Hand me another Pop Tart, I’m on a diet: An overview of flexible dieting

Please Note: This is an older article on flexible dieting, for a newer and more comprehensive article click here.

Disclaimer: I am not a nutritionist or a registered dietition, I will be refering to scholarly sources and my own personal experience for this article, please take any recommendations with a grain of salt.

Diet. A word synonymous with “attempting to lose weight.” I’d like to take this exposition as an opportunity to state that a diet is what you eat regularly for nourishment (Webster’s Dictionary). That being said there is an addendum to that definition stating that it can be a prescribed nourishment with a specific goal. That goal is typically weight loss.

This shouldn’t be surprising, approximately 30% of American adults are obese (A. Meule et. Al, 2012). How did we get here? Well it’s simple when you think about the fact that most of us have no idea how much we eat in a day. As a society we’ve become extremely talented at taking massive amounts of calories and condensing them into bite sized pieces. For instance: I didn’t pack my lunch the other day, so instead of enjoying my Hellfire Burrito Bowl I went to a nearby Pei Wei and got a Spicy Chicken and Rice dish of comparable size. It was nearly double the calories and I didn’t feel nearly as full afterwards.

I believe this to be the main cause of America’s obesity epidemic. We simply aren’t aware of how many calories our food contains. To solve this problem we need a way to conveniently measure our food while simultaneously educating ourselves on our foods content. Enter Flexible Dieting, also known as “If It Fits Your Macros” (IIFYM).

Flexible Dieting, what is it?

Flexible dieting is based around the concept that your body needs a given amount of protein, carbs and fats to accomplish a goal.  The source is not as important as the result; a gram of carbs from sugar will be treated the same as a gram of carbs from a wheat source. .

Commonly flexible dieters will track their macronutrient and caloric intake using apps like MyFitnessPal, MyMacro+, MyPlate, etc… (App developers should stop starting these names with ‘My’). Their goal is to hit a set amount for each macronutrient and caloric threshold. This is done by logging the contents of each meal into these programs and although this may sound stressful flexible dieters have been shown to have lower body mass, decreased instances of overeating, lower levels of depression and lower levels of anxiety than other dieters. However in the same study calorie counting was associated with increased body mass and regular binge eating (C.F. Smith et. Al, 1999).  So it’s not a simple black and white distinction.

A calorie is a calorie. But are all calorie sources created equal?

For 10 weeks Mark Haub ate food exclusively procured from a local convenience store (like twinkles and junk food) and managed to lose 27 pounds in 10 weeks (read more here).  Haub demonstrated that to lose weight the quantity of calories mattered the most, however there is no telling how much muscle mass was lost in addition to fat. While this diet managed to lower his weight, and even help improve his cholesterol, Haub has stated that he would not recommend this diet. Despite that measuring and manipulating calorie intake can result in desired effects on bodyweight it will certainly not optimize health related results on its own.

Additionally Dr. Eades wrote an article (read here) in 2008 comparing the famous 1944 Ancel Keys starvation study and a study from the late 1960’s conducted by John Yudkin. In both studies participants consumed about 1500-1600 calories a day. In Keys’ study participants experienced starvation, constantly thought about food, one man even experienced psychological issues to the point he cut off his own fingers. While in Yudkin’s study participants said they felt great and voluntarily ended up eating around the same number of calories.

The difference? The distribution of calories. In the starvation study the diet was primarily low fat, low protein and high carb. In Yudkin’s study the only limitation was that carbohydrates were restricted to 50 grams a day. Yudkin’s participants were encouraged to eat as many vegetables and lean meats as possible, since vegetables do not have high caloric density the amount of food eaten may have been much larger than the amount consumed by the participants in the starvation study.  Indeed, Dr. Eades’ comparison shows the dramatic difference distribution of macronutrients within a set amount of calories can have.

The Macronutrients: Carbohydrates, Protein and Fat

Need to know:

  • Proteins have 4 calories per gram
  • Carbs have 4 calories per gram
  • Fat has 9 Calories per gram
  • Bonus: Alcohol has 7 Calories per gram


Proteins are the building blocks of you, basically your fat and bones aren’t protein based and even then their enzymes are. Assuming you are physically active you are going to need a decent amount of protein in your diet. According to WebMD 10-35% is adequate and consuming “too much” can result in fat gain (assumingly because it is extra calories).  This is misguided advice.

Protein should be assigned as a function of body weight. Why? Because the amount of protein needed to facilitate healthy synthesis (muscular or otherwise) is dependent on the size of the person.  The RDA for protein is 0.8 grams per kilogram of bodyweight, however that means this is the bare minimum to prevent deficiency. RDAs are not designed for optimization. The optimal amounts of protein, according to the American College of Sports Medicine and the Academy of Nutrition and Dietetics, range from 1.2 to 1.7 g per kg for athletes.

In my experience the most common prescription is 1 g per pound of bodyweight (2.2 g per kg) and honestly this is a pretty good recommendation. In a study testing the effects of consuming 4.4 g per kg researchers found that there was not any significant fat gain and no ill effects, except a few participants did complain about intestinal pain (J. Antonio et. Al, 2014).  So there’s really no danger to the 1 g per pound of body weight recommendation, however there is much debate between this prescription and 1 gram per pound of lean mass.


Eating a lot of carbs is synonymous with performance, picture marathoners “carb-loading” with pasta or powerlifters eating endless potatoes, and eating less carbs is synonymous with weight loss. Believe it or not this is actually a pretty good way to look at things.

For instance reconsider the comparison of experiments from before. At a caloric deficit with a high level of carbohydrates participants were starving, at the same calorie level with less carbs participants felt great. If these two groups were simply dieters we would be able to tell rather quickly that the low carb group would be the more likely group to lose weight and keep it off.  With this in mind it would be unwise to attempt to enact a zero carb diet.  The consumption of carbohydrates is associated with serotonin production, a key neurotransmitter without whom it becomes very difficult to sleep.

On the other side carbohydrates aid in performance and even in muscle hypertrophy. Carbohydrate demands for trained athletes are 5-8 g per kg (2.3-3.6 g per pound) for trained athletes (Kreider et. Al, 2010) however the level of training this is assuming is akin to the training of college athletes.  Normal gym goers do not need to consume anywhere near this amount of carbohydrates and should be in the range of 3-5 g per kg (1.4 -2.3 g per pound) (Kreider et.Al,2010).

The body can oxidize approximately 1 gram of carbohydrate per minute during exercise (Kerksick et. Al, 2008) thus an acceptable window for simple carbohydrate (sugar) consumption is during and shortly after a workout, the amount should be based on the time spent exercising. However, from my own personal experience most sessions that involve about 60 minutes of heavy lifting merit only around 30-40g of sugar afterwards.


It is natural to think “If I don’t want to be fat I shouldn’t eat fat” and while this is understandable it is misguided. Fat, like carbohydrates, is an energy source. In addition to supplying energy fat is also an insulator and helps with the body’s hormonal homeostasis. Additionally people engaging in high fat diets have better testosterone  circulation than those on low fat diets (Dorgan et. Al, 1996).  So it’s definitely worthwhile to maintain a decent fat content in your diet.

According to the Mayo Clinic website approximately 20-35% of calories should come from fat. Up to 50% of calories can come from fat in individuals engaging in high volume training (Venkatramen et. Al, 2000).   Training allows for a slight increase in fat intake however it should ideally remain in the 30-35% range. Some diets, such as Atkins or Paleo, promote higher fat content with minimal carbs, I will not be getting into these diets in the scope of this article.

Determining your macros and tracking your intake

As I stated earlier flexible dieting involves determining a ratio of these 3 macronutrients within a set amount of daily calories.  To find this ratio  we first need to determine the Total Daily Energy Expenditure (TDEE).  There are a variety of calculators available online to calculate TDEE.  Once you have your TDEE the next step is to decide the amount of protein you will consume.  Let’s assume 1 g per pound fro a 200 lb man whose TDEE is 2600 calories per day.  So far he has 200 g of protein (800 cal).

Next we need to determine Fat content.  This typically takes some self-experimentation to find the optimal level but for safety we will assume 30% caloric intake will come from fat.  This means a total of 780 calories need to come from fat (87 g).

Lastly carbohydrate content is determined from the difference between the TDEE and the sum of protein calories and fat calories.  With our example this gives us: 2600 – 800 (protein) – 780 (fat) = 1020 calories.  Which gives us 255 g of carbohydrate.

Finally our breakdown looks like:

  • 2600 Calories
  • 200 g protein
  • 87 g fat
  • 255 g carb

This is a breakdown reflecting maintenance of current weight.  The macronutrient distribution should vary based on new goals.  For instance for the common goal of weight loss decreasing the amount of total calories by decreasing the amount of carbs is quite common, while when trying to gain muscle individuals will usually increase carbs and protein.

There are many many may variations on how people distribute their macronutrients.  Some aim to hit protein and calories, allowing the rest to be filled by fat and carbs (the two are viewed as interchangeable since they are both energy sources), while others aim to nearly eliminate carbs or fats.  The method I have laid out in this article will work for the majority of people decently, however to truly optimize your experience with IIFYM it will take some level of self-experimentation.  Some people run better with next-to-no carbs and a very high fat content while some others must keep their fat intake under a specific level to avoid undesired effects.


The positive aspects flexible diets are numerous. First of all, it’s flexible… You’re never going to have to look at some sort of delicious pastry and think “If I want to look good I can never have that again…” in fact many people utilize intermittent fasting so that they may indulge in carb-loaded treats at the end of the day.  Next up, it’s educational: someone that has been a flexible dieter for a while is more likely to look over a menu of food choices and have an accurate idea of the nutritional value of each option than most people.  Lastly, it is an effective accountability system: it’s hard to remain unaware of how bad frequent little indulgences can be when you see the numbers adding up right before you.

Flexible dieting allows for eating foods that may not be commonly associated with dieting: Pop Tarts, Donuts, etc… And that is much of the allure.  In interviewing multiple individuals for this article most said that after logging long enough you stop trying to fit junk food every day because you can.  About 15% of what you eat is something people may refer to as “unhealthy.”  Which personally I find to be a great ratio, it’s mindful yet not overly exclusive.  This kind of transition in eating habits shows why flexible dieting can be a great tool for introducing people to mindful nutrition.

The only negative aspect  of flexible dieting that I would like to bring up is the resources for learning about it.  Many online groups and websites providing information to newbie flexible dieters tend to be bodybuilding centered.  This is not a problem in and of itself, I think bodybuilding is great but many newcomers may not be aware that when people say things like “a gram of carbs from candy is the same as a gram of carbs from pasta” they are referring to the effect it will have on your body composition.  Meeting your macros by any means is a good method for achieving your goal physique however it is not the most health-minded approach.

To wrap it all up: I think flexible dieting is a great tool to create mindful eating habits and educating yourself on the content of your food.  Many of the flexible dieting advice found online is bodybuilding oriented and this should be kept in mind when reviewing it.  A high-protein diet will not make you fat if you don’t metabolize all the protein you consume, likewise carbs and fat won’t cause weight gain on their own; the amounts matter the most.  A high carb, high protein approach may be optimal when your goals are hypertrophy or performance oriented while a high fat, high protein distribution may help the dieter feel more satiated while trying to lose weight.