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Health A to Z  —  9 minutes

Fasting to lose weight: Is it safe and effective?

July 12th, 2021
Biron Team
Biron Team

Diet fads come and go, but some are more popular than others. In 2019, intermittent fasting was one of the most searched topics on Google in Canada, and this approach has been endorsed and popularized by many celebrities. But is it really effective? And how risky is it to cut back on your food consumption so dramatically? Don’t we eat three meals a day because we need them?

What is intermittent fasting?

Intermittent fasting is more a way of life than a diet. It consists of adopting a systematic eating pattern alternating eating and fasting periods. The main difference from traditional weight-loss diets is that it is not principally based on restriction of calories, but on the most appropriate timing of caloric intake.

Is intermittent fasting more effective than other weight-loss regimes?

Losing weight seems to be a rather simple equation: calories consumed minus calories spent. If the result is greater than zero, you gain weight. If the result is negative, you lose weight. You can achieve a better result by increasing your physical activity or reducing the total number of calories in your diet. There are hundreds of groups and websites that can propose programs to help you with this. Of course, you can combine both, and it will be easier if you develop a program tailored for you with the help of a doctor, nurse, kinesiologist, dietitian, psychologist or any other qualified health care professional.

If we consider only the reduction of calories in your diet, intermittent fasting is as good as any other regime, and probably better than many others, as long as you can sustain it. A December 2019 article in the New England Journal of Medicine (NEJM) [1] presented a systematic review of hundreds of studies on animals and humans and showed how intermittent fasting positively affects general health indicators such as blood pressure and cholesterol levels. The article found that intermittent fasting consistently produces – at least in animal models – additional beneficial effects besides weight loss: for one, a positive effect on life expectancy! It also found that intermittent fasting seems to slow or reverse the aging process and that there is a very high probability that this is also the case in humans. Losing weight is more than just calories in minus calories out: it also depends on the timing of the caloric intake.

What are the other beneficial effects of intermittent fasting?

Although it is difficult to separate beneficial effects of intermittent fasting from those of weight loss, intermittent fasting has been proven effective in humans to:

  • Stabilize blood sugar levels
  • Increase resistance to stress
  • Suppress inflammation
  • Decrease blood pressure
  • Decrease cholesterol levels
  • Improve resting heart rate
  • Improve brain health and memory [2]

In animal models in the NEJM study, intermittent fasting was shown to decrease the resistance of cancer cells to therapeutic agents (promising ongoing studies in humans) and to extend significantly the lifespan.

A method suited for everyone?

No. Except under strict medical supervision, intermittent fasting is not recommended for children, pregnant or breastfeeding women, people with a very low BMI (less than 18), those with a history of eating disorders (bulimia or anorexia) seniors or people who are under treatment for diabetes. [3]

How does intermittent fasting work?

Dietary glucose and fatty acids are the two main energy sources for our cells. In general, after a meal, glucose is used as an energy source, and fatty acids are stored in fat tissue. When you do not eat, glucose is not immediately available, so the body turns instead to its emergency solution: burning fat. That said, the replacement of glucose as the main cellular energy source occurs after only a few hours of fasting. To be effective, fasting periods should last at least 14 to 16 hours.

How to initiate your diet

There are several techniques for intermittent fasting, including the following:

  • Time-restricted eating: all food to be consumed within a 10 hour time frame daily, 5 days a week, then gradually reduce this time frame to 6 hours daily, 5 days a week;
  • The 5:2 intermittent fasting diet: reduce calorie consumption to 1000 calories per day, 2 days a week and then, over a period of 4 months, reduce this consumption to 500 calories per day, 2 days a week.

Neither approach has any particular advantages over the other. The choice between the two depends on what is easiest for you to maintain, which you should consider with the help of a professional.

As an example, to restrict your eating to a 10-hour period, you could eat from 7 a.m. - 5 p.m. or 10 a.m. - 8 p.m. For a six-hour period, that could be from 7 a.m. - 1 p.m. or 10 a.m. - 4 p.m.

One of the elements for successful transition to intermittent fasting is to proceed GRADUALLY. Do not try the six-hour feeding period seven days a week or the 500-calorie diet two days a week at the beginning. These are goals that should be attainable only after four months of transition. Typical transition plans for either regime could be established with the help of a health care professional (physician, dietician, nurse).

Intermittent fasting table

Your intermittent fasting diet must differ from your habitual diet: you will need to increase the amount of protein and fibers. There are numerous proposals on the web indicating to eat high-fibre (nuts, beans, fruits and vegetables) and high-protein foods (meat, fish, tofu, etc.) The key is to develop a diet plan that is sustainable for you. With the help of health professionals, choose an eating approach that suits you and is sustainable in the long term.

How about this abstinence?

Dry fasting, or absolute fasting, restricts both food and liquids. There are many websites describing the benefits of intermittent dry fasting. Contrary to food-only intermittent fasting, there are actually no studies on the added benefits or secondary effects of repeatedly restricting water intake during intermittent fasting. However, avoiding water for significant periods is known at the very least to involve increased hunger, tiredness and irritability. Urination will decrease. For coffee lovers, it may involve headaches and poor focus [4].

Are there any side effects to intermittent fasting?

The side effects of the fasting are hunger rages, irritability, lack of concentration, hypoglycemia, lack of energy and a decrease in basal metabolic rate. These side effects should disappear during the first month of intermittent fasting.

As long as an appropriate balance of nutrients is maintained, there are no known serious long-term side effects to intermittent fasting in the majority of individuals. Our typical North American diet consists of three meals plus snacks every day and getting out of this habit may be a major hurdle for people wanting to try intermittent fasting.

What to take away from this method

Intermittent fasting can be an effective weight-loss technique if you approach it carefully and gradually. However, it may bring dramatic short-term changes accompanied by their share of side effects. Therefore, take a cautious and thoughtful approach if you choose to try it, and seek the help of professionals you trust.

  1. Cabo, Rafael De, and Mark P. Mattson. “Effects of Intermittent Fasting on Health, Aging, and Disease.” New England Journal of Medicine 381, no. 26 (2019): 2541–51.
  2. Thompson, Dennis. “'Intermittent Fasting' Diet Could Boost Your Health.” WebMD. WebMD, December 26, 2019.
  3. Tello, Monique. “Intermittent Fasting: Surprising Update.” Harvard Health Blog, February 10, 2020.
  4. Lefebvre, Myriam. “Jeûne Sec : Une Diète Inquiétante Qui Suggère De Ne Pas Boire D'eau.” Le Journal de Montréal, February 6, 2020.
Biron Team
Biron Team