• Dr. Sarah Penney, ND, MSc

Fasting and Weight - Worth The Hunger?

Can Fasting Help Me Lose Weight?

Weight loss is often defined as calories in vs calories out, so it makes sense that cutting your calories would help shed the pounds. Many in this field however advise against skipping meals or eating very little due to the effect it may have on your metabolism. Your metabolism is the fire that converts your food into energy, and plays a role in how many calories your body is burning even at rest. One common theory is that fasting puts the body into ‘starvation mode’, where the metabolism is decreased to conserve resources, and thus weight loss efforts will be stunted. The available research does not seem to demonstrate a significant negative impact on metabolic rate in the first few days of fasting, although it may be decreased when long term fasting is undertaken. (1-4)

Contrary to concerns about metabolism, new research suggests that intermittent fasting may produce positive results for weight loss and markers of heart health. One recent study using fasting one day per week combined with nutritional counselling in obese women showed reductions in both body weight and LDL (‘bad’) cholesterol levels after 10 weeks (5). Interestingly, intermittent calorie restriction to about 25% of regular daily calorie intake 1-2 days per week may also be as effective as long-term calorie restriction for weight loss (6). A review published in 2015 that looked at 40 trials investigating weigh loss and intermittent fasting comfirmed that although this method seems to be effective for weight loss, it is not superior to the usual dieting template of daily calorie moderation (7). Proponents of intermittent fasting suggest that the benefit of this method over other restrictive diets may lay in the black and white instructions– less choice breeds less temptation. That is not to say intermittent fasting doesn’t require dedication and willpower – it can be a difficult practice to integrate into your life on a weekly basis.

When using intermittent fasting for weight loss, the foods you eat on your days off can also impact your success, as one benefit of intermittent fasting may be the overall weekly decrease in calories from skipping a day of meals. One study has however found that when subjects eat whatever they want after a day of fasting, food choices tend to be high calorie carbohydrate foods, which may cancel out any calorie deficit from their previous day of fasting (8). Another issue may be maintaining a dietary pattern of frequent fasting long term, as some results show that hunger during a type of fasting that is done every other day did not improve with prolonged practice (9).

What Are Some Possible Complications?

It is important to consider potential risks associated with the type of fasting you choose. No fasting should ever completely restrict water intake, as this can result in a serious risk of dehydration especially in the summer months. Symptoms may include dry mouth, headache, dizziness, decreased urine output and low energy. Other symptoms may include short-term withdrawal from certain substances like caffeine or sugar, causing temporary cravings, cranky mood, fatigue and headaches. Long-term fasts have the potential to impair nutrient intake and cause deficiencies of vitamins, minerals or protein if food is not carefully planned on the days you are eating. Juice fasts or partial fasts inherently have less of these risks as you are generally delivering high doses of nutrients to the body, and short term intermittent fasting is unlikely to disturb overall nutrient balance.

Is Fasting Safe For Me?

Several medical conditions pose a higher risk of health complications to those fasting. It is not recommended that people with Type 1 Diabetes, or insulin dependent Type 2 Diabetes undertake a fast unless they are medically supervised, as a change in blood sugar levels caused by restricted calorie intake can have serious health consequences. Pregnant and nursing mothers should not perform fasting due a potential effect on their child, and children under the age of 18 should not fast as nutrient intake is essential for development. Fasting is not recommended for individuals with certain heart conditions like heart failure or defects, rhythm disturbances or those who have had a heart attack. Individuals suffering from malnutrition, eating disorders, significant liver or kidney disease, infectious diseases, metastatic cancers, and many other medical conditions should not perform a fast. It is best to talk to you your medical professional before staring a fast to make sure it is safe for you. Never discontinue any prescription medications during a fast, and talk to your pharmacist to see if the action of your medications would be affected by a putting the body in a state of fasting.

Always talk to your doctor about what is right for you when it comes to weight loss - this article is meant to provide information only and is not individualized medical advice for you! I spend 1.5 with each patient in their initial visit investigating the cause of health concerns like weight management issues - book online anytime through my website!

Dr. Sarah Penney, ND, MSc



  1. http://ajcn.nutrition.org/content/81/1/69.full

  2. http://www.ncbi.nlm.nih.gov/pubmed/2180482

  3. http://www.ncbi.nlm.nih.gov/pubmed/8172872

  4. http://www.ncbi.nlm.nih.gov/pubmed/20484315

  5. http://www.nutritionandmetabolism.com/content/9/1/98

  6. http://www.ncbi.nlm.nih.gov/pubmed/20921964

  7. http://www.ncbi.nlm.nih.gov/pubmed/26384657

  8. http://www.ncbi.nlm.nih.gov/pubmed/22732752

  9. http://ajcn.nutrition.org/content/81/1/69.full

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© 2020 by Dr. Sarah Penney, ND