You’ve probably heard about intermittent fasting which, according to Google, was the most searched diet trend of 2019. The practice of fasting has been around since ancient times. Hippocrates, widely considered the father of modern medicine, was a staunch supporter. Fasting for spiritual purposes is widely practiced and is a component of virtually every major religion in the world, considered to be a cleansing and purifying process for both the body and the spirit. Numerous studies suggest intermittent fasting has a beneficial effect on a wide range of chronic disorders including cardiovascular disease, diabetes, cancer, autoimmunity, and neurodegenerative brain diseases such as Alzheimer’s and Parkinson’s. But can it also help us lose weight and, more importantly, keep the weight off permanently?
What is intermittent fasting?
The basic principle of intermittent fasting is simple: abstain from eating during specific time periods and eat during others. The 3 most common variations are:
- Daily time-restricted eating: this involves eating all your meals for the day within a certain window of time, typically anywhere from 4-12 hours (meaning you’ll be fasting for 12-20 hours of the day). For example, if you stop eating by 8 p.m. then have your first meal at noon, you will have created a 16-hour fasting state.
- Alternate day fasting: this involves alternating “feed days”, where you eat normally, with “fast days”, where eating is restricted to one meal of ~500 calories, ideally consisting of protein, healthy fats, and vegetables (i.e. no refined starches or sugars)
- 5:2 intermittent fasting: in this plan, you eat normally five days of the week and drastically reduce calories on the other two days, typically 500 calories for women and 600 calories for men.
Which plan is best for you? There is no single right answer and in fact, most people find it beneficial to switch things up periodically. In general, it is recommended to start slowly and gradually increase your fasting window in order to avoid unpleasant side effects that can discourage you from continuing. There is some evidence to suggest that timing your eating window earlier in the day, say from 8a.m. to 2 p.m., can help normalize cortisol expression and circadian rhythms.
How does intermittent fasting work?
At its most superficial level, intermittent fasting can reduce the total number of calories we consume. Although there are no specific calorie limits set, restricting our eating to a limited number of hours naturally tends to limit the amount of food we eat, particularly in the evenings when many of us plunk ourselves down on the couch to watch TV while mindlessly munching on high calorie, highly processed snack foods.
In a prolonged fasting state, once the body has used up its reserves of glucose, it starts burning fat for fuel. Decreasing the body’s excess fat stores, particularly the visceral fat deposits (around the abdominal organs) is integral to healthy and long-lasting weight loss.
One common misconception that makes many people reluctant to try fasting is the idea that it causes our basal metabolism to decrease—the so-called “starvation mode”, making it harder to lose weight in the long run. In fact, fasting causes an increase in metabolic rate, mediated by an increase in adrenaline as well as growth hormone. Presumably, this serves as an adaptive response to give us the energy to go out and find food in times of scarcity. Humans as a species would not have survived very long if the body’s response to missing a few meals was to shut down completely. Growth hormone also helps the body utilize fat for fuel and preserve muscle mass and bone density. Ironically, daily calorie reduction, not fasting, does in fact lead to decreased metabolism, as the body matches lower food intake with lower energy expenditure. When food intake goes to zero, however, the body switches its energy source from ingested food to its own stored fat reserves which provide ample energy for maintaining metabolism. Alternating between a fed state and a fasting state is therefore more important than simply reducing overall calories.
Decreased insulin and insulin resistance:
The root cause of obesity in many cases is persistently high levels of insulin leading to insulin resistance, increased fat storage, and a higher body weight set point. The body weight set point is monitored by the hypothalamus but appears to be set higher by higher levels of insulin. When we attempt to lose weight by the standard method of eating less and exercising more, the hypothalamus acts to maintain the set point by decreasing our energy expenditure (by decreasing metabolic rate, body temperature, etc) and increasing appetite. Therefore, further weight loss becomes more and more difficult, and over time we tend to regain that weight and then some. Breaking the insulin-resistance cycle and lowering the body weight set point to allow long-term weight loss requires recurrent periods of very low insulin levels. Since all food stimulates an insulin response to some degree (with refined starches and sugars being the worst offenders), the best way to achieve this is via fasting.
Contraindications for fasting
Although intermittent fasting can be a powerful tool for losing weight and improving overall health, it is not for everyone. Women who are pregnant or breastfeeding should not practice intermittent fasting so as not to compromise their baby’s nutritional needs. Likewise, children under 18 should not fast for extended periods, nor should anyone underweight (e.g. body mass index (BMI) less than 18.5). The strict eating schedule may also act as a trigger for those with a history of anorexia or other eating disorder.
Individuals taking medications, especially diabetics, must exercise caution and require medical supervision when undertaking an intermittent fasting program, as does anyone with pre-existing medical conditions such as kidney disease, cardiovascular disease, and adrenal issues.
Susan Goto, ND
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