Skip to content
Link copied to clipboard

Everything you need to know about Intermittent Fasting

As a registered dietitian, it's up to me to weed through the hype, dig up the research, and uncover the truth.

In a world of information overload, we are constantly inundated with nutrition advice coming from all directions, most of which is conflicting, confusing, and contradicting. As a registered dietitian, it's up to me to weed through the hype, dig up the research, and uncover the truth.

Today's nutrition investigation is on Intermittent Fasting. Gaining popularity thanks to celebrities likes Hugh Jackman, Jimmy Kimmel, Jennifer Lopez, and Beyonce, it's time to set the story straight: Is Intermittent Fasting (IF) a safe and effective health intervention, or just another fleeting fad? Read on to discover the hard and fast truth behind IF.

Types of Fasting

Fasting regimes come in all shapes and sizes...

  1. 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.

  2. Modified Fasting (MF):  Limits caloric intake to 25 percent of daily requirements (~500 calories/day) for two nonconsecutive days a week, and ad libitum — or "as you please" — eating the remaining five days of the week.

  3. 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.

Theories Behind IF

The concept of IF 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. Longevity researchers suggest IF serves as a mild acute stressor, or "fire drill", conditioning the body to grow stronger and more resilient in anticipation for future stressors. 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.

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

The Evidence

Though limited, there are in fact scientific studies that support the above theories. The critical question becomes, how strong is the evidence? The answer: pretty weak. Here's why…

  1. Animal studies: The majority of research on IF is based on animal studies, specifically rodents like mice and rats.  While animal studies can provide important scientific insight, unfortunately, the science does not always translate to humans due to the drastic differences in anatomy, biology and physiology.  Extrapolating scientific findings from animal research to humans is the epitome of making an "apples-to-oranges" comparison.

  2. Human studies: There a handful of IF human studies but 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, and conversely, 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 needs to be designed in a manner that specifically isolates "fasting" as the experimental variable. It's 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.

The Cross Examination

The case for Intermittent Fasting has been analyzed from all angles. Here are the pros and cons to consider…

The Cons of IF

  1. Skipping Meals: Fasting requires skipping meals, which can induce hypoglycemia, ketogenesis, brain fog, fatigue, and mood swings.   

  2. Dysfunctional Eating: Extreme restriction during fasting promotes mindless and uncontrollable binge eating on feeding days, creating an unhealthy "all or nothing", yo-yo approach to eating.

  3. Quantity over Quality: With so much focus on meal timing, IF tends to lose sight of what's most important: the quality of your food.  The emphasis becomes more about when you eat rather than what you eat.

  4. Calorie Restriction in Disguise: Fasting involves significant calorie restriction, which is associated with slower metabolism, malnutrition, disordered eating, and unsustainable weight loss followed by a significant relapse of weight gain.

  5. Impracticality: IF requires resisting human nature and ignoring our instinct to eat in response to hunger. Following an IF regime for a lifetime is not only unenjoyable and illogical, it's also completely unrealistic.  A true health solution is one that yields lasting results in a practical, reasonable, and realistic manner.

On a more positive note, there is a valuable lesson to learn from IF, and that's the relationship between food and time. Today, eating has become a 'round-the-clock activity, snacking our way from one meal to the next. Gaining a better understanding of how your food choices, meal timing, and eating frequency interact can help guide you towards a healthier, more personalized eating pattern. Here's how:

  1. Food Stamina: Pay attention to the composition of your meals, and note which foods provide long staying power versus which foods provide quick bursts of energy followed by abrupt crashes.

  2. Pace Yourself: Get to know the ideal meal structure that supports your personal digestive pace. One person may function better on three-square meals/day, while another may benefit from 4-6 smaller meals.

  3. Sneaky Hours:  Keep track of not just when you eat, but why you are eating.  You'll probably be surprised to find that you repeatedly snack out of boredom during certain times of the day (mostly mid to late afternoon, and in the evening).  Anticipate these hours by arming yourself with non-edible antidotes, such as calling a friend, or going for a walk.

  4. Eating Curfew: Most of our late night eating involves mindlessly munching on nutrient deplete nonsense.  Take a page out of the IF playbook and aim to cap all food intake at least two hours before bed.

The Final Verdict

When it comes to IF, the jury is still out. But rather than wait around for stronger and more conclusive evidence, take advantage of the strategies already proven to optimize health and well-being: Eat real food. Sleep. Hydrate. Move. Relax. And, be mindful. Not only is this formula logical, practical and enjoyable, it's also effective and science-backed. Case closed!

Lindsey Kane is a Registered Dietitian from Philadelphia. For more nutrition tips and recipes, visit her blog


Read more Goal Getter for healthy eating, weight loss and more.