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Fasting, Effective or Risky?

Fasting is abstaining from eating any food, sometimes including water, for one or several days. Some wish to get therapeutic benefits: reduction of pain, purification and detoxification of the organism, reduction of blood pressure, etc. Others simply want to lose a few kg in record time, anticipating a special evening or wedding. Is fasting effective, and most of all, is it safe?

Every day, our body needs a certain amount of calories to accomplish its basic functions, i.e., maintaining tissues, body temperature, blood circulation and filtration, breathing, etc. This is what we call basal metabolism1. Picture this: the brain alone uses 120 g of carbs every day2! Of course, the brain can’t function without this vital energy. Therefore, fasting or not eating enough carbs forces the body to draw within its own glycogen reserves. However, their supply is limited, and usually completely consumed within 24 h.

Beyond this period, the body converts fat into carbohydrates. This is what happens when we fast, or follow a restrictive diet that provides less than 50 g of carbohydrates per day. How is using fat as an energy source harmful to the organism? In practical terms, the transformation of fat into carbs leads to the formation of toxic waste (ketonic acid) that accumulates in the blood supply. One of the effects of blood acidification is a feeling of euphoria and well-being3, which makes us feel light on the first days of a fast. However, nausea and headaches, constipation, muscular cramps, dehydration, and even loss of appetite can also happen4.

When the body does not have enough fuel, it can also convert protein into carbs. In order to do so, it must break down muscle tissue. Therefore, abstaining from food for a few days involves a loss of muscle tissue, as well as a reduction of the basal metabolism. It is the ideal combination to regain a few extra kg quickly once we return to a normal nutrition.

In short, despite the surprising weight loss, and the feeling of happiness and relaxation associated with fasting, it would be wiser to turn to long-term dietary practices. You will avoid undesirable effects on your health and weight (read the article The Dangers of Popular Diets, by Karine Larose).

By Vanessa Martin


  1. Dubost, Mireille. 2006. La nutrition. 3e édition. Montréal : Chenelière Éducation, 367 p.
  2. Institut canadien du sucre. 2004. Apports nutritionnels de référence pour les sucres. En ligne. < http://www.sugar.ca/francais/pdf/carbohydratenews/CarboNews2004.pdf>. Consulté le 5 octobre 2012.
  3. McClernon FJ, Yancy WS, Eberstein JA, Atkins RC, Westman EC. 2007. The effects of a low-carbohydrate ketogenic diet and a low-fat diet on mood, hunger and other self-reported symptoms. Obesity. 15;1:182-187.
  4. Bray GA. 2012. Dietary therapy for obesity. En ligne. In UpToDate. <http://www.uptodate.com/contents/dietary-therapy-for-obesity?> Consulté le 5 octobre 2012.

Vanessa Martin

A newcomer to Nautilus Plus, Vanessa Martin holds a degree in nutrition from the Université de Montréal and is a member in good standing of the Ordre professionnel des diététistes du Québec. She also works in the hospital setting and loves to blog in her spare time. Passionate and versatile, Vanessa plans on enhancing her knowledge in the field of psychology with an eye to better guiding and motivating the habit-changing endeavors of her clients. Member of a running club, she enjoys taking part in the competitions organized in her area. Vanessa is currently training for a 21 km race and would like to run her first marathon!

Fasting, Effective or Risky? is a post from I'm taking charge. I'm taking charge is a blog that aims to help people in their journey to fitness through articles on training, nutrition, motivation, exercise and healthy recipes.
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