Pros and Cons of Intermittent Fasting

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 eliminated on certain days to protocols where food is only restricted at certain times of the day.

Even people who are already healthy may use these dietary patterns to reach and maintain a healthy weight and gain wellness benefits. Research is ongoing to understand the pros and cons of intermittent fasting fully. Long-term studies are lacking to know for sure if this eating style provides lasting benefits.

Pros

Easy to Follow

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

No Calorie Counting

Some people who are trying to reach or maintain a healthy weight prefer to avoid calorie counting. The process of measuring portion sizes and tabulating daily counts either manually or on a smartphone app can be tedious.

A study published in 2011 found that people are more likely to follow plans when pre-measured calorie-controlled foods are provided. Commercial diets such as 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.

No Macronutrient Limitations

Some popular eating plans 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 ones. This may require new cooking skills and learning to shop and stock the kitchen differently.

None of these skills are required when intermittent fasting. There is no target macronutrient range, and no macronutrient is restricted or forbidden.

Unrestricted Eating

Anyone who has ever changed their diet knows that you crave foods you are told not to eat. A study published in 2017 confirmed that an increased drive to eat is a major contributor to unsuccessful weight loss attempts.

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

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

Could Boost Longevity

One of the most widely cited benefits of intermittent fasting involves longevity. According to the National Institute on Aging, 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 have not yet confirmed this benefit.

According to a review published in 2010, there has been observational research linking religious fasting to long-term longevity benefits. Still, it was hard to determine if fasting provided the benefit or if related factors played a part.

Promotes Weight Loss

In a review of intermittent fasting research published in 2018, 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 efficient in reducing weight, irrespective of the body mass index (BMI). The article also noted that although researchers reviewed short-term weight loss, longer-term studies are needed.

Intermittent fasting may be no more effective than other diets that restrict calories on a regular basis. A 2018 study compared intermittent fasting with traditional diets (defined as continuous energy restriction) and found that weight loss benefits are similar.

In a large meta-analysis published in 2018, scientists reviewed 11 trials lasting between 8 and 24 weeks. Study authors concluded that intermittent fasting and continuous energy restriction achieved comparable results when weight loss and metabolic improvements were the goals. Longer trials are needed to draw conclusions.

It is also possible that weight loss results may depend on age. A study published in the journal Nutrition in 2018 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

In 2018, some intermittent fasting researchers proposed that this eating style may help those with type 2 diabetes manage blood sugar through weight loss in overweight or obese people—but may worsen insulin sensitivity in others.

A case series published in 2018 demonstrated the effectiveness of fasting (with medical supervision and nutritional training) to reverse insulin resistance while maintaining control of blood sugars over a 7-month period. Patients were able to stop insulin therapy, lose weight, reduce waist circumference, and have an overall improvement in blood glucose.

However, a study published in 2019 showed a less impressive impact on blood glucose control with a larger sample size. 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 groups.

These findings were consistent with 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.

However, the study authors note that intermittent energy restriction may be superior to continuous energy restriction for maintaining lower HbA1c levels. But more studies with larger sample sizes 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 notes that more research is needed to understand the benefit fully.

For example, research published in 2018 determined that intermittent fasting during Ramadan reduced total cholesterol, LDL, triglycerides in study participants. The participants also benefited from an increase in HDL ("good" cholesterol) levels.

Another study found intermittent fasting (specifically time-restricted feeding) may effectively combat systemic low-grade inflammation and some age-related chronic diseases linked to immune function without compromising physical performance. This has only been studied in 40 men, and a larger variant study is needed.

Cons

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 or tired or experience heartburn, constipation, dehydration, diminished sleep quality, or anemia.

If you have hypertension, high levels of LDL cholesterol, abnormally high levels of uric acid in the blood, hyperglycemia, cardiovascular disease, and liver and kidney diseases, intermittent fasting may be detrimental.

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 to fail to meet daily step goals and may even change their regular exercise routines. Continuing research has been proposed 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.

Medications

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 to be taken with food. Therefore, taking medications during fasting may be a challenge.

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 nutritious, balanced diet.

If you are following a short-term intermittent fasting program, 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 foods high in fat, calories, sodium, or added sugar) on days when "feasting" is allowed.

Long-Term Limitations

While the practice of intermittent fasting is not new, much of the research 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 provider can monitor your progress, including both benefits and concerns, to ensure that the eating style is healthy for you.