London - What if everything we thought we knew about calories was wrong? In recent months, several studies have thrown open the debate about calories, questioning conventional wisdom about which foods are really making us fat.
Not only are many of the calorie contents listed on food labels and in diet books inaccurate, but the calories from certain foods affect the body in different ways. The discrepancies really could add up - a recent study at the University of California found that people who had just 19 more calories a day than usual gained 2lb of weight in a year.
Here, the experts help unravel the calorie myths…
It’s food texture, not calories, that matters
We’re consistently told that the most effective way to maintain a healthy weight is to take in no more than 2 000 calories per day.
But calories work differently in the body depending on which food they come from. Protein foods such as chicken are estimated to use 10 to 20 times as much energy to digest as fats. And many processed or sugary foods seem to barely tax the digestive system at all, meaning no extra calories are needed to eat them.
But this isn’t accounted for on food packaging. So while a lemon muffin and a flapjack may contain the same calories, the body uses more calories to break down the flapjack, so you’ve notched up fewer after eating it.
Similarly, a sandwich of wholemeal bread and peanut butter might have the same calories as one with white bread and smooth peanut butter, but it takes more energy to eat so the calorie count from your meal will be lower.
Rick Miller, a clinical dieticianand spokesman for the British Dietetic Association, says: “Soft and highly processed foods require less effort to chew, so you use fewer calories. High-fibre foods require more chewing and are more difficult to digest, so you use up more calories eating them.”
“The texture and consistency of a food influences the amount of energy you need to digest it.”
That’s why raw food is less fattening
There is plenty of evidence that cooking makes food easier and less time-consuming to digest by altering its structure, meaning you take on board more calories.
Some experts have even suggested that our ancestors, who had to hunt for food, invented cooking partly as a way to access as many calories as quickly as possible.
Rachel Carmody, a researcher at Harvard University’s department of human evolutionary biology, has shown that sweet potatoes provide more calories when cooked because the starch they contain is better digested by the body.
In her latest study, she gave raw and cooked beef to mice and found that, unsurprisingly, the cooked meat was easier to digest.
The mice lost 2g of body weight on a raw meat diet but just 1g on cooked meat.
It may also be possible that because the heat killed bacteria, the immune system had less work to do – another energy saver.
So lightly steamed vegetables or medium-to-rare cooked meat could cut calories, while well-cooked food could add them.
Bridget Benelam, a scientist at the British Nutrition Foundation, says: “There are a lot of variables when it comes to measuring accurate calorie content.
“Foods vary in the way they are produced and cooked, which can affect their calories.
“It can become very difficult to measure calories in a mixed food such as a ready meal.”
It’s the quality, not the quantity
In her controversial book The Obesity Epidemic, obesity researcher Zoe Harcombe reported that despite the UK National Food Survey confirming that we ended the last century eating 25 percent fewer calories than in the Seventies, the obesity rate has increased six-fold since then.
“It is insane that we ignore these facts and stick resolutely to calorie counting,” Harcombe says.
“There is a lot seriously wrong with calorie advice.”
So how can we be eating fewer calories yet be getting fatter?
It’s probably down to our love of fast food and microwave meals - which take no calories at all to digest but are proportionately high in the most ‘fattening’ types of calories, sugar and fat.One of the problems with calorie counting is that it focuses too much on the quantity of food rather than the quality, say experts.
Why low fat is bad for dieters
Scientists at the New Balance Foundation Obesity Prevention Centre at the Boston Children’s Hospital compared the effects of three popular diet approaches over four weeks.
These were a low-fat diet that limited fats to 20 percent of total calories, a low carbohydrate diet based on the Atkins approach (cutting carbs to 10 percent of total calories) and a low glycaemic index (GI) diet containing 40 percent fat, 40 percent carbohydrate and 20 percent protein. All the dieters ate the same number of calories.
The results were telling. Those on the low-fat diet burned the fewest calories of all three groups. Their triglycerides (blood fats) rose while their ‘good cholesterol’ levels dropped, raising the risk of heart disease.
Those following the low-carbohydrate diet burned around 300 extra calories a day than those on the low-fat diet - but they also had raised levels of the stress hormone cortisol and other markers for heart disease and diabetes.
The most effective plan was the low glycaemic one, which led to an extra 150 calories being burned than on the low-fat diet but had no negative impact on hormone or blood-fat levels.
David Ludwig, the professor of nutrition who led the study, concluded that the beneficial effects boiled down to the type of carbohydrates consumed in the low GI diet - i.e., minimally processed foods that are slow to be digested such as beans, pulses, and non-starchy vegetables like cauliflower and broccoli.
Don’t trust the food labels
You shouldn’t always trust the number of calories printed on labels, say experts. The calorie tables used by manufacturers were put together more than 100 years ago by an agricultural chemist called Wilbur Olin Atwater. He literally burned samples of food, then measured the amount of energy released from the heat they produced. He worked out that every gram of carbohydrate and protein produced four calories, and every gram of fat produced nine.
What concerns experts today is that Atwater’s figures are estimates based on averages that don’t take into account variations in food make-up, preparation and processing techniques. Many of his measurements were based on food in its raw state.
Miller says: “We’ve known for some time that the calculations for certain foods are inaccurate. The calorie figure you see on a food label isn’t always the amount you will ingest.”
As research into calories begins to escalate, so more irregularities are unearthed.
Take nuts, for example. Peanuts, pistachios and almonds seem to be less completely digested than previously thought - possibly because of their tough cell walls - a study by the U.S. Department of Agriculture has found.
So while most packaging will say a 30g handful of pistachios provides 170 calories, the reality is a more waist-friendly 160.
And when you eat a similar serving of almonds, you are likely to get just 128 calories rather than the 170 on the label.
Professor Michael Rosenbaum, of New York’s Columbia University, recently showed that the key to successful dieting could be to permanently cut 300 calories from your daily food intake.
So when a few extra daily calories can contribute to weight gain, how on earth are dieters meant to navigate the increasingly complex calorie maze?
“If you adhere to calorie counting and reading labels, then there is a chance you could be getting more than you imagined,” says Bridget Benelam.
“What’s important is to balance out the foods you eat, so there is less refined produce, more fresh food and plenty of fibre.” – Daily Mail