Wednesday, December 12, 2012

Study Finds Low-Fat Diets Lead To Weight Loss

There's a new, big study in the BMJ that delivers "high quality, consistent evidence" showing cutting fat leads to weight loss. (Coincidentally, the last post gave a graphic description of the energy density of 2 tablespoons of fat.)

Effect Of Reducing Total Fat Intake On Body Weight: Systematic Review And Meta-Analysis Of Randomised Controlled Trials And Cohort Studies, British Medical Journal, 6 December 2012

It analyzed 33 randomised controlled trials (73,589 participants) and 10 cohort studies (107,624 participants). Not a small undertaking. Note that the bulk of the data was from controlled intervention trials, the gold standard of studies, "the best scientific way of testing cause and effect," say the authors.

They found that a low-fat diet was associated with:
  • Lower body weight
  • Less weight gain
  • Lower body mass index (BMI)
  • Smaller waist circumference
  • Lower LDL cholesterol
  • Lower total cholesterol
  • Lower systolic and diastolic blood pressure

The numbers:
"Meta-analysis of 33 randomised controlled trials in adults suggested that diets lower in total fat on average reduced body weight by 1.6 kg, body mass index by −0.51, and waist circumference by 0.3 cm. These effects were from randomised controlled trials in which weight loss was not an intended outcome, suggesting that they occur in people eating normal diets and the direction of effect on weight was consistent regardless of subgroups or sensitivity analyses."
They excluded studies where weight loss was the goal, making their findings all the more compelling:
"Populations recruited specifically for weight loss studies and interventions intended to result in weight loss would be excluded. This was because they were potentially confounded by the implicit objective of reducing calorie intake to produce weight loss."
There was criticism that the effect was minimal. The authors said:
"We disagree that loss of 1.6kg is pointless. The effect on health of an individual reducing his or her body weight by 1.6 kg is likely to be small, but the effects of a whole population doing so would be noticeable. In a man of average height (1.75 m) and weighing 80 kg a loss of 1.6 kg will reduce body mass index from 26.1 to 25.6, a reduction of 0.5. Moving the whole population distribution (remember, we are talking about a sensible way to eat for the whole population, not an individual diet) to the left by 0.5 BMI units would have a serious impact on risks of weight-related illnesses including respiratory problems, infertility, diabetes, some cancers, some forms of arthritis and high blood pressure."
But that 1.6 kg was an average. Many experienced greater losses. The lower the fat intake, the lower the weight:
"There was evidence of a dose-response gradient between total fat intake and change in weight."
The mechanism? It may be that eating less fat leads to eating fewer calories. May be:
"Although further metabolic studies may reveal a mechanism of action, most studies that reported energy intake suggested lower energy intake in the low fat group than in the control or usual fat groups, and subgroups suggested that a greater degree of energy reduction in the low fat group (compared with control) was related to greater weight loss. This suggests that weight reduction may be due to reduced energy intake in those on low fat diets, rather than a specific effect of the macronutrient composition of the diet."
There's a lot of good literature that says macronutrients do matter. I think they do. These authors say that those who ate low-fat ate more carbohydrates (and a little more protein). There is a profound and complex impact on the body from carbohydrates. The effects of resistant starch and fiber are just two carbohydrate-based lines of study that might have contributed to the the weight loss seen above.
Photo of my fat-free cabbage and white bean soup.


Hurikane El Swiss said...

isn't this obvious though ?

Angela and Melinda said...

It's hard to imagine that this is surprising! As you point out, fat is the most caloric-dense food there is!

Angela and Melinda said...

My problem, when I tried this some years ago, was that without any fat, I tended to eat more sweet things (without fat in them), which then gave me yeast infections.

Trond Gynt said...

I'm not surprised.

Bix said...

Hurikane, there's a passionate low-carb movement, fueled by MDs such as Atkins and writers such as Gary Taubes, that argue it's the carbs making us fat, not the fat. (Taubes, 2002: What If It's All Been A Big Fat Lie?)

RB said...

Why is it that focus of diets is just weight loss? Diets should be about total health. The post mentions lower LDL cholesterol, lower total cholesterol and lower systolic and diastolic blood pressure. These health item should be also be measures of a good diet. Is their more info on how much cholesterol and blood pressure was lowered?

I think low-fat diets are the way to go. The low-carb diets I see as just wrong. We need to distinguished between processed carbs (refined sugar, wheat, etc.) and good carbs found in whole fruits, vegetables, seeds, grains and legumes. A low-carb diet would necessarily eliminate or at least greatly reduce healthy foods needed for good nutrition.

Bix said...

More comment from the study's author;

"The US population between 1971 and 2000, during the period that obesity levels rose from 15 to 31%, did not reduce total fat (it stayed very constant), so it was not fat reduction that caused the rise in obesity over this period. Americans started eating more food, from increased portion sizes, more salty snacks, soft drinks and pizza between 1971 and 2000 (3). Although people didn’t reduce the amount of fat they ate, the proportion of calories from fat fell (as they at more calories). This made it possible for observers to suggest that the fall in the percentage of energy from fat had caused the weight rise (as the time trend seemed confirmatory). However, we know that just because two things happen at the same time, doesn’t mean that one caused the other.

Conversely, the evidence that cutting down on the amount of fat we eat causes a fall in weight is very strong (4). It is a consistent message from 33 long term randomised controlled trials (the best scientific way of testing cause and effect) conducted in men and women, on several continents, for up to 11 years duration, in people who were well and those who had chronic illness. Evidence in children confirms the adult evidence.

When we cut down on the amount of fat we eat from animal sources and very processed foods (saturated and trans fats), we can eat a bit more starchy food (bread, pasta, potatoes, rice, cereals) to make up the energy, make sure that the fat we are still eating is mainly mono- and poly-unsaturated, but need to make sure that we don’t eat more food overall. This will still work to help us lose weight, and reduce our risk of cardiovascular events."

Claudia said...

OMG who said they weren't surprised? Arghhhhh. That's like my nightmare phrase. No but seriously who would be surprised? I'm with Hurikane on this. This is obvious!

Bix said...

Claudia, you make me laugh. Now when I hear that phrase, I think of you. Boy, it's common, so many permutations ...

"I'm not surprised"
"Doesn't surprise me"
"You shouldn't be surprised"
"I knew that"
"I know! I know!"
and Homer's famous "Duh!"

Where are all the curious people anymore? Where are all the scientists?

Matthew said...

People aren't curious because that means they don't know something. They have to know and be right. It's narcissism, Bix. And it's on the rise especially in young people.

"Just so we are all on the same wavelength, narcissism is a personality characteristic associated with self-absorption, egocentrism, an overestimation of one's own importance and abilities, a sense of entitlement and a disregard for others.

Think of all the qualities that enable us to form a functioning and vital nation -- respect, compassion, tolerance, selflessness -- and you will see that they don't exist in the narcissistic personality (or culture).

The indifference, egotism, disrespect and lack of consideration that are central to narcissism are also reflective of the increasingly polarized and vitriolic tone of our current body politic, recent unethical corporate behavior, the rise in cheating among students in school and the gamut of bad behavior among professional athletes. As Pogo noted so famously, "We have met the enemy, and he is us."

Bix said...

Good article, Matthew. Music lyrics *have* changed. Back in the 60s and 70s lyrics were... "He ain't heavy, he's my brother. His welfare is my concern. You've got a friend. Lean on me. Life is for learning. Teach your children. Reach out and touch somebody's hand, make this world a better place, if you can. Take some time out of your busy day to lend some encouragement to someone who's lost their way. We can change things if we start giving. Remember his shoes could fit your feet." I know I sound nostalgic! But these ideas mold people. And they have changed over the years.

Why is there not more joy in others' accomplishments? It's like humans are losing their emotional intelligence.