Why Diets For Weight Loss Don't Work

By Julia Katcher, Registered Dietitian and PhD Nutrition Science Student

CONTENT WARNING: For researchers to obtain funding to carry out studies, most investors and grants require biological measurements such as height and weight instead of self-reported data alone. Because of the simplicity behind Body Mass Index (BMI), many of these papers mention BMI classes. This does not mean we support BMI, as we believe BMI is not a good indicator of health. If BMI references are triggering for you, please do not click on the links to the research and just read this blog post. 

Like most people, I used to think it was simple for people to lose weight. All they need to do is eat less and exercise more. As a qualified Personal Trainer, the calories-in-vs.-calories-out equation for weight loss (the energy balance equation) made sense to me. But as I researched this further, I realized this was wrong. Let's break down the energy balance equation and what it really means for weight loss. 


A calorie is a unit of energy, also known as the kilocalorie. Energy balance is the relationship between ‘calories in’ which refers to the energy consumed and ‘calories out’ which is energy expended through physical activity and the body’s daily needs to function. This relationship is defined as: ENERGY BALANCE = CALORIES IN - CALORIES OUT

So to lose weight, all you need to do is eat fewer calories than you use (a “calorie deficit”). And whenever the body has excess calories, it converts them to be stored as fat, and so you gain weight (a “calorie surplus”). Simple, right?

Actually, it’s not so simple! The energy balance equation doesn’t account for all of the complexities of the human body. Look, we're not disputing that a calorie deficit can lead to weight loss. But our bodies are complex - there are a multitude of factors influencing how they consume and use energy outside of how much we eat and exercise. We explore the two components of the energy balance equation in detail below.


Did you know that two people can eat the exact same foods and their bodies will respond very differently? In fact, even identical twins, who share the same genetics, respond differently to food! In this study, it was found that despite having the same genes and exposure to similar environments, identical twins often had very different glucose (sugar) responses to set meals. In the same study, it was also revealed that the information on the foods' nutritional labels — such as fat, protein, and carbohydrate content — accounted for less than 35% of the differences between people's glucose  responses to foods with a similar calorie content. These results suggest that factors including individual differences in people's metabolism, gut microbiome, schedules, sleep, medications, meal timings, and physical activity levels are just as important as the nutritional content of the food consumed.

Some of the variables that influence how we consume, absorb, and use our energy include:


Leptin and Ghrelin are two hormones that act on the hypothalamus, the part of our brain controlling our body weight. Leptin helps to reduce our hunger when we are full and satisfied. When we lose weight, leptin levels typically drop and this increases our appetite and cravings while also lowering our energy expenditure (AKA less calories are going ‘out’). Leptin signalling can also be disrupted in individuals with more body fat, which can cause leptin resistance and result in an increase in hunger and lower energy expenditure. The hormone ghrelin helps to increase our hunger. When we restrict our caloric intake, levels of ghrelin increase, which in turn increases our hunger. 

These are not the only hormones influencing our energy balance; adiponectin, resistin, cholecystokinin, PYY, PP, amylin, oxyntomodulin, GLP-1, GIP, insulin, and thyroid hormones all play a role in energy regulation.

Studies have shown that the hormonal changes that occur with weight loss do not return to normal levels, even 1 year following initial weight loss. These mechanisms partly explain why it’s so difficult to maintain a diet and weight loss - our hormones are literally FIGHTING us to stop restricting our caloric intake and to eat more!

Genetics and Epigenetics

Research has determined that genes play a role in a person’s predisposition to holding onto fat tissue, such as inheriting certain risk alleles, lacking the expression of a gene, or having a gene interact with the environment in a particular way. There are also genetic diseases, such as Prader-Willi Syndrome, which results in an appetite that is near impossible to satisfy, which can result in weight gain. Overall, certain genetic variations can increase an individual’s susceptibility to higher levels of fat mass.


There are many medications that have an effect on weight by increasing or decreasing appetite, retention of fluid, increasing storage of fat, or lowering metabolic rate. Some of the medications leading to weight gain (and in some cases, weight loss) include anti-depressants, anti-psychotics, anti-convulsants, nerve pain medications, and diabetes medications. 

Disease processes

Disease processes and conditions can largely impact our energy expenditure and play a huge role in energy balance. Disease processes including cancer, burns, deep wounds, gastrointestinal disorders, or other traumatic injuries and disorders can result in hyper-metabolism, a massive increase in basal metabolic rate, and these individuals can require double (and sometimes more!) of their typical daily energy needs.

These variables that influence how we consume, absorb, and use our energy all provide evidence that there are many factors outside the control of an individual’s behaviours that affect how the body responds to calories in.



There are three major ways our body uses energy:

1. Basal metabolic rate (BMR)

Our BMR is what our body would need to survive and carry out all bodily functions (such as breathing and pumping blood), in a fasted state, if we were to do nothing but lie in bed all day.1,2 BMR accounts for about 60% of the average person’s daily energy used

2. The thermic effect of food (TEF)

TEF is the metabolic response to the foods eaten and the increase in work the body must do to digest, absorb and process the foods we consume. This leads to an increase in energy expenditure, with protein being the most metabolically active. TEF accounts for around 10% of the average person’s daily energy use.1

3. Activity

The movement we engage in is the most modifiable component of energy used. This component is highly variable depending on how sedentary or how active a person is and is also influenced by intensity, duration, and frequency of exercise. Knowing this variation, it’s estimated that about 30% of the average person’s daily energy is used for activity1,2,3. But again, it’s highly individualized!

You can see from the above that at least 70% of the way that our bodies use energy/calories is outside of the individual's control!


Our bodies are much more complex than balancing what we eat with how much we exercise due to many other factors that are outside of our control. 

Although many people who go into a caloric deficit can lose weight, it’s important to remember: 

  1. Any weight loss is typically short-lived due to the many other factors that influence energy balance. 
  2. The vast majority of people who intentionally lose weight will regain their initial weight loss - and biology directs this process.
  3. Even if someone is able to sustain a caloric deficit over a longer period of time, that doesn’t mean the weight loss will be sustained, as the physiological changes in your body can make it more difficult to do so.

DISCLAIMER: This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified healthcare provider with any questions you may have regarding your medical condition.


  1. Gropper, SS, Smith, JL. Advanced nutrition and human metabolism. 6th ed. Belmont, CA: Wadsworth Cengage Learning; 2013.
  2. Institute of Medicine, Food and Nutrition Board. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Washington, DC. National Academies Press. 2005 pp. 119–21.
  3. Food and Nutrition Board. Dietary Reference Intakes for Energy, Carbohydrates, Fiber, Fat, Protein, and Amino Acids. Washington, DC: National Academy Press. 2002