In the tug-o-war between low carb or low fat, which one wins the war-o-weight?
It's a question various experts are keen to understand.
There are two basic arguments around weight-gain or loss. The first is that it doesn't matter exactly what you eat because a calorie-is-a-calorie regardless of where it comes from. The second is that the macronutrient breakdown (fats, proteins, carbs) does matter because they have different effects on the hormones that determine when fatty acids are absorbed by fat cells and when they are released for energy.
Kevin Hall, a senior investigator at the National Institute of Diabetes and Digestive and Kidney Diseases, designed two studies in an attempt to test both hypotheses.
"The argument is that people are consuming too many carbohydrates, which drive up insulin levels in the blood," Hall told the Centre for Science in the Public Interest last week.
"Insulin causes the body's fat cells to suck in too many calories, and because calories are trapped in the fat cells, the rest of the body is starving. That makes you hungrier, so you eat more calories."
Meanwhile, the body, thinking it is starving, "slows down its metabolic rate, so it burns fewer calories".
Hall's first study was titled: Do carbs drive you to gain more body fat because they boost levels of the hormone insulin?
In it,19 participants lived in-house for one week so that the researchers could control what they ate. Their diet cut about 3300 kilojoules of fat or carbs.
"When we cut carbs, daily insulin secretion went down," says Hall. This was contrary to what they expected.
If the carbohydrate-insulin theory was correct, the carb-cutting should have boosted fat loss "while relieving the internal starvation and therefore causing calorie burning to go up".
Instead, they burned fewer kilojoules Hall explained.
In the second study, participants ate a high-sugar, high-carbohydrate diet for one month, before their carb intake was cut to just five per cent, while they "cranked the fat up to 80 per cent, and kept protein and calories constant".
It again failed to show that cutting carbs sped up weight-loss more than cutting fat.
"The rate of fat loss actually slowed down for the first two weeks, and then picked back up to the normal rate again for the last two weeks," Hall said, adding:
"If anything, there is a statistically significant greater fat loss and calorie burning on a low-fat diet. But the effects are so small that they're physiologically meaningless. Sometimes you can't see any significant difference, and sometimes you can see a few pounds difference that is clinically meaningless."
In a new meta-analysis of 32 other "controlled feeding" studies, Hall found similar findings.
So calorie-is-a-calorie then? Not quite.
Harvard's David Ludwig, a proponent of the low-carb, high-fat movement, argued there were design flaws in Hall's research and noted the importance of the type of carb (or fat) we eat.
The carb-insulin problem, he said was the result of "all the fast-digesting, processed carbohydrates that flooded into our diet during the low-fat craze of the last 40 years — white bread, white rice, prepared breakfast cereals, potato products, crackers, cookies, and of course concentrated sugar and sugary beverages."
In another new study, which is yet to be fully published, 699 overweight participants were prescribed either a low-fat diet or a low-carb diet for one year.
They were not told to cut calories, but to eat until they were full. They were however given one other instruction, regardless of which diet they were on: to eat as healthily as they could.
"We told everyone in both groups to eat as little white flour and sugar and as many higher-fibre vegetables as possible," said lead author, Christopher Gardner, a professor of medicine at Stanford University.
Still, by eating healthily, both groups cut about 2000 kilojoules and, after a year had lost, on average six kilograms.
"We assumed that insulin-resistant people would do better on a low-carb diet - as they did in some earlier studies - but they didn't," Gardner said.
He thought that the "healthy" foods may be responsible for the results and was more significant than whether they were low-carb or low-fat.
"In some older studies, when researchers told people to eat less fat, they weren't particular about which low-fat foods. Coke and white flour and sugar are low-fat," Gardner said.
In a new article for the New Yorker, Jerome Groopman agrees its not clear-cut.
"The problem with most diet books, and with popular-science books about diet, is that their impact relies on giving us simple answers, shorn of attendant complexities: it's all about fat, or carbs, or how many meals you eat (the Warrior diet), or combinations of food groups, or intervalic fasting (the 5:2 diet), or nutritional genomics (sticking to the foods your distant ancestors may have eaten, assuming you even know where your folks were during the Paleolithic era). They hold out the hope that, if you just fix one thing, your whole life will be better," he writes.
And of course, none of it takes into account other factors that influence fat including genetic differences, and microbiome differences, excess intake of anything, sleep deprivation, stress and a sedentary lifestyle.
But, although it may not be clear-cut, it is relatively simple.
"With regard to weight loss, calories count and the relative proportions of fat, protein, and carbohydrate do not matter much (although low-carb diets may help with eating less)," wrote Marion Nestle, a professor of nutrition, food studies and Public Health at New York University, in response to Hall's study.
"With regard to health, the food sources of calories matter very much indeed, and nearly everyone would be better off eating less sugar - at the very least because sugars provide calories, but no nutrients."
Groopman adds that while the researchers battle it out, we should keep our sanity.
"What this means for most of us is that common sense should prevail. Eat and exercise in moderation; maintain a diet consisting of balanced amounts of protein, fat, and carbohydrates; make sure you get plenty of fruit and vegetables. And enjoy an occasional slice of chocolate cake."
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