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Debunking Fad Diets

Submitted by Amy Sercel MS RD CD

Edited by Marcia Bristow MS RDN CSSD CD

After the new year we’re reminded of the “great new diets” that will make this the year to reach our goals.  With so many trends on social media, in health magazines, and sometimes even recommended by medical professionals, it can be difficult to sort through the clutter of fact vs. fiction.

If you find yourself wondering, “Do any of these diets even work?” you are not alone!  In this post, we have highlighted some of the most common diet trends you’re likely to keep hearing about in 2019.

The Ketogenic Diet

Probably the most popular fad diet right now, the ketogenic diet requires you to cut out carbohydrates until you’re in ketosis.  Your brain and muscles prefer to use carbohydrates as a source of fuel.  When you don’t provide this to your body through your diet, your body has to break down fatty acids and convert them into ketones, the only other source of fuel your brain can use.   This process is called ketosis.1

What You Can Eat: On the ketogenic diet, you can eat as much as you want of foods that don’t provide carbohydrates.  This means seafood, poultry, cheese, avocadoes, eggs, Greek yogurt, and oils are encouraged in large quantities.  You can also still eat small amounts of carbohydrate-containing foods and remain in ketosis.

What You Can’t Eat: The exact amount of carbohydrates you can eat and remain in ketosis varies from one person to the next, but in general people on the ketogenic diet usually eat less than 50 grams of carbohydrates per day.1 To put it into perspective, there are about 15 grams of carbohydrates in a slice of bread, 12 grams of carbohydrates in 1 cup of skim milk, and 27 grams of carbohydrates in a medium banana.

It’s also important to remember that high-protein diets may push you out of ketosis, because some proteins can be converted into glucose.1

What the Research Says: Over the short term, ketogenic diets have been shown to result in weight loss more quickly than traditional diets.  After about 6 months, the weight loss levels out, and ketogenic diets are not shown to be any more effective than traditional diets for long-term weight loss.  At the same time, this restrictive, high-fat diet can increase your risk for nutrient deficiencies and high cholesterol.1

If you’re an athlete, the ketogenic diet has been shown to limit anaerobic exercise performance.  In one study, athletes experienced decreased exercise performance after following a low-carbohydrate diet for only four days.2

The Verdict: There is not enough evidence that the ketogenic diet has any long-term health benefits for the average person.  It restricts entire food groups that are important sources of essential nutrients and is likely difficult to follow in the long-term.  At the same time, you still need to think about your overall calorie intake if you want to lose weight.  High-fat foods also tend to be high in calories, and it is your overall calorie intake that will promote weight loss, not just the amount of carbohydrates you eat.

The Paleo Diet

The Paleo diet encourages you to adopt the eating habits of your Paleolithic ancestors from about 2 million years ago.

One major flaw of this diet is that people don’t know exactly what foods were available to humans 2 million years ago.  The foods eaten at that time would have depended on where in the world those humans were living.  It is also very difficult to say that the foods available in our grocery stores today are the same as they would have been in prehistoric times.

What You Can Eat: In general, the Paleo diet allows you to eat meat, fish, shellfish, eggs, nuts, seeds, fruits, vegetables, olive oil, coconut oil, and some honey.3 You are encouraged to eat unprocessed foods in their natural state.

What You Can’t Eat: As people in the Paleolithic times were hunters and gatherers, the Paleo diet discourages you from eating anything that would not grow wild on its own.  This means avoiding both whole and refined grains, sugar, dairy products, beans, peanuts, lentils, alcohol, coffee, and refined vegetable oils.3

What the Research Says: Many of the studies that examine the Paleo diet are short-term and involve fewer than 40 participants.  Despite this, they indicate that the Paleo diet may result in more positive health changes than the control diets.  In general, people who followed the Paleo diet tended to lose more weight and improve their blood pressure, insulin sensitivity, and cholesterols more significantly.3,4

The Verdict:  The Paleo diet’s recommendations to choose unprocessed foods and encouragement of lean protein, fruits, vegetables, nuts, and seeds are all health-supportive.  However, there need to be some long-term, larger-scale studies that evaluate its effectiveness before any conclusions can be drawn about the Paleo diet being better than a general balanced meal pattern.  Furthermore, unless you are allergic to grains, dairy, beans, peanuts, or lentils, there is absolutely no reason to avoid them entirely.  These foods provide a variety of nutrients that are essential for your health, including fiber, protein, B vitamins, calcium, and potassium.  Furthermore, people who eat these foods are at a lower risk for many different diseases, including heart disease and diabetes.

The Alkaline Diet

Proponents of the Alkaline diet claim that you need to eat foods that will not make your body become overly acidic, as an acidic environment can promote cancer development.  They claim that you can prevent your body from becoming acidic by choosing foods whose breakdown products are alkaline, or have a pH above 7.5

What You Can Eat: Fruits, vegetables, legumes, and nuts are considered alkaline foods (their breakdown products will increase your body’s pH), and fats, starches, and sugar are considered neutral (they have no impact on your body’s pH).

What You Can’t Eat: Meat, poultry, fish, dairy, eggs, grains, alcohol, coffee, soda, and other stimulants are considered acidic, and the Alkaline diet advises you to avoid them.5,6

What the Research Says: When the US News and World Report reviewed the Alkaline diet, they indicated that it may have a positive impact on weight loss or maintenance, reduced risk of diabetes, and improved heart health.  However, any health improvements seen while following the Alkaline diet are more likely caused by reducing your intake of processed foods and choosing more fruits, vegetables, and plant-based protein.6

The rationale behind the Alkaline diet is based on a fundamental misunderstanding of how the human body works.  Your body regulates your blood pH level within the small range of 7.36-7.44; if you are otherwise healthy, your blood pH stays within this range regardless of what you eat because moving outside of this range can lead to organ dysfunction or death.  You may excrete more or less acid in your urine in order to keep blood pH at a safe level, but the pH of your urine is not solely determined by the acidity or alkalinity of your diet.5

The Verdict: You can improve your health and may experience some weight loss by choosing a diet high in fruits, vegetables, and plant-based protein, and limiting your intake of processed foods, sugar, and alcohol.  These health improvements will not be related to changing your body’s pH.  At the same time, acidic foods like meat, poultry, fish, eggs, and whole grains are also health promoting, and absolutely fit into a balanced meal pattern.

Intermittent Fasting

In short, Intermittent Fasting involves setting specific times when you will and will not eat.  These time limits can occur within a day or within a week.  Some people give themselves 8-hour windows to eat every day (10 am – 6 pm), limit themselves to 500-700 calories every other day, or even go as far as spending two days each week fasting completely.7

What You Can Eat: With Intermittent Fasting, you are able to eat whatever you’d like within the time limits you set yourself.

What You Can’t Eat: Intermittent Fasting only restricts you from eating at specific times during the day or week.

What the Research Says: Many of the studies on Intermittent Fasting are short-term (lasting between 2 and 6 months).  This research indicates that the diet can be safe for the average person, does not lead to “compensatory overeating” during the days or times when you are allowed to eat, and can contribute to decreased insulin resistance.7–9 That being said, people who practiced Intermittent Fasting did not lose significantly more weight than people who took a more traditional approach to weight loss and limit what they eat overall.9,10

At the same time, Intermittent Fasting may also be associated with guilt, social isolation, irritability, obsession with food, and lethargy, all of which mimic the signs of an eating disorder.  Although fasting alone hasn’t been shown to directly cause eating disorders, it could definitely trigger a relapse in someone who has recovered from an eating disorder, and does not promote a positive, healthy relationship with food.

The Verdict: If weight loss is your goal, you may find it helpful to limit yourself to eating at certain times of the day.  For example, some people like to consider the kitchen “closed” after 8 pm.  More extreme versions of fasting are not likely to result in better outcomes than simply limiting what you are eating overall.

If It Fits Your Macros (IIFYM)

If you tend to be numbers-focused, you may be interested in the Macro Diet.  This diet goes beyond calorie counting by encouraging people to track their carbohydrate, protein, and fat intake every day to make sure they’re sticking to a goal.  To be successful with this diet, you’d have to carefully track the calorie and macronutrient content of everything you eat, either with a smartphone app or by looking up nutrition fact information and logging that by hand.11,12

What You Can Eat

You can eat anything you want on this diet – as long as it fits within your macronutrient allowance!  If you aren’t working with a Registered Dietitian, the diet explains how to calculate this yourself.  First, use an online calculator to estimate your resting metabolic rate, or the number of calories you need to eat in a day to keep your body functioning without accounting for physical activity.  The calculator will help you determine your final calorie needs depending on your activity level and weight goal.

To find your macronutrient goals, first calculate protein using the recommended 0.7-1.0 grams per pound of your body weight.  Your fat goal will be within 0.25-0.4 grams per pound of body weight.  Whatever calories you have remaining will be for carbohydrates.12

What You Can’t Eat

This diet doesn’t advise people to stay away from any specific foods, as long as they are eating within their macronutrient goals.

What the Research Says

Critics of this diet point out that it doesn’t address diet quality.  For example, it could be possible for someone to stay within their calorie and macronutrient goals every day without eating any fruits, vegetables, or whole grains, and therefore become nutrient-deficient or remain at an increased risk of disease.11 At the same time, studies have shown that calorie-counting apps don’t provide enough support to help their users create lasting healthy habits.13 Instead, users may lose weight as they become more aware of their overall calorie intake and choose smaller portions of high-calorie foods, but will likely return to their previous eating habits when they stop tracking their diets.

At the same time, calorie counting is not effective, or even safe, for everyone.  You can find many personal accounts of people who developed an eating disorder after starting to track their calorie intake by worsening the rigid, numbers-focused thinking often seen in disordered eating.  One study that assessed disordered eating behaviors in college students found that those who tracked their calorie intakes were more likely to engage in disordered eating behaviors.14 It is also commonly known that calorie counting can trigger a relapse in someone who has recovered from an eating disorder, as it often leads to more preoccupation with food choices and the perception that higher-calorie foods must be limited.

The Verdict

While it is often helpful to be aware of your calorie and macronutrient intake, if you set goals without considering the overall quality of your diet you put yourself at risk for vitamin and mineral deficiencies.  At the same time, logging every food, every day, may lead to unhealthy preoccupation with your calorie intake that could turn into disordered eating.  If you are curious about your calorie and macronutrient goals, work with a Registered Dietitian who will help you set realistic goals and learn how to turn them into long-term behavior change, so you will feel confident that you’re eating to promote health without needing an external guide for the long term.

The Mediterranean Diet

Consistently rated the best diet for overall health, the Mediterranean diet was also rated the easiest diet to follow in 2019 by US News and World Report.  In addition to emphasizing meals rich in fruits, vegetables, whole grains, and lean protein, the Mediterranean diet also encourages you to get active regularly and to slow down and savor your meals.15,16

What You Can Eat: This diet encourages you to make fruits and vegetables the basis of your meals.  In addition, you can have olive oil and whole grains every day, and, if you’d like, a 3-5 ounce serving of red wine every night.  Legumes, nuts, fatty fish, whole grains, lean meat, eggs, low-fat dairy products are also encouraged, but limited to three servings per week each.17

What You Can’t Eat: The Mediterranean diet encourages you to avoid added sugars, saturated fats like butter or coconut oil, fried foods, and refined grains.  You may have dessert once per week.

What the Research Says: Many large-scale, long-term studies support the health benefits of the Mediterranean diet.  This diet is associated with a lower risk of heart disease, high cholesterol, and diabetes.  In addition, some research suggests the Mediterranean diet may reduce your risk for Alzheimer’s disease.15

The Verdict: If you are looking for a diet to follow in 2019, the Mediterranean diet is the one.  It has been consistently rated one of the healthiest and easiest to follow diets since it gained popularity, and it is supported by a wide base of well-designed research studies.  Furthermore, this diet is flexible, allows for customization according to your lifestyle and food preferences, and includes meals that will keep you full and satisfied.15

There are pros and cons to any diet, and there is no one meal pattern that will work exactly the same for every person.  When it comes to selecting the one that’s right for you, consider how sustainable it will be, whether it will fit into your lifestyle, and whether it allows foods that you enjoy.  Most importantly, make sure you are eating a variety of foods from each food group, choosing meals that keep you full and satisfied, and creating a lifestyle that will allow you to maintain your health well into the future.


  1.  Sumithran P, Proietto J. Ketogenic diets for weight loss: A review of their principles, safety and efficacy. Obes Res Clin Pract. 2008;2(1):1-13. doi:10.1016/j.orcp.2007.11.003
  2.  Weiss E, Wroble K, Trott M, Schweitzer G, Rahman R, Kelly P. Low-carbohydrate, Ketogenic Diet Impairs Anaerobic Exercise Performance. J Acad Nutr Diet. 2018;118(9):A46. doi:10.1016/j.jand.2018.06.176
  3.  Boston 677 Huntington Avenue, Ma 02115 +1495‑1000. Diet Review: Paleo Diet for Weight Loss. The Nutrition Source. https://www.hsph.harvard.edu/nutritionsource/healthy-weight/diet-reviews/paleo-diet/. Published July 24, 2018. Accessed January 14, 2019.
  4.  5 Studies on The Paleo Diet – Does it Actually Work? https://www.healthline.com/nutrition/5-studies-on-the-paleo-diet. Accessed January 14, 2019.
  5.  The Alkaline Diet: An Evidence-Based Review. Healthline. https://www.healthline.com/nutrition/the-alkaline-diet-myth. Published October 2, 2018. Accessed January 14, 2019.
  6.  Alkaline Diet: What To Know | US News Best Diets. https://health.usnews.com/best-diet/acid-alkaline-diet. Accessed January 14, 2019.
  7.  Mattson MP, Longo VD, Harvie M. Impact of intermittent fasting on health and disease processes. Ageing Res Rev. 2017;39:46-58. doi:10.1016/j.arr.2016.10.005
  8.  MPH MT MD. Intermittent fasting: Surprising update. Harvard Health Blog. https://www.health.harvard.edu/blog/intermittent-fasting-surprising-update-2018062914156. Published June 29, 2018. Accessed January 14, 2019.
  9.  Collier R. Intermittent fasting: the science of going without. CMAJ Can Med Assoc J. 2013;185(9):E363-E364. doi:10.1503/cmaj.109-4451
  10.  The Fast Diet: A Fast Route to Disordered Eating? Psychology Today. http://www.psychologytoday.com/blog/hunger-artist/201411/the-fast-diet-fast-route-disordered-eating. Accessed January 14, 2019.
  11.  The macros diet takes calorie counting to the next level: Does it really work? Washington Post. https://www.washingtonpost.com/lifestyle/wellness/the-macros-diet-does-it-really-work/2016/06/07/d38a5434-2bf6-11e6-9b37-42985f6a265c_story.html. Accessed January 28, 2019.
  12.  IIFYM (If It Fits Your Macros): A Beginner’s Guide. https://www.healthline.com/nutrition/iifym-guide. Accessed January 28, 2019.
  13.  Davis SF, Ellsworth MA, Payne HE, Hall SM, West JH, Nordhagen AL. Health Behavior Theory in Popular Calorie Counting Apps: A Content Analysis. JMIR MHealth UHealth. 2016;4(1). doi:10.2196/mhealth.4177
  14.  Simpson CC, Mazzeo SE. Calorie counting and fitness tracking technology: Associations with eating disorder symptomatology. Eat Behav. 2017;26:89-92. doi:10.1016/j.eatbeh.2017.02.002
  15.  Mediterranean Diet: What It Is & What to Know. https://health.usnews.com/best-diet/mediterranean-diet. Accessed January 14, 2019.
  16.  Mediterranean Diet | Oldways. https://oldwayspt.org/traditional-diets/mediterranean-diet. Accessed January 14, 2019.
  17.  How You Can Get Started on the Mediterranean Diet. Health Essentials from Cleveland Clinic. https://health.clevelandclinic.org/can-get-started-mediterranean-diet/. Published October 15, 2015. Accessed January 14, 2019.

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