‘Diets don’t work.’ You’ve probably heard this claim many times, and for most people, statistics say it’s true. The question is, why do diets fail? Is it because they are badly thought out? Or, are we just misunderstanding how to follow them? Food scientists at William and Mary University, VA, USA, speculate that low-carb and low-fat diets fail because many of us just don’t know what’s in our food.
Dr. Zach Conrad and his colleagues presented their findings earlier this year in an article in the Journal of the Academy of Nutrition and Dietetics. The research team, including scientists and registered dietitians from William and Mary, Ohio State University, and Oak Ridge Institute, investigated whether people’s food diaries reflected the diet that they claimed to be following.
Understanding the dynamics behind why diets fail is essential to winning the battle of the bulge. With rates of metabolic disorders soaring, it has never been more important to figure out why it’s so hard to stick to a plan. Are diet researchers overestimating how good we are at decoding food labels? Could our tendency to give ourselves the benefit of the doubt when we report our eating habits be causing more trouble than we think?
Conrad and his team believe it’s a possibility. What people say they eat influences everything, from national dietary guidelines to your doctor’s nutrition advice. If that information isn’t accurate, it can lead to poor treatment recommendations, flawed research, and widespread public confusion about what works and what doesn’t when it comes to weight loss and healthy eating.
Forgetful or confused?
So what exactly makes Conrad and Co. believe that people don’t know what they are eating? It all comes down to how we ask the question. Typically, when your doctor asks if you are sticking to your diet, of course, you’ll say yes, and as far as you are aware, you are! The team investigated whether folk are as good at self-reporting as weight loss researchers expect them to be.
The researchers analysed data from 30,219 adults aged 20 and over who took part in the National Health and Nutrition Examination Survey (NHANES) between 2007 and 2018. NHANES collects health and dietary data from a nationally representative sample of Americans through two 24-hour food recalls, one done in person and a second by phone a few days later.
Participants were first asked: ‘Are you currently on any diet, either to lose weight or for another health reason?’ If they said yes, they were prompted to choose a specific diet from a list that included low-carb, low-fat/low cholesterol, diabetic, low sugar, gluten-free, high protein, and more.
Their responses were then compared to what they actually ate, based on their 24-hour food logs. The researchers defined a low-carb diet as getting less than 26% of total energy from carbohydrates, and a low-fat diet as consuming less than 30% of total energy from fat. Both definitions are grounded in guidelines from the National Lipid Association.
Low Diet Low Down
Let’s break that down.
- We typically define low-carb diet as getting less than 26% of your daily calories from carbohydrates. On a 2,000-calorie diet, that works out to around 130 grams of carbohydrates per day, roughly the amount in two bagels and a banana. This type of diet is designed to shift your body away from using glucose as its main energy source and toward burning fat, sometimes even leading to a state of ketosis (though that demands still stricter limits). Many popular weight-loss plans, like keto or Atkins, fall under this category.
- The definition of a low-fat diet, on the other hand, is consuming less than 30% of your total daily calories from fat. That’s about 67 grams of fat per day if you’re eating 2,000 calories. This approach was widely recommended in the 1990s and early 2000s for heart health and weight control and focuses on limiting high-fat foods like oils, butter, cheese, and fatty cuts of meat, while promoting more fruits, vegetables, and whole grains.
These specifications are based on guidelines from the National Lipid Association’s Nutrition and Lifestyle Task Force, which reviewed multiple studies to establish meaningful benchmarks for ‘low’ intake.
Are we asking too much?
These diets are founded on quantitative measures of carbohydrate and fat content, but is it realistic to expect us to follow them? Despite their clinical definitions, many people don’t count grams of carbs or fat when they eat, often relying on general impressions or diet branding. For example, someone might assume a turkey sandwich on whole wheat bread is ‘low-carb’ because it seems healthy, even though it might exceed the daily carb limit for a true low-carb plan.
Despite the low success rate most people have in sticking to these regimes, these diet patterns remain hugely popular. In the International Food Information Council’s 2023 Food and Health Survey, over 50% of American adults said they had followed a special diet within the past year. Low-carb and low-fat diets were the third and fourth most common, behind only calorie-restricted and diabetic-friendly diets. That’s millions of us trying to stick to a nutritional label that, as the study shows, we may not fully understand.
And it’s not necessarily their fault. The terms ‘low-fat’ and ‘low-carb’ are used so often in marketing and pop culture that they’ve lost some of their clinical meaning. Without clearly defined boundaries and without education on how to measure what you’re eating, it’s easy to mislabel your diet. Add to that the influence of influencers, apps, and fad diets, and you’ve got a recipe for confusion.
Force also reviewed studies that defined low-fat diets as
Who sticks to the plan?
Here’s where the numbers get interesting.
- Two hundred and ninety-seven people in the survey reported being on a low-carb diet. When researchers analysed their actual intake, only 12 (that’s just 4.1%) truly consumed fewer than 26% of their calories from carbohydrates.
- On the other hand, 704 people said they followed a low-fat diet. Among them, 162 (or 23%) ate under the fat threshold.
This shows a clear gap between self-reported and actual behaviour. It also reveals that participants were more likely to understand and follow a low-fat diet than a low-carb one.
Interestingly, some people who didn’t say they were on a diet still met the criteria. That suggests either accidental adherence or a misunderstanding of the diet labels themselves.
Out of the initial 57,414 NHANES participants, only 30,219 were included in the final analysis. Researchers excluded the rest due to missing dietary data, pregnancy, or unreliable recall reporting.
When the Diets Don’t Work
This isn’t the first time diet misreporting has been flagged. Previous studies using techniques like doubly labelled water (a gold standard for tracking energy intake) show that people consistently underreport what they eat, often without realizing it. Factors like shame, confusion over portion sizes, and poor nutrition literacy all contribute.
In fact, in some weight-loss studies, participants underreported their calorie intake by over 250–450 kcal per day by the end of the first year. And more than half of those claiming to follow a diet dropped off by the 6-month mark.
Why Diets Fail
Misreporting what we eat doesn’t just make it harder to lose weight; it can skew national nutrition data and make dietary advice less effective. Healthcare providers might base recommendations on false assumptions. Most importantly for us as individuals, we get frustrated when our so-called perfect diet isn’t working.
But it’s not about blame. Most people, this data shows, simply don’t have the tools to assess their eating accurately. That’s where education, support, and reliable tracking materials can make a difference.
Diets don’t always fail because of a lack of willpower. Sometimes, we’re just not eating the diet we think we are.
References: Kowalski C, Dustin D, Ilayan A, et al. Are people consuming the diets they say they are? Self-reported vs estimated adherence to low-carbohydrate and low-fat diets: National Health and Nutrition Examination Survey, 2007–2018. Journal of the Academy of Nutrition and Dietetics. 2025;125 (2):239–246. https://doi.org/10.1016/j.jand.2024.07.006
‘Diets don’t work.’ You’ve probably heard this claim many times, and for most people, statistics say it’s true. The question is, why do diets fail? Is it because they are badly thought out? Or, are we just misunderstanding how to follow them? Food scientists at William and Mary University, VA, USA, speculate that low-carb and low-fat diets fail because many of us just don’t know what’s in our food. Dr. Zach Conrad and his colleagues presented their findings earlier this year in an article in the Journal of the Academy of Nutrition and Dietetics. The research team, including scientists and registered dietitians from William and Mary, Ohio State University, and Oak Ridge Institute, investigated whether people’s food diaries reflected the diet that they claimed to be following. Understanding the dynamics behind why diets fail is essential to winning the battle of the bulge. With rates of metabolic disorders soaring, it has never been more important to figure out why it’s so hard to stick to a plan. Are diet researchers overestimating how good we are at decoding food labels? Could our tendency to give ourselves the benefit of the doubt when we report our eating habits be causing more trouble than we think? Conrad and his team believe it’s a possibility. What people say they eat influences everything, from national dietary guidelines to your doctor’s nutrition advice. If that information isn’t accurate, it can lead to poor treatment recommendations, flawed research, and widespread public confusion about what works and what doesn’t when it comes to weight loss and healthy eating. So what exactly makes Conrad and Co. believe that people don’t know what they are eating? It all comes down to how we ask the question. Typically, when your doctor asks if you are sticking to your diet, of course, you’ll say yes, and as far as you are aware, you are! The team investigated whether folk are as good at self-reporting as weight loss researchers expect them to be. The researchers analysed data from 30,219 adults aged 20 and over who took part in the National Health and Nutrition Examination Survey (NHANES) between 2007 and 2018. NHANES collects health and dietary data from a nationally representative sample of Americans through two 24-hour food recalls, one done in person and a second by phone a few days later. Participants were first asked: ‘Are you currently on any diet, either to lose weight or for another health reason?’ If they said yes, they were prompted to choose a specific diet from a list that included low-carb, low-fat/low cholesterol, diabetic, low sugar, gluten-free, high protein, and more. Their responses were then compared to what they actually ate, based on their 24-hour food logs. The researchers defined a low-carb diet as getting less than 26% of total energy from carbohydrates, and a low-fat diet as consuming less than 30% of total energy from fat. Both definitions are grounded in guidelines from the National Lipid Association. Let’s break that down. These specifications are based on guidelines from the National Lipid Association’s Nutrition and Lifestyle Task Force, which reviewed multiple studies to establish meaningful benchmarks for ‘low’ intake. These diets are founded on quantitative measures of carbohydrate and fat content, but is it realistic to expect us to follow them? Despite their clinical definitions, many people don’t count grams of carbs or fat when they eat, often relying on general impressions or diet branding. For example, someone might assume a turkey sandwich on whole wheat bread is ‘low-carb’ because it seems healthy, even though it might exceed the daily carb limit for a true low-carb plan. Despite the low success rate most people have in sticking to these regimes, these diet patterns remain hugely popular. In the International Food Information Council’s 2023 Food and Health Survey, over 50% of American adults said they had followed a special diet within the past year. Low-carb and low-fat diets were the third and fourth most common, behind only calorie-restricted and diabetic-friendly diets. That’s millions of us trying to stick to a nutritional label that, as the study shows, we may not fully understand. And it’s not necessarily their fault. The terms ‘low-fat’ and ‘low-carb’ are used so often in marketing and pop culture that they’ve lost some of their clinical meaning. Without clearly defined boundaries and without education on how to measure what you’re eating, it’s easy to mislabel your diet. Add to that the influence of influencers, apps, and fad diets, and you’ve got a recipe for confusion. Force also reviewed studies that defined low-fat diets as Here’s where the numbers get interesting. This shows a clear gap between self-reported and actual behaviour. It also reveals that participants were more likely to understand and follow a low-fat diet than a low-carb one. Interestingly, some people who didn’t say they were on a diet still met the criteria. That suggests either accidental adherence or a misunderstanding of the diet labels themselves. Out of the initial 57,414 NHANES participants, only 30,219 were included in the final analysis. Researchers excluded the rest due to missing dietary data, pregnancy, or unreliable recall reporting. This isn’t the first time diet misreporting has been flagged. Previous studies using techniques like doubly labelled water (a gold standard for tracking energy intake) show that people consistently underreport what they eat, often without realizing it. Factors like shame, confusion over portion sizes, and poor nutrition literacy all contribute. In fact, in some weight-loss studies, participants underreported their calorie intake by over 250–450 kcal per day by the end of the first year. And more than half of those claiming to follow a diet dropped off by the 6-month mark. Misreporting what we eat doesn’t just make it harder to lose weight; it can skew national nutrition data and make dietary advice less effective. Healthcare providers might base recommendations on false assumptions. Most importantly for us as individuals, we get frustrated when our so-called perfect diet isn’t working. But it’s not about blame. Most people, this data shows, simply don’t have the tools to assess their eating accurately. That’s where education, support, and reliable tracking materials can make a difference. Diets don’t always fail because of a lack of willpower. Sometimes, we’re just not eating the diet we think we are. References: Kowalski C, Dustin D, Ilayan A, et al. Are people consuming the diets they say they are? Self-reported vs estimated adherence to low-carbohydrate and low-fat diets: National Health and Nutrition Examination Survey, 2007–2018. Journal of the Academy of Nutrition and Dietetics. 2025;125 (2):239–246. https://doi.org/10.1016/j.jand.2024.07.006
Forgetful or confused?
Low Diet Low Down
Are we asking too much?
Who sticks to the plan?
When the Diets Don’t Work
Why Diets Fail
