 male athletes in 2016 found that those assigned to a TRF group of 8 hours feeding / 16 hours fasting found that compared to the control group that had no constraints on eating over 8 weeks, those fasting experienced a 20.7% drop in testosterone (11). Testosterone plays a key role in increased muscle and bone mass. Despite this evidence, the majority of literature still seems to promote IF as a “healthy” intervention.
Our bodies cannot tell the difference between this diet and actual food scarcity/ starvation (which to be honest, is what we do to our body when we diet), so to compensate for the lack of food, our metabolism slows down (8). Following a restrictive diet can lead to overeating and bingeing, as when hungry, the brain sends out hormones like ghrelin and neuropeptide Y which stimulate appetite and will build up throughout the day. Once you are finally ‘allowed’ to eat again, you will feel as though you can’t stop, leading to overeating and bingeing (9).
When eating is wrapped up with rules and restrictions we can have feelings of guilt and shame when the diet inevitably fails. Living without rules or restrictions around food can feel quite daunting at first, especially in a culture where the behaviours are normalised and even encouraged under the guise of “wellness”. The sooner we realise that diets do more harm than good, the sooner we can get on with our lives and nourish our bodies in a way that truly enhances wellbeing.
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