As a nutritionist, Kathleen Cornmell was horrified when she rapidly piled on nearly 2st while going through the menopause.
The 56-year-old – who’d never been overweight – found herself weighing nearly 12st which, at only 5ft 4in tall, was heavier than she’d ever been.
At first she was confused. ‘I wasn’t eating dramatically differently and was still as active as before, so it felt frustrating and quite unfair,’ says Kathleen, who has three adult children and lives near Basingstoke with husband Andrew, 60, an engineer.
Although she realised her body was changing hormonally, it was the speed of change that shocked her.
Initially, Kathleen tried a combination of calorie-counting and intermittent fasting – only eating 800 calories two days every week and having between 1,200 and 1,500 calories on the other five days. An active woman of her height needs about 2,200 calories a day, meaning she was eating at least 700 calories fewer than she needed.
In theory, that meant Kathleen should have lost around 2lb a week – but the scales didn’t budge. ‘If anything I ended up gaining a few pounds,’ she says.
Even when she went to her GP to ask for hormone replacement therapy (HRT), still the pounds wouldn’t shift.
It wasn’t until she stopped focusing on calories and instead focused on the quality of her food – eating more protein and healthy fats (such as ‘calorific’ avocados) – that she turned a corner.
‘I was shocked how rapidly the weight was coming off without a lot of effort,’ she says. ‘Within two months I started to feel comfortable in my body again.’
There are many dieters who have had similar experiences. Calorie-counting was once seen as the mainstay of weight loss, but increasingly scientists believe this approach is fundamentally flawed.
Kathleen Cornmell says it felt ‘frustrating’ to gain weight during menopause as she ‘wasn’t eating dramatically differently and was still as active as before’
An active woman of Kathleen’s height needs about 2,200 calories a day – and she was eating at least 700 calories fewer than she needed
‘In my opinion, calories are a useless measure,’ says Giles Yeo, a professor of molecular neuroendocrinology at the University of Cambridge.
‘They tell us nothing about [a food’s] nutritional content – and the impact on our bodies differs depending on what we are eating, even if foods have the same amount of calories.’
He is one of many experts who believe the way calories are calculated is based on outdated science, meaning many of the calorie counts of foods are misleading.
In fact, it seems much of what we know about calories may be wrong.
There’s even evidence that calorie-counting can have the opposite of the desired effect, leading to increased hunger and a lower metabolic rate (where your body burns fewer calories), which could also lead to a decline in bone density and delayed wound healing.
That was the finding from a 2025 review by the Chinese Academy of Sciences in Beijing, published in the journal Nature Reviews Endocrinology, which looked through the findings of research where people were on calorie-reduced diets deemed enough to provide adequate nutrition.
A calorie is actually a unit of heat energy (defined as the amount of energy required to raise one gram of water by one degree Celsius).
Calories are extracted from food and converted into energy by the mitochondria, the ‘powerhouse’ in each of our cells, then transported around the bloodstream after digestion – this is the process known as metabolism.
But the calorie counts we rely on today are based on calculations made more than 200 years ago.
They were made by US chemist Wilbur Olin Atwater, who at the end of the 19th century used a bomb calorimeter, which works out the energy in a food by burning it in a sealed oxygen-filled container and measuring how much heat it gives off.
However, many of the foods he used to do this – such as mutton, tongue and turnip – are nothing like what we commonly eat today, says Professor Yeo.
More importantly, Atwater’s calculations didn’t take into account the energy our bodies expend in extracting calories from different food types. ‘Metabolism (i.e. the rate at which we burn calories) was never considered when Atwater made these calculations – so calorie counts are probably about 10 per cent inaccurate,’ says Professor Yeo.
In fact, researchers at the University of Toronto in Canada discovered people eating 75g of almonds a day absorbed 40 to 60 fewer calories (roughly 20 per cent) than would be predicted by Atwater’s system. The findings, published in the journal Mayo Clinic Proceedings in 2021, were based on analysing stool samples.
‘The fundamental thing that a lot of people don’t realise is that we don’t eat calories, we eat food – and then our body has to extract the calories from that food,’ explains Professor Yeo. ‘Once calories are in our cells, giving them energy to function, then one calorie is equal to another, regardless of the food it came from.
‘But the problem is getting the calories out of the food – it takes varying amounts of energy from the body depending on the food. That’s why it makes a difference if you’re eating a carrot, a doughnut or steak.’
Foods high in fibre or protein require more energy to extract their calories compared with processed foods, which have already partially broken down nutrients, he explains.
Amanda Avery, a dietitian and an associate professor at the University of Nottingham, says counting calories ‘takes away some of the enjoyment of food’
That’s because cells of high-fibre foods such as certain vegetables, wholegrains and legumes are actually ‘wrapped up’ in a membrane containing fibre, ‘which means your body has to work harder to access all the calories in them’, says Gary Frost, a professor of nutrition and dietetics at Imperial College London.
What’s more, around half of the energy from fibre is used by gut bacteria (the healthy bugs that play a role in everything from preserving the gut lining to our immune system, for example) instead of our body’s cells.
This means that if the cell structure of the plant remains intact, you may only absorb a fraction of the given calories of a high-fibre food, explains Professor Frost.
For example, almonds have very tough cell walls that protect the inner cell contents, which means that the fat in almonds is not absorbed as well when eaten whole compared with when eaten as almond flour.
Similarly, whether a food is raw or cooked can make a difference to the amount of calories our bodies can extract from it.
‘A classic example is a stick of celery, which only has around six calories available to us raw,’ says Professor Yeo. ‘If you cook it in a stew you can get up to 30 calories from it – because cooking breaks down the fibre, making it easier for us to get the calories.’
Meanwhile, modern food processing is also changing how many calories we absorb, as our bodies can extract more calories from nutrients that have already been broken down than from natural sources, says Professor Yeo.
‘You’ll get more calories from an ultra-processed 400-calorie ready meal than a 400-calorie piece of steak,’ he says.
Underlining this idea that not all calories are equal, a study found that eating refined carbohydrates makes you more prone to weight gain than other food groups.
For the research, mice were split into two groups and given different foods – but with the same number of calories. One group received refined bread, wheat flour and rice flour, and the other was given high‑fat foods. After ten weeks the researchers, from Osaka Metropolitan University in Japan, found that those in the carbohydrate group gained weight, while the mice on the high-fat diet did not.
These findings, published in the journal Molecular Nutrition & Food Research in April, will not necessarily translate into humans – but it tallies with what we know to be true, says Professor Yeo.
‘Carbohydrates are chemically the least complex of the three macronutrients – a calorie of protein makes you feel fuller than a calorie of fat, than a calorie of carb, in that order, which might influence how much you eat,’ he adds.
The reason a calorie of protein makes you feel fuller than a calorie of fat or carbohydrate is because it triggers the release of hormones in the gut including GLP-1, the hormone mimicked by weight-loss injections (such as Wegovy and Mounjaro) which signals to the brain that you’re full and slows down emptying of the stomach.
What’s more, whereas if you eat excess fats and carbohydrates they are easily turned into fat, if you eat more protein than needed then the nitrogen it contains needs to be removed as uric acid in our urine before it can be converted into fat, and this process uses up a lot of energy. ‘This explains why for every 100 calories of protein we eat, we only get around 70 because the remaining 30 are given off as heat to process protein,’ says Professor Yeo. ‘But none of this information is written on the side of a pack.’
Not that what’s written on food packaging seems to make a big difference to people’s food choices.
Calorie labels may only lead to a 1.8 per cent reduction in calories consumed – the equivalent of 11 calories in a 600-calorie meal – according to a Cochrane review of 25 studies by scientists from University College London, the Universities of Oxford and Cambridge and others in the UK, whose findings were published in the BMJ last year.
‘Calorie-counting takes away some of the enjoyment of food,’ adds Amanda Avery, a dietitian and an associate professor in nutrition and dietetics at the University of Nottingham.
Kathleen says that It wasn’t until she stopped focusing on calories and instead focused on the quality of her food that she turned a corner
And just as not every calorie consumed is equal, how our bodies process calories differs between us too. Those who have a higher proportion of fat to muscle ‘will burn fewer calories than someone who has the same overall weight but has more muscle’, says Amanda Avery.
That’s because muscle has more mitochondria – so burns more calories – than fat. This is why maintaining muscle mass is an effective way to prevent weight gain, says Amanda Avery, and also explains why we tend to gain weight in mid-life.
‘It’s not necessarily that our metabolism slows down, but because we lose muscle over time – often becoming more sedentary – so burn fewer calories than we used to,’ she explains.
And hormone changes post-menopause can influence a woman’s body composition and therefore her energy expenditure.
There’s also evidence that the composition of our gut microbiome – the community of micro-organisms that plays a vital role in digestion and immunity – affects how we use calories.
Scientists at Arizona State University developed a new model to follow food through the digestive tract to see how much is absorbed in the upper digestive tract and what happens to the remaining material when it is broken down by gut microbes in the colon.
This process produces molecules called short-chain fatty acids which are absorbed through the colon and are also used by the body as calories. The model estimated that this equated to 140 calories per day, or 7.4 per cent of total usable energy.
The research, published in the journal PLOS One last month, involved participants eating either a microbiome-nourishing diet filled with fibre and few processed foods, or a typical Western diet with more processed foods.
The results showed that those eating the Western diet absorbed 116 more calories per day overall.
The team concluded that the model could play a role in developing personalised diets for people with obesity, diabetes or metabolic disease.
Studies also show ‘good’ bacteria flourish by feeding on high-fibre foods, and that when short-chain fatty acids such as butyrate are produced, they trigger the release of appetite-suppressing hormones and regulate the rate of metabolism, explains Amanda Avery.
‘People with a very high-fibre diet [more than 30g per day] tend to be of lower body weight,’ adds Professor Frost.
It’s not fully understood why, but research suggests these short-chain fatty acids stimulate the release of gut hormones including peptide YY (PYY) and GLP-1 that suppress appetite and make you feel fuller for longer, he explains.
So how should we approach food choices if calorie counts can’t be relied on?
Professor Yeo says the secret is to improve the quality of what we eat. ‘We need to eat foods that are higher in protein and fibre, not only because it’s more difficult to extract calories from them, but also because these foods are higher quality in terms of the nutrients they contain,’ he explains.
This gets back to the avocado example – which, despite being considered calorific (a medium-sized one can contain over 300 calories), are filled with good-quality fats, fibre and vitamins.
If swapping to a higher-quality diet doesn’t help with your weight loss then Professor Yeo suggests cutting back on quantity evenly across every part of your meal.
Someone who wanted to cut calories by a third who was eating a roast dinner, for example, would cut a third from each food type – meat, potatoes, gravy and vegetables – not just one.
It’s also important to remember that a lower body weight will need fewer calories to maintain it than a higher one, so diet and lifestyle changes need to be maintained, says Amanda Avery. ‘That’s why people gain weight when they return to their old foods and portion sizes after being on a diet.’
Eating moderate amounts of lean meats, nuts, seeds, eggs, milk, fruits and vegetables rather than sugary snacks or low-calorie foods which provide little in the way of nutritional content will help, says Amanda Avery.
She also recommends using smaller plates to aid with portion control and reducing ‘empty calories’ from alcohol.
Kathleen reduced her intake of carbohydrates, boosting her consumption of protein and healthy fats (such as avocado and oily fish) along with cutting out snacks.
Rather than eating calorie- counted portions of porridge and pasta, she freely eats eggs for breakfast, salad with oily fish such as salmon for lunch and chicken curry with extra vegetables for dinner. Now, 18 months after changing her diet, Kathleen weighs 9st 12lb, and her blood sugar levels – which had increased to a near pre-diabetes level – are back in a normal range.
‘Eating this way is much nicer than counting calories because I don’t weigh or measure anything,’ she says.
‘When I was reducing calories, I was often choosing foods because they were low-calorie but not filling. That meant I could end up hungry, thinking about food, needing snacks, or feeling like I was constantly battling willpower.
‘I would never count calories again. For me, the question now is not “how many calories does this contain?” but “will this nourish me, keep me full and help me feel well?” ’
