Pros and Cons of Intermittent Fasting

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Intermittent fasting (IF) has been used for centuries to treat a range of illnesses. There are many different styles of IF ranging from programs where food is completely eliminated on certain days to protocols where food is only restricted at certain times of the day. In recent years, the various dietary patterns have received more attention as a way to reach and maintain a healthy weight and to gain wellness benefits even in already healthy individuals.

Research is ongoing to fully understand the pros and cons of intermittent fasting. For now, long-term studies are lacking to know for sure if this eating style provides lasting benefits.


  • Easy to follow

  • No calorie counting

  • No macronutrient limitations

  • Unrestricted eating allowed

  • Might boost longevity

  • Promotes weight loss

  • May help with glucose control

  • May provide other health benefits


  • Side effects on fasting days

  • May reduce physical activity

  • May cause severe hunger

  • Concerns for those on medications

  • Does not encourage nutritious eating

  • May promote overeating

  • May not be advisable long-term


Easy to Follow

Many dietary patterns focus on eating particular foods and limiting or avoiding other foods. Learning the specific rules of an eating style can require a substantial time commitment. For example, there are entire books devoted to understanding the DASH diet or learning how to follow a Mediterranean-style meal plan.

On an eating plan that incorporates intermittent fasting, you simply eat according to the time of day or day of the week. Once you've determined which intermittent fasting protocol is best for you, all you need is a watch or a calendar to know when to eat.

No Calorie Counting

Not surprisingly, people who are trying to reach or maintain a healthy weight usually prefer to avoid calorie counting. While nutrition labels are easily found on many foods, the process of measuring portion sizes and tabulating daily counts either manually or on a smartphone app can be tedious.

Studies have found that people are more likely to follow plans when all pre-measured calorie-controlled foods are provided. Commercial diets such as WW, Jenny Craig, and others provide these services for a fee. But many people don't have the resources to pay for these types of programs—, especially long term.

Intermittent fasting provides a simple alternative where little or no calorie-counting is required. In most circumstances, calorie restriction (and subsequently weight loss) occurs because food is either eliminated or significantly restricted on certain days or during certain hours of the day.

No Macronutrient Limitations

There are popular eating plans that significantly restrict specific macronutrients. For example, many people follow a low-carb eating plan to boost health or lose weight. Others follow a low-fat diet for medical or weight-loss purposes.

Each of these programs requires the consumer to adopt a new way of eating—often replacing favorite foods with new and possibly unfamiliar foods. This may require new cooking skills and learning to shop and stock the kitchen differently.

None of these skills are required when intermittent fasting simply because there is no target macronutrient range and no macronutrient is restricted or forbidden.

Unrestricted Eating

Anyone who has ever changed their diet to achieve a medical benefit or reach a healthy weight knows that you start to crave foods that you are told not to eat. In fact, studies have confirmed that an increased drive to eat is a key factor during a weight loss journey.

But this challenge is specifically limited on an intermittent fasting plan. Food restriction only occurs during certain limited hours and on the non-fasting hours or days of the plan, you can generally eat whatever you want. In fact, researchers sometimes call these days "feasting" days.

Of course, continuing to eat unhealthy foods may not be the healthiest way to gain benefits from intermittent fasting, but cutting them out during certain days limits your overall intake and may ultimately provide benefits.

Could Potentially Boost Longevity

One of the most widely cited benefits of intermittent fasting involves longevity. Rodent studies have shown that when mice are put on programs that severely restrict calories (often during fasting periods) many showed an extension of lifespan and reduced rates of several diseases, especially cancers.

So does this benefit extend to humans? According to those who promote the diets, it does. However, long-term studies are needed to confirm the benefit. There has been observational research linking religious fasting to long-term longevity benefits, but it was hard to determine if fasting provided the benefit or if related factors played a part.

Promotes Weight Loss

In a published review of intermittent fasting research, authors report that the studies they examined showed a significant decrease in fat mass among subjects who participated in clinical trials. They also found that intermittent fasting was found to be efficient in reducing weight, irrespective of the body mass index.

It is possible, however, that IF is no more effective than traditional calorie restriction.

Intermittent fasting may be no more effective than other diets that restrict calories on a regular basis. Many studies have compared intermittent fasting with traditional diets (defined as continuous energy restriction) and found that weight loss benefits are similar.

In a large meta-analysis, scientists reviewed the conclusions from 11 trials lasting between 8–24 weeks. Study authors concluded that both intermittent fasting and continuous energy restriction achieved comparable results when weight-loss and metabolic improvements were the goal. They indicated that longer-term trials are needed to draw definitive conclusions.

It is also possible that weight loss results may depend on age. A study published in the journal Nutrition examined the effects of intermittent fasting (time-restricted feeding) on young (20-year-old) versus older (50-year-old) men. Intermittent fasting slightly decreased body mass in the young, but not in the older men. However, muscle power stayed the same in both groups.

Glucose Control

Some intermittent fasting researchers propose that this eating style may help those with type 2 diabetes manage blood sugar. But the findings have been inconsistent.

Some published case studies have demonstrated the effectiveness of fasting to reverse insulin resistance while maintaining control of their blood sugars. In these cases, patients were able to stop insulin therapy.

However, another study showed a less impressive impact on blood glucose control. Researchers conducted a 24-month follow-up of a 12-month intervention comparing intermittent fasting with continuous calorie restriction in people with type two diabetes. They found that HbA1c levels increased in both the continuous calorie restriction and intermittent groups at 24 months.

These findings were consistent with results from other studies showing that despite a range of dietary interventions it is not uncommon for blood glucose levels to increase over time in those with type 2 diabetes. The study authors do note, however, that intermittent energy restriction may be superior to continuous energy restriction for maintaining lower HbA1c levels, but noted that more studies are needed to confirm the benefit.

Other Health Benefits

Some studies have associated intermittent fasting with a host of other health benefits. However, almost every study author notes that more research is needed to fully understand the benefit.

For example, one study determined that intermittent fasting led to the reduction of total cholesterol, LDL, triglycerides in study participants. The participants also benefited from an increase in HDL levels.

Another study found that intermittent fasting (specifically time-restricted feeding) may be an effective strategy to combat systemic low-grade inflammation and some age-related chronic diseases linked to immune function without compromising physical performance.


Side Effects

Studies investigating the benefits of intermittent fasting also point to certain side effects that may occur during the fasting stage of the eating program.

For example, it is not uncommon to feel moody, tired, and experience headaches when your calories are severely restricted. It is more likely that these side effects will happen when food is entirely eliminated (for example, during programs like alternate day fasting) and less likely to occur when food intake is reduced (such as on the 5:2 diet when 500–600 calories are consumed during fasting days).

Reduced Physical Activity

One notable side effect of intermittent fasting may be the reduction of physical activity. Most intermittent fasting programs do not include a recommendation for physical activity. Not surprisingly, those who follow the programs may experience enough fatigue that they fail to meet daily step goals and may even change their regular exercise routines.

Ongoing research is underway to see how intermittent fasting may affect physical activity patterns.

Severe Hunger

Not surprisingly, it is common for those in the fasting stage of an IF eating plan to experience severe hunger. This hunger may become more extreme when they are around others who are consuming typical meals and snacks.


Many people who take medications find that taking their prescription with food helps to relieve certain side effects. In fact, some medications specifically carry the recommendation that they should be taken with food. Therefore, taking medications during fasting may be a challenge.

Anyone who takes medication should speak to their healthcare provider before starting an IF protocol to be sure that the fasting stage will not interfere with the medication's effectiveness or side effects.

No Focus on Nutritious Eating

The cornerstone of most intermittent fasting programs is timing, rather than food choice. Therefore no foods (including those that lack good nutrition) are avoided and foods that provide good nutrition are not promoted. For this reason, those following the diet don't necessarily learn to eat a healthy diet.

If you are following a short-term intermittent fasting program for weight loss or to gain a medical benefit, it is not likely that you will learn basic healthy eating and cooking skills, including how to cook with healthy oils, how to eat more vegetables, and how to choose whole grains over refined grains.

May Promote Overeating

During the "feasting" stage of many intermittent fasting protocols, meal size and meal frequency are not restricted. Instead, consumers enjoy an ad libitum diet. Unfortunately, this may promote overeating in some people. For example, if you feel deprived after a day of complete fasting, you may feel inclined to overeat (or eat the wrong foods) on days when "feasting" is allowed.

Long-term Limitations

While the practice of intermittent fasting is not new, much of the recent investigating the benefits of this eating style is relatively recent. For that reason, it is hard to tell if the benefits are long-lasting. Additionally, researchers often comment that long-term studies are needed to determine if the eating plan is even safe for more than several months.

For now, the safest course of action is to work with your healthcare provider when choosing and starting an IF program. Your health care team can monitor your progress, including health benefits and concerns to make sure that the eating style is healthy for you.

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