The #1 Predictor of Weight Loss Success: Dietary Adherence

You can lose weight on any type of diet that restricts calories.  People have lost weight on a McDonalds diet, an ice cream diet, a potato diet, meal replacements (Slim Fast), premade meals (Nutrisystem) and any other type of diet that you can think of.

The problem is that diets work fairly well in the short-term (<3 months) but not in the long-term (>6 months).  Diets generally work pretty well when you’re on them.  But how long does that last?  The McDonalds diet for weight loss lasted 6 months and the ice cream diet lasted 100 days (much to my astonishment, the potato diet lasted an entire year!).  Most people can only stick to a diet for about two to three months before adherence tanks (Figure 2).

Figure 1. Diets Work in the Short-Term (<3 Months) but Not in the Long-Term (>6 Months).  Source: Dansinger, ML; 2005, JAMA.

Figure 2. Dietary Adherence (How Well You Can Stick to a Diet) Rapidly Decreases Over Time and is Highly Correlated to Weight Loss Success. Source: Dansinger, ML; 2005, JAMA.

People find it impossible to stick to a diet.  But why?  Researchers, health professionals, and more than enough quacks and charlatans have been trying to answer this question for years.  In a classic, I’m a hammer and every problem is a nail fashion, most of these same nutrition obsessed individuals have looked at WHAT we’re eating (the nuts and bolts of nutrition) rather than WHY we are eating (psychology, decision making, and the food environment) what we are eating to solve the adherence problem.

If you can increase dietary adherence, you can increase the amount of weight lost.  There are hundreds and thousands of unique, individual factors that go into dietary adherence but three of the most commonly cited factors are

  1. Hunger
  2. Deprivation – not allowed to eat good tasting food; diet food doesn’t taste good
  3. Restriction – not enough calories; leads to feeling hungry and deprived

Traditional weight loss advice is to eat a calorie-restricted low-fat diet.  Although debatable, it is generally thought that fat and protein are more satiating (keeps you fuller, longer) than carbohydrate.  By their very nature, low-fat diets are typically also slightly lower in protein since fat and protein are found in many of the same food sources.

In theory, by staying fuller, longer, you will be less likely to consume additional calories and hence lose weight.  As you can see in figure 1, eating a high fat/high protein diet (Atkins) doesn’t necessarily lead to more weight loss (this is one example of many).  There is some evidence that a purely high fat diet (Keto Diet) does suppress appetite more than other diet types.  But if weight loss was just about controlling hunger, the Keto diet would be more effective than other diets.  It really isn’t. 

A quick side note on ketosis.  You can enter ketosis (producing ketones or ketone bodies through the breakdown of body fat or dietary fat) by severely restricting carbohydrate and protein intake (a nearly 100% fat diet) or by eating a very-low-calorie diet (VLCD; <800 calories/day).  Either will put you into ketosis.  Personally, I think the Keto diet is gross.  If I’m going to try to go into ketosis, I’m choosing the VLCD over the Keto diet.

Table 1.  An Overview of the Diets that Supposedly Reduce/Control Hunger, an Explanation of Why They Purport to do So, and the General Effectiveness of Each.

Hunger is one piece of the puzzle, but it is clearly not the only driver of eating.  People eat when they’re not hungry (for pleasure, boredom, etc.) and when they are hungry how many times do they eat an indulgence (bag of chips) rather than a healthy snack or small meal?  People also have a difficult time not eating too much food when they are hungry – to the point to quench hunger without overdoing it.

Hunger during weight loss is unavoidable.  We’ve tried every conceivable combination of macronutrients (fat, carbs, protein) and meal timing but they all have minimal to no effectiveness (Table 1).  You are not going to control hunger through nutritional strategies.  With all that being said, I would like to outline some of the methods for dealing with hunger at a later date (this post is already too long).


People report feeling deprived on a weight loss diet.  The food they were eating was likely sweet (carbs) and savory (fat).  Low-fat diets deprave you of fat, while low-carb diets deprave you of carbs.  What if there was a way to let you eat carbs and fats without having to limit them?

Answer: intermittent fasting (IF).  On traditional diets (continuous energy restriction), you decrease your calorie intake by the same amount every single day.  For example, you may consume only 75% of the calories you need to maintain your body mass. 

On an intermittent fasting diet you vary your daily caloric intake.  Although there are many forms of intermittent fasting, let’s take a look at alternate day fasting (ADF) as an example.  ADF can be broken up into two-day time blocks.  On day 1, you will consume 125% of your calorie needs (feast).  On day 2 you will consume 25% of your calorie needs (famine).  Collectively, between day 1 and day 2 you will have consumed 150% of your calorie needs (125% + 25% = 150%).  You needed 200% (100% per day x 2 days) to maintain your body mass.  You’re going to lose weight.

On an intermittent fasting diet you do not have to reduce carbs or fat on feast (feed) days, provided you reduce your total calories on fasting days.  Intermittent fasting has proven to be to as effective as continuous energy restriction for weight loss.  But is it more effective?  Not really.

Another form of intermittent fasting that can kind of be in a its own category is time restricted feeding (TRF).  As we saw in our Meals, Snacks, and Indulgences: What’s the Difference and Why Does It Matter? post, humans of the western world eat from the time they get up until the time they go to bed.  This equates to 16+ hours of semi-continuous eating and only 8 possible hours of fasting. 

In TRF you severely and intentionally reduce the number of eating hours by restricting your eating to a certain time window.  For example, you may choose to only eat between the hours of 10:00am and 4:00pm.  There are only so many calories you can physically consume within that time frame.  Carbs and fats are not off-limits.  Is time restricted feeding more effective than other forms of dieting in losing weight?  Not really.

Too Restrictive

There are two types of restriction.  Calorie restriction and macronutrient restriction (carbohydrates, fats, and proteins).  We already covered low-fat and low-carbohydrate diets above.  Here we will go over calorie restriction.

The more calorically restrictive the diet, the more weight you are going to lose.  Outside of weight loss (bariatric) surgery and medications, a very low-calorie diet (VLCD; < 800 calories/day) is going to lead to the greatest rate of weight loss and total amount of weight loss over time.

Most people cannot tolerate a VLCD for any longer than 8 weeks before they will need to transition to a less restrictive diet.  VLCDs are made up of meal replacements.  “Real food” isn’t consumed. The transition to a less restrictive (real food) diet type is super difficult to come out of (and get right).  In my next post I will discuss some of the strategies for doing so.

Table 2. Diet Types, Level of Restriction, and Rate of Weight Loss.

If VLCD’s are so difficult to sustain, what if we allowed people to eat a few more calories?  This is, in essence, a low-calorie diet (LCD) (Table 2).  On low calorie diets real food and meal replacements are consumed and the calorie level varies between 800-1200.  Some studies (but probably a minority) show that LCDs can be as effective as VLCDs for weight loss.  But 800 – 1200 calories is still incredibly difficult to stick to!

In an attempt to increase dietary adherence on a VLCD or LCD we have given people all their food (so they don’t have to shop, cook or calorie count), provided them with low to no calorie snacks to decrease their levels of hunger and deprivation, and have utilized intermittent energy restriction (IER).  IER sounds very similar to IF but instead of altering what you eat in two-day time blocks as you do in alternate day fasting, you will vary your calorie intake over the course of several weeks.

Just like hunger and deprivation, it is very difficult to prevent someone from feeling like a diet is too restrictive, regardless of the nutritional intervention.

We’ve Tried Everything!

It doesn’t matter how you slice it, STAYING ON A DIET IS SUPER FREAKING HARD!  We have tried low-carb diets, high protein diets, high fiber diets, low-glycemic diets, intermittent fasting, time restricted feeding, intermittent energy restriction, snacks, food provision, individual and group counseling, spousal support, you name it, we’ve done it.  Researchers are super smart and hyper focused.  They’ve tried everything.  Anything that is branded as “new” or “cutting edge” has probably been tried before.

There is No “Right” Diet for You

There are people out there willing to tell you that you just need to find the diet that is right for you.  That if you are able to match your dietary intake to your metabolic machinery (i.e., you’re a good carb or fat burner) that you’ll lose weight.  They will tell you that the problem is you haven’t found the diet that meets your individual genetics, physiology, tastes, and preferences.

So far this has not proven to be true.  In randomized controlled trials (RCTs) on weight loss, the gold standard for weight loss research, people are usually placed into different diet groups in a “random” manner to protect against biasing one diet or the other.  So in theory, some people will be put on a diet not of their preference.  For example, they may really like meat/fat but be put on a low-fat diet.  It turns out, that even if you let people choose their own diet, they don’t lose any more weight than if they were put on a random diet.

In figure 3 below, individuals were put on 1 of 4 diets: Atkins, Weight Watchers, Slim Fast or a Control Group.  Each circle represents one individual.  As you can see, a few people lost a lot of weight, most people lost a moderate amount of weight, and some people even gained a little bit of weight. 

Figure 3. Individual Responses to Any Given Diet Varies Considerably. Each dot represents one individual’s weight loss while on an Atkins, Weight Watchers, or Slim Fast diet.  The Control Group did not diet. Source: Truby, H; 2006, BMJ.

While it is tempting to say that if one diet didn’t work for someone, that maybe another type of diet better suited to their tastes and preferences would.  But I’m not buying it.  I think that the individuals who were successful on Atkins could have been equally successful on Weight Watchers or Slim Fast and the people who failed were going to fail regardless of the diet type.

What Works

Dietary adherence is the number one predictor of weight loss success.  No matter the diet type, the more closely you adhere, the more weight you are generally going to lose.  On VLCD and LCDs, the more meal replacements you take (and the less real food you eat), the more weight you lose.  The more weight loss meetings/counseling sessions you attend, the more weight you will lose.  The more frequently you food log and the longer you food log – over the course of weeks and months, the more weight you lose.  The more frequently and the longer – over the course of weeks and months you weigh yourself, the more weight you will lose.  The bottom line is


There is WAY too much focus on diet type and not enough focus on how you are going to implement and relentlessly execute your day-to-day plan.  Diet type doesn’t matter.  Dieting is not about the food, it’s about your relationship with the food.  Paying too much attention to diet type distracts us from what really matters. Execution of the diet/plan.

Staying on a diet is super freaking hard.  Life is complicated, messy, and unpredictable.  Things continually come up and you’ve got to find a way to work around them.  I know this all too well in my own life.

Staying on a diet will require you to take a comprehensive, systematic approach to weight loss.  In my next post, How to Increase Dietary Adherence, I want to outline the strategies that I think you should use to streamline and automate as much of your plan as possible.

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