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    A Dietitian's Take on Keto vs. Intermittent Fasting -

    Imagine a scenario where you just attended your annual check-up with your physician, where you learned that you put on a few extra pounds since this time last year. You decide that enough is enough. This is the year! You’re going to start making meaningful changes to your fitness and nutrition routine.

    Because you genuinely care about your health, you want to make sure the steps you are taking are the healthiest and safest ones possible. So, you do what many of us would do in the same situation…you consult the internet, only to become overwhelmed by the sheer amount of information available out there.


    Sound familiar? If so, you are far from alone. Here are the facts:

    • According to the Centers for Disease Control, more than half of American adults are considered overweight or obese.
    • Research strongly suggests that being overweight or obese is associated with an increased risk for developing several chronic diseases including heart disease, type 2 diabetes, stroke, and many types of cancer. Furthermore, individuals with obesity are at a heightened risk of death from all causes compared to those who are overweight or at a normal weight.
    • While positive messages about body acceptance and whole person health and wellness are on the rise, many Americans are still trying to lose weight, with nearly 50% of us reporting trying at least one weight loss diet in the past 12 months. Evidence of this is reflected by the weight loss and diet control market, which raked in a record breaking $72 billion dollars in 2018. and is projected to continue increasing over the next 5 years.


    You would think that with unlimited access to information, it should be easier than ever to find the perfect DIY diet method without having to shell out hundreds of dollars for meal replacements or professional guidance. Instead, we are bombarded with crash diets, anecdotes from under-qualified “experts,” and companies looking to profit off of our desperation to lose weight, be healthy, or fit a certain body type ideal. Frankly, it’s exhausting.

    For now, let’s focus on two of the most popular weight loss methods of 2020: intermittent fasting and the ketogenic diet, also known as keto.


    What is Intermittent Fasting?

    Intermittent fasting is an umbrella term describing any meal pattern where the time spent eating and drinking is confined to set times during the day or week, while the rest of the time is spent abstaining from food. In other words, while many diets focus on what you can eat, intermittent fasting focuses on when you can eat.

    There are 3 main types of intermittent fasting: alternate day, periodic, and time-restricted fasting.

    01 – Alternate Day Fasting

    Alternate day fasting involves switching between “fast” days, where very small amounts of food are eaten, and “feast” days, where eating is largely unrestricted. This style switches every other day and is considered quite aggressive, since there are 3-4 days per week where little to no food is consumed.

    02 – Periodic Fasting

    Periodic fasting involves periods of 24 hours or more of fasting followed by normal, unrestricted eating for a set amount of time. Some may choose to consume up to 25% of their daily requirements during fasting periods, making this style slightly easier to stick to. The 5:2 method is an example of periodic fasting in which you would eat normally for 5 days a week and fast for 2.

    03 – Time-Restricted Fasting

    Time-restricted fasting focuses on restricting eating for a set number of hours during a 24 hour period. The bulk of the fast takes place during sleeping hours. 16:8 is a common method, where a person fasts for 16 hours a day and confines eating to 8 hours a day. Skipping meals or limiting eating to one meal a day are also examples of time-restricted fasting.

    Fasting has been practiced throughout human history as a part of religious and cultural traditions. It’s only been during the past 10 years or so that intermittent fasting has been used as a popular weight loss method.

    So, what make’s intermittent fasting so appealing?

    In order to lose weight, you must burn more energy than you bring in. Intermittent fasting is useful in this regard in that there are substantial periods of time where no energy is consumed at all, and the total amount of time spent bringing in energy is cut short. Additionally, since the focus of intermittent fasting is time-based, there are no restrictions on what types of food you can eat as long as you maintain a negative energy balance.

    What do the experts say?

    According to a 2019 review,  intermittent fasting produces “equivalent weight loss when compared to continuous energy restriction” with “no differences between groups in weight or body fat loss.” Basically, this means that when it comes to weight loss, intermittent fasting provides no added benefit compared to just eating fewer calories throughout the day, provided you are burning more calories than you’re eating.

    Intermittent fasting may also give rise to additional health benefits. For example, periods of fasting have been shown to improve insulin resistance, glucose metabolism, and inflammation, decreasing the risk for developing cardiovascular disease and type 2 diabetes. But as with weight loss, these effects appear to be due to the calorie restriction and not the fasting itself.

    Please note – because intermittent fasting as a weight loss method is a relatively new topic, there is not a lot of research on the long-term effects and potential complications of it. Furthermore, many of the existing studies have been done using animal models or were purely observational, meaning the evidence is not very strong. Randomized control trials using human subjects is the gold standard for strong research.

    It’s also important to mention that intermittent fasting should not be practiced by everyone. Pregnant women, children, people with a history of disordered eating, and those with diabetes should consult with their doctor, as intermittent fasting could have negative consequences for these populations.

    Bottom line, intermittent fasting is a relatively safe and effective method for weight loss as long as a modest caloric deficit is maintained. It may also improve insulin sensitivity, glucose metabolism, and inflammation. While intermittent fasting has yet to prove better than calorie restriction, it may be easier to stick to since there are no limitations on the types of food you eat.


    What is the ketogenic diet?

    The ketogenic diet, commonly known as keto, is a high fat, low carb diet that was originally used to treat epilepsy in children. In order for keto to be effective, at least 70% of total daily calories must come from fat. 10-20% of calories come from protein, while the remaining 5-10% come from carbohydrates (5-10%). This is considered a very low-carb diet; the current Dietary Guidelines for Americans recommend 45-65% of total calories come from carbohydrates.

    While many types of low-carb diets exist, the keto diet is unique in that due to the low availability of carbohydrates, the body is forced to shift from using glucose to drive energy production, to using fat as the body’s main fuel source. This metabolic shift generates compounds called ketones, which are then used by cells to create energy. This metabolic state is called ketosis.

    It can take the body anywhere from 2 days to 2 weeks to reach ketosis, depending on a variety of factors, including level of physical activity and the amount of carbs included in the diet before attempting keto. Furthermore, eating too much protein or carbohydrates can knock the body out of ketosis, as both of these nutrients can be converted to glucose for energy, causing the body to shift back to glucose-driven metabolism.

    What makes keto so appealing?

    In theory, a body in ketosis is better suited to use its own fat reserves for fuel compared to a higher carb diet, leading to weight loss without  losing muscle tissue. Furthermore, fatty foods tend to be highly palatable and can keep you feeling full for longer, making this diet easier to stick to for those trying to maintain a calorie deficit in order to lose weight.

    What do the experts say?

    Like intermittent fasting, weight loss with keto is likely a result of eating fewer calories than you burn, with keto being equally as effective as regular old calorie restriction. The few studies that show keto being better than higher carb diets typically allow subjects to eat ad libitum, meaning calorie totals are not controlled. This doesn’t invalidate the study –  it just demonstrates that keto has a favorable effect on reducing appetite.

    It is commonly known that during the first few weeks of starting keto, dieters experience rapid weight loss. However, most of this is water weight due to the diuretic effect of ketones and as a result of the body using up stored carbohydrates for energy. After this point however, research regarding how keto effects body composition is less clear. One study involving overweight and obese men demonstrated that there was no significant difference in the amount of fat lost while on keto compared to higher carb diet with the same amount of calories.

    On the other hand, a different study exploring the same thing, but in resistance trained men found the opposite; subjects following a keto diet had greater reductions in fat mass compared to the non-keto group. However, this study was also examining the effect of keto on gaining muscle, so the subjects were also lifting weights and eating more calories than they were burning. This may have contributed to the larger decrease in fat mass experienced by the keto group.

    Most studies agree that any form of weight loss comes with an undesirable decrease in muscle mass as well. This is particularly true for low-carb diets compared to more balanced diets. However, muscle mass tends to be better preserved when the diet is higher in protein. While keto is primarily a high fat diet, it is also high in protein, providing 1 to 1.5 grams per kilogram of body weight. Therefore, a standard keto diet is likely sufficient to preserve muscle mass, especially when paired with regular resistance training.

    Beyond Weight Loss

    Beyond weight loss and body composition, keto may produce other health benefits as well. While in ketosis, insulin levels are kept at a minimum, which may improve insulin sensitivity, promote fat loss, and reduce inflammatory markers.

    However, keto does come with a great deal of potential downsides. For example, followers of keto are very limited in the types of food available to them due to the avoidance of healthy carbohydrates. This puts dieters at a risk of vitamin and mineral deficiencies and drastically reduces the amount of fiber in the diet, which could lead to constipation.

    Furthermore, diets high in saturated fat are known to raise total cholesterol, increasing the risk of developing heart disease. Few studies show that keto can promote favorable reductions in total cholesterol, but these effects are likely short lived.

    Like intermittent fasting, keto has not been around long enough to study the long term effects so it may be a while before we know if keto is a safe method for long term weight loss. What we do know is that people with liver, kidney, or pancreas issues should not attempt keto without discussing it with their doctor first. This goes for people with type 2 diabetes as well. People with type 1 diabetes however should avoid keto completely, as it can lead to a life threatening condition called ketoacidosis.

    Bottom line, keto is effective at promoting weight loss while preserving muscle mass and may improve insulin sensitivity. The high amount of fat in the diet can help dieters feel full for longer, making keto easier to stick to in the short term. However, as with any overly restrictive diet, keto is likely not sustainable long term due to the exclusion of so many foods. We also do not know the long term effects of using this diet for weight loss, since it hasn’t been around long enough to produce much research. Sticking to a moderate calorie-restricted diet that includes a variety of healthy foods from all 3 macronutrient groups while getting plenty of exercise


    So which is best for weight loss? Intermittent fasting or keto?

    • Both diets promote weight loss, provided you’re eating fewer calories than you burn
    • Both diets have the potential in increase insulin sensitivity, leading to improved glucose utilization and reduced risk for developing diabetes and heart disease
    • Intermittent fasting is more flexible than keto when it comes to the types of food you can eat. This increases the pool of available vitamins and minerals obtainable from the diet
    • Intermittent fasting can be modified for those who require or desire shorter fasts and is not mandatory to fast every day. Keto is all or nothing: you’re either in ketosis or not and keto requires strict daily adherence.
    • Keto may produce quicker results compared to certain types of intermittent fasting


    Bottom line

    There is no best diet for everyone, as everyone has nutrition requirements as unique as they are. The best diet is one that is varied, colorful, sustainable, meets all your nutrition needs, and allows you to have a healthy and joyful relationship with food.


    Have you tried Intermittent Fasting or the Keto Diet? Tell us about your experience in the comments!


    A Dietitian’s Take on Keto vs. Intermittent Fasting

    written by:

    Evangeleena Manzanares

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