Intermittent fasting is touted for its ability to promote weight loss and improve longevity. Consider the evidence before diving into starvation mode.

As the ultimate restrictive diet, the act of limiting food for large stretches of time through intermittent fasting has gone mainstream in recent years. With promises of health benefits, mental clarity, and longevity, what’s really true? 


Gaining popularity thanks to celebrities and mainstream sources, it’s time to set the story straight: Is intermittent fasting (IF) a safe and effective health intervention, or just another fleeting fad?

But first, what is intermittent fasting?

Fasting regimes come in all shapes and sizes…

Alternate Day Fasting (ADF)

Alternates between eating days, where food is consumed as usual, and fasting days, where no calorie-containing foods or beverages are consumed,

Modified Fasting (MF)

Limits caloric intake to 25% of daily requirements (~500 calories/day) for 2 nonconsecutive days a week, and ad libitum eating the remaining 5 days of the week (ad libitum = “as you please”)

Time Restricted Fasting (TRF)

No limits on daily calorie intake, but rather time limitations are placed on meals. Typically, ad libitum eating occurs during an 8 hour window, and fasting occurs during the surrounding 16 hours of the day.

Where did the concept of IF come from?

The concept of intermittent fasting was sparked by an assortment of theories.  Some propose that long bouts of fasting more closely resemble the eating patterns of our fellow mammalian species as well as the typical meal frequency of our human ancestors during hunting and gathering eras.  Others argue that time-restricted feeding helps synchronize our eating patterns with our biological circadian rhythm, so that oscillations in behavior, physiology, sleep, and metabolism are all synchronized in harmony.

Longevity researchers suggest extreme caloric restriction and IF serve as mild acute stressors, or “fire drills”, that condition the body to grow stronger and more resilient in anticipation for future stressors.  These advantageous adaptations are hypothesized to boost disease resistance and promote longevity.

Another theory is that IF creates ketosis, a state in which the body has fully exhausted its carbohydrate sources, and is forced to use fat as fuel.  Research also suggests that ketosis improves cognitive function, increases insulin sensitivity, inhibits cancerous cell growth, and reduces inflammatory markers linked to chronic disease.

While proponents of IF tout its ability to support weight loss, reduce the risk of cardiovascular disease, type 2 diabetes, Alzheimer’s, cancer, and potentially prolong life, it’s important to take an objective look at the research before you go starving yourself.

Though limited, there are in fact scientific studies that support these theories.

However, the critical question becomes: how strong is the evidence?  The answer: pretty weak.

Here’s why:

The majority of research on IF is based on animal studies, specifically mice and rats.  While animal studies can provide important scientific insight, unfortunately, the science does not always translate to humans.  THis is mainly due to the drastic fundamental differences in anatomy, biology, physiology in humans versus rodents.  Extrapolating scientific findings from animal research to humans is the epitome of making an apples to orange comparison.

So, are there any human studies?

While there are a handful of IF human studies, this research has several shortcomings.  Of the limited human research that does exist, the results are inconsistent: some studies link IF to weight loss but show no effect on the risk of chronic disease, and vice versa.

Small sample sizes, short study durations, and poorly constructed methodology significantly weaken the results derived from human research. Because the existing human studies are so short in duration, it’s nearly impossible to determine whether the potential benefits of IF are temporary or lifelong. Conversely, its difficult to know whether there are any long-term risks associated with IF.

In order to draw a definitive conclusion about IF, additional human research is required, using larger sample sizes, better research practices, and a longer duration of data collection.  The research methodology also needs to be designed in a manner that specifically isolates “fasting” as the experimental variable, and accounts for all confounding variables.

It’s also still unclear whether the weight loss observed using IF in preliminary studies is specifically due to time restricted feeding, or if it’s an indirect result of the caloric restriction that naturally accompanies IF.  Finally, more research is needed to tease apart the three types of IF in order to determine the optimal frequency and timing of meals, as well as identify the safest and most effective regime.

No matter what your take on intermittent fasting is, remember that most restrictive dietary regimens are difficult to maintain for the long-term. Keep it simple by eating real food and a balanced, active lifestyle.

That is undisputedly the most effective way to help you lead a sustainable, healthier life.

Adapted from the original article.

Lindsey Kane, MS, RD, LDN is a San Francisco-based Registered Dietitian helping others live a stress-free, balanced, and thriving life. By getting to know her clients inside and out, Lindsey identifies the opportunities within their everyday lifestyle to integrate subtle changes that create lasting, impactful results. Learn more at Bite For Change!


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