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Why Do Most Diets Fail in the Long Run?

An introduction to the psychology of sustainable weight loss.

 Natasha Spencer/Pexels
Source: Natasha Spencer/Pexels

One of the most asked questions on Google is “how to lose weight." There is a staggering amount of information (and misinformation) on the internet. However, weight gain is increasing at an alarming rate, with obesity levels in the United States increasing by over a third in the past 20 years alone. You might be surprised to read that an increase in weight loss attempts mirrors rising obesity rates.

Around two out of three adults are trying to control their weight each year. Women are significantly more likely to try to lose weight. Scientific analysis shows that any type of diet can be effective in the short term. Some people on diets can lose weight relatively quickly. However, as you may have personally found out the hard way, the story rarely ends there.

The majority of dieters gradually regain any lost weight. Multiple studies have found this to be the case for all diets, whether changing macronutrient proportions of carbohydrate, fat, or protein. Research has shown that the more diet attempts you make, the more likely you are to gain weight in the future.

Scientists at UCLA conducted a review to investigate the long-term outcomes of diets to assess whether dieting is effective. The studies revealed that one- to two-thirds of dieters regained more weight than they lost on their diets. The researchers concluded: “In sum, there is little support for the notion that diets lead to lasting weight loss or health benefits.”

There are multiple biological, psychological, and social reasons why diets end with weight regain. Addressing all of these factors is outside the scope of this short blog. However, we’ll cover two major ones, starting with our physiology.

Our Physiology

When you diet and lose weight, you invariably lose a mixture of muscle and fat, both of which are metabolically active: i.e., they burn calories (with muscle burning more calories than fat). Your body requires a certain amount of energy to keep all its cells functioning so that you stay alive (known as your resting metabolic rate). With less muscle and fat, the amount of energy that your body needs to take in decreases. At the same time, your metabolism becomes more efficient, essentially running on fewer calories. As a result, to maintain weight loss in your lighter, more efficient body, you now have to consume fewer calories on a daily basis than you did previously.

In order to maintain the weight you’ve lost, it’s crucial to maintain changes that have resulted in this weight loss. However, most diets are rigid and unsustainable. On average, weight loss attempts last four weeks for women and six weeks for men. You may be able to relate to this (i.e., following a strict diet plan for a few weeks, but then going back to your old eating habits, with the weight soon returning).

 Karolina Grabowska/Pexels
Source: Karolina Grabowska/Pexels

Our Psychology

From a psychological perspective, diets usually demonize a particular food group, such as carbohydrates or fats. Foods with "too many carbs," "too much fat," or "too many calories" are seen as "bad." Unfortunately, banning actually backfires.

A study published in the scientific journal Appetite investigated the role that psychologically banning foods, such as chocolate, crisps, sweets, and biscuits, may play in dieting failures. The researchers found that those with a tendency to overeat who were told to abstain from eating their favorite snacks for 24 hours ended up consuming around 133 percent more, compared to those who were given no instructions.

The Alternative to Dieting

The first step to ending the diet cycle is to recognize that short-term measures lead to short-term results. To improve our health, lose weight, and keep it off, we have to make real, sustainable, long-term changes to our actions.

This may not seem like a startling revelation. However, it’s worth reminding ourselves, since most of us are still tempted by the lure of crash diets and quick fixes. Any weight you’ve gained did not appear overnight, and it will not disappear overnight. Few people in the diet industry are prepared to publically admit this: There are no shortcuts. Long-term health and weight loss is an ongoing process.

Lasting Behavior Change

Changing our behavior isn’t as easy as it sounds. If it were straightforward, you would have figured it out on your own. We might know that we need to eat more vegetables and be more active. Knowing the theory is different from being able to put it into practice. Right now, there’s probably a gap between what you know and what you do. What’s between this gap? Your psychology, including your beliefs, outlook, and emotions.

A key to sustainably losing weight and living a healthier life is to understand how your psychology impacts your behaviors. N.B. It might be appropriate to touch briefly upon "weight loss," which can sometimes be a misnomer. Whilst researchers tend to use the term "weight loss," most people are concerned with losing fat, rather than weight. Indeed you may even wish to gain lean muscle at the same time. What’s more important is, therefore, body composition. However, for ease of reading and since we’ll be referring to the scientific literature, we’ll use the terms "weight loss" and "body composition" in this post and the following ones.

Thankfully, experts have identified the key cognitive, behavioral, and external factors involved in changing behaviors and maintaining behavioral changes over time and in different contexts. The three key elements of the psychology of behavior change are self-regulation, motivation, and habits. Over the next several posts, you will learn about the basics of these areas. By shifting your mindset and behaviors in a science-based and sustainable way, you’ll be able to change your body composition in line with your goals.

*This post has been prepared from information in Dr. Aria’s contributing chapter, “The Psychology of Health-Related Behaviour Change,” in the medical book "A Prescription for Healthy Living” (Elsevier Publishing, 2021).


Lowe, M. R., Doshi, S. D., Katterman, S. N., & Feig, E. H. (2013). Dieting and restrained eating as prospective predictors of weight gain. Frontiers in psychology, 4, 577.

Mann, T., Tomiyama, A. J., Westling, E., Lew, A. M., Samuels, B., & Chatman, J. (2007). Medicare's search for effective obesity treatments: diets are not the answer. The American psychologist, 62(3), 220–233.

Soetens, B., Braet, C., Van Vlierberghe, L., & Roets, A. (2008). Resisting temptation: effects of exposure to a forbidden food on eating behaviour. Appetite, 51(1), 202-205.

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