Last updated on July 19, 2021.
Intermittent Fasting has been one of the most popular weight loss methods in recent years. People pay more attention to their eating habits, as they generally become more health-aware. Here is where intermittent fasting comes in as a trendy weight-loss method — not only for the typical keep-fit gurus, the office ladies, but more and more men are also willing to jump into the bandwagon to make changes to their eating pattern. This diet is easy to follow with no restriction on what you eat, but when you eat.
Introduction to Intermittent Fasting
Intermittent fasting is an eating pattern adapted throughout human evolution history, characterized by consecutive eating and fasting sessions. In ancient times, hunter-gatherers had to bear with an empty stomach for hours or up to days until they could hunt for food in the wild. This process has trained the human body to adapt to maintain its functions without food for an extended time. Fasting is also a common practice amongst countries with different religious backgrounds and for political reasons (i.e. hunger strike). For example, in Ramadan month, Muslims fast from dawn to dusk, to reflect spiritually by abstaining from physical pleasure like eating and drinking. Also, in Roman Catholics, gluttony (over-indulgence and over-consumption of food and drink) is considered to be a sin, so fasting is practiced from time to time to detoxify oneself from all the indulgence. In modern days, weight loss has become the biggest motivation for people to adopt fasting.
Methods of Intermittent Fasting
There are many variations of intermittent fasting, in terms of fasting interval and calorie intake. There is no so-called “best” method: every method can be effective, but what works best for each individual might be different. Here are some common methods of intermittent fasting:
- Alternate Day Fasting
It involves fasting for 24 hours once or twice per week on non-consecutive days in the week. E.g. fasting on Monday and Thursday, eating for the rest of the week.
- Modified Fasting
This method, also known as the 5:2, requires a reduced calorie intake i.e. 20-25% of normal calorie intake (normally 500 calories for women and 600 calories for men) on 2 non-consecutive days, and normal calorie intake for the remaining 5 days of the week. E.g. a reduced calorie intake on Monday and Thursday, and eating normally for the rest of the week.
- Time-restricted Fasting
There are different types of time-restricted fasting, i.e. 12/12, 14/10, 16/8, etc. The number before the slash is the number of consecutive hours of fasting, and that after the slash is the number of hours of eating. This method is practiced throughout the week. E.g. For the 16/8 method, eat during 12pm–8pm, and fast from 8pm until 12pm on the second day.
Although there is no restriction on food choices during the eating session, over-eating will inevitably lead to weight gain. One of the main purposes of intermittent fasting is to reduce calorie intake by fixing eating sessions within a certain time frame. Normally, people who practice the 16/8 method can only fit in 2 meals within the 8 hours window. Cutting an entire meal can significantly lower calorie intake, and thus bring about weight loss.
Benefits of Intermittent Fasting
During intermittent fasting, the body first utilizes glucose in the blood for energy production. When blood glucose is running out, liver’s glycogenolysis will be speeded up to break down more glycogen into glucose in order to regulate normal blood glucose levels. The glycogen store is usually used up within 8–12 hours during the fasting period, after which the body will start to break down fat to produce ketones as a source of energy.
Similar to the keto diet, weight loss is achieved when the body uses up its sugar stores and starts burning fat after hours without food. A review article revealed that a 2–12 week intermittent fasting regime can bring about a 3-8% reduction in body weight, which is more significant than traditional calorie restriction methods. The same article also showed comparable reductions in visceral fat mass (fat around the abdominal area, also a risk factor of cardiovascular diseases) by both intermittent fasting and the traditional methods. This implies that intermittent fasting might be an effective weight loss method without the need to intentionally restrict calorie intake.
Gut health & Immunity
Intermittent fasting might be able to boost beneficial bacteria (i.e. probiotics) in gut microbiota, according to the Turkish Journal of Gastroenterology. The changes in gut microbial composition show positive effects on metabolic markers, such as lower “bad” cholesterol and fasting glucose levels due to the action of gut microbiota fermentation products (e.g. acetate and lactate) during fasting, which prevents obesity and insulin resistance.
Studies showed that intermittent fasting, in particular the alternate day fasting method, can reduce inflammation, a leading cause of many chronic diseases, such as cardiovascular disease, atherosclerosis, arthritis, diabetes and cancers. Recently, animal testing suggested that intermittent fasting may potentially minimize cancer risk by decreasing oxidative stress, that is, the accumulation of free radicals in the body that leads to DNA and protein damage.
Intermittent fasting may alter gene expression and enhance genes related to longevity and disease protection. Studies revealed that intermittent fasting can extend the life of rats to 36-83% longer. Intermittent fasting also initiates the cell repair mechanism — autophagy, in which cells digest waste materials, engulf dysfunctional cells and produce new cells. This implies that intermittent fasting can potentially extend life span and slow down aging.
Research showed that intermittent fasting increases the production of the brain hormone brain-derived neurotrophic factor (BDNF), and stimulates the renewal of nerve cells, increasing resistance to different neurodegenerative conditions, in specific memory loss and Alzheimer’s disease.
Intermittent fasting can have a multiplicative effect on calorie reduction by lowering calorie intake while burning more calories by an enhanced metabolic rate. The metabolism of the body is increased due to alterations in hormonal levels during fasting. A 2005 clinical trial showed that insulin levels drop significantly but the body’s sensitivity to insulin increases upon alternate day fasting. As a result, the body burns and metabolizes stored fat instead of simple sugar as the energy source. Another study stated that 48-hour fasting may bring an increased epinephrine production, hence promoting fat metabolism. According to a US study, human growth hormone increases 5-fold during fasting, which enhances the breakdown of fat while preserving muscle mass.
Sustainability of Intermittent Fasting
Same as any other diet plans, when you stop the intermittent fasting regime and go back to your “not-so-healthy” normal diet, your body weight will slowly climb up again. Continual effort is crucial to maintain your desired weight. Let’s say you’ve gone through 6 months of intermittent fasting, what’s the next step?
Many other diet plans may not be feasible to keep for life, as they may involve giving up some of your favorite foods, or that they may be disrupted by different life events of yours: taking care of kids, working for long hours, just to name a few. Nonetheless, intermittent fasting is a relatively easy regime to practice indefinitely as a lifestyle change — it should get easier when you grow used to a fixed routine (start eating at 12pm and stop eating at 8pm, for instance). Alternatively, you can gradually expand the eating time frame from 8 hours to 10 hours or 12 hours. But do remember to continue eating healthily, and complement it with regular exercise for the optimal weight management outcome.
Is intermittent fasting suitable for everyone?
No. For people who have special medical conditions e.g. diabetes, cancer, lower blood pressure, cardiovascular disease, eating disorder, underweight, as well as for pregnant/breastfeeding women, it is recommended to consult the doctor or dietitian beforehand to assess whether this diet is suitable for your body condition.
The longer the fasting period, the greater the weight loss effect?
Not necessarily, an extended period of fasting (more than 3 days) may trigger the protective mechanism of the body to store more fat and slow down metabolism.
I feel tired after trying intermittent fasting, should I give up?
It is normal to feel slightly tired in the first 2 weeks of intermittent fasting because your body takes time to accustom to the new eating pattern. However, if you experience adverse symptoms, such as nausea, headache and mood swing, you should stop fasting and resume eating progressively to replenish your energy and nutrients. Consult a doctor if you want to continue the practice.
What can I eat during intermittent fasting?