Is fasting safe as a weight-loss solution?


Fasting for spiritual reasons is a concept frequent in many religions such as Christianity, Buddhism and Islam. An entirely different approach that is becoming more and more popular is fasting to lose or maintain weight as well as other reasons, such as body cleansing. This subject is still controversial mostly because related studies have been conducted only on short periods of time and because the methods used for this practice are opposed to the ones used in diets.

Fasting vs. low-calorie diets

What is fasting? Fasting means willingly abstaining from some or all food and/or drink for a period of time.

There are many types of fasting designed for various purposes, such as the “water fast” meaning during fast days you can only drink water, cleansing fasts that allow drinking fresh juices made from various fruits and vegetables and partial fasts that imply excluding specific types of foods from your meals such as meat, rice and wheat.

Fasting for weight-loss usually means abstaining from all food for a period of time, usually 24-48 hours and drinking only non-caloric liquids. The most popular eating pattern is known as intermittent fasting (IF), which means alternating fasting and non-fasting periods.

Traditional dieting and fasting have one common denominator: the goal of both methods is to reduce calorie intake, thus create a calorie deficit to force the body into consuming stored fat. However, the means of reaching the goal are completely different.

Weight-loss diets usually have an eating pattern of 5-6 meals/day, consisting of breakfast, lunch and dinner with snacks in between. There are some benefits of splitting daily calories into smaller meals but most of them are at least exaggerated. The first one would be that eating increases your metabolism so the more often you eat, the higher it will be. The increase of metabolic rate caused by energy consumed to break down eaten food is known as the thermic effect of food (TEF). It is true that TEF increases every time you eat, but its increase is proportional to the number of calories you’ve eaten. As a result 3 larger meals will cause the same metabolic rate increase as 6 smaller meals.

Another promised benefit of eating small, frequent meals is the increased sensation of satiety that makes us feel full for a longer period of time, thus prevent overeating. Research studies performed on this matter had very different results. Moreover, many of them were only short-term studies or highly artificial (meaning they resembled little with real-life scenarios). As a result the “6 daily meals” method is more likely a possible solution rather than a universal one in terms of appetite control.

Eating small, frequent meals is also promoted to keep levels of blood sugar under control and provide the energy we need to perform our daily tasks at optimal capacity. The truth is that blood glucose level is maintained within normal range in healthy individuals even during a 48-hour period of fasting according to this study. Moreover, cognitive performance and mood states have not been affected in any way.

Another study, also performed on young, healthy subjects  researched the effects on blood sugar and insulin levels of eating the same amount of calories from 3 high carbohydrate meals (3CHO), 6 high carbohydrate meals (6CHO) and 6 high protein meals (6HP). The results showed that blood sugar levels where higher throughout the day in the case of 6CHO meals compared to 3CHO meals, with no difference in insulin levels. While the 6HP meals had the lowest levels of both insulin and blood glucose we consider the high-carbohydrate meals to have a closer resemblance with eating habits of most people.

This actually means that in terms of regulating blood sugar levels, eating 6 smaller meals every day instead of 3 larger ones will do exactly the opposite. The very least, blood sugar levels depend too much on the ingested nutrients to define the best approach based solely on meal frequency.

Potential benefits of intermittent fasting

Just to be clear, IF doesn’t mean you abstain from food during the fast periods so you can eat anything during non-fasting periods. You still have to make sure you eat quality foods. The goal is a healthier body, not just weight loss or maintenance and it cannot be done by eating burgers and French fries. Nonetheless, you won’t be forced to eat only low-calorie foods. Instead you be able to eat nutritious meals while compensating only partially for the calorie deficit created during the fast periods. Occasionally you will also be able to indulge yourself with deserts and other highly caloric foods which are usually forbidden in diets.

Compared to dieting, IF is a time-saver and much easier to achieve in terms of time spent eating, cooking and shopping for food. This is actually a reason why many people are having a hard time to follow a diet. A busy schedule makes it difficult to find both time and concentration needed to buy and cook all those meals as well as eat 5-6 times every day.

Several studies combined have revealed that IF weight-loss compared with diet weight-loss preserves more muscle mass. In other words the weight you lose by dieting will consist of a larger percentage of muscle mass than by intermittent fasting. However, the studies didn’t take exercise into account, which normally leads to gaining muscle mass in either case, but most likely at different rates.

Another study suggests that IF may have additional health benefits, such as lower risk of cardiovascular disease, diabetes and improvement of cognitive function. While it may be true to a certain extent, take it with a grain of salt since several tests have been performed only on animals.

Types of intermittent fasting

Sensible IF plans usually have fasting periods not longer than 30 hours alternated with 4-12 hours feeding periods and work even better when they include a workout schedule.

Fasting periods can also be used occasionally; such as before or after “cheat days” (that means any day when you overeat). It is a popular belief that regular fasting may force the human organism to starve.

The fasting phases show the opposite: the first phase lasts approximately 6 hours, time during which the body uses nutrients from the last meal, absorbed from the intestinal tract. The second phase can last up to 48 hours and even more. During this period the body uses glycogen stored in muscle and liver tissue to fuel the brain with glucose.

Fat (adipose tissue) starts to be converted into fatty acids a few hours later. Within 10 hours, half of the muscle fuel comes from fat.  Starvation only occurs when the body’s fat stores are depleted. That is when our body starts using protein (found in muscle tissue) and can have serious negative health effects starting with the function of organs. This process usually begins after several days of fasting and cannot be “forced” by any IF regimen.


The objective of intermittent fasting is to make the body use stored fat without negative health effects. IF plans are not “diets”. They were designed as a choice of lifestyle not a temporary solution from which you can go back to the former habits which caused the problems in the first place.

Intermittent fasting may be a solution for many people, nonetheless consulting a physician before adopting any form of IF is advisable.  Moreover, fasting is not recommended in certain cases, such as children under 18 years, diabetics and pregnant women. Choosing and following the right plan for you should be done under the supervision of a trained nutritionist.