A new study by Yunsheng Ma and colleagues in the Annals of Internal Medicine attempts to answer this question (1). The research team randomized 240 obese adults with the metabolic syndrome to one of two diets:
- An American Heart Association diet. This diet recommends "consuming vegetables and fruits; eating whole grains and high fiber foods (≥30 g/d); eating fish twice weekly; consuming lean animal and vegetable proteins; reducing intake of sugary beverages; minimizing sugar and sodium intake; maintaining moderate to no alcohol intake; consuming 50% to 55% of calories from carbohydrates, 15% to 20% of calories from protein, and 30% to 35% of calories from fat; and limiting saturated fat to less than 7% of energy, trans fat to less than 1% of energy, and cholesterol to less than 300 mg/d." In addition, participants were counseled to reduce calorie intake by 500-1000 calories per day.
- A high-fiber diet. All diet advice focused solely on increasing fiber intake.
As is often the case, diet adherence wasn't great, with the AHA group making only modest dietary changes, and the high-fiber group only increasing its fiber intake by 38 percent (to 23.5 g/day).
Over the course of one year, both groups lost weight. The AHA group lost 6 lbs (2.7 kg), while the high-fiber group lost 5 lbs (2.1 kg). Only the AHA group lost abdominal fat, although the change was small. Although the weight loss was modest, there was very little weight rebound over the course of the 12 months.
Blood pressure and fasting insulin level improved by the same amount in both groups. A circulating marker of inflammation, TNF-alpha, also decreased by the same amount.
Interestingly, LDL ("bad") cholesterol was unaffected by either diet, despite the fact that the AHA diet is specifically designed to reduce the risk of cardiovascular disease, and saturated fat intake declined in both groups.
It was a simple study, but its primary implication is surprising. I'll let the authors state it in their own words:
No clear between-group differences were found, suggesting that a dietary intervention focusing on a targeted fiber goal may be able to achieve clinically meaningful weight loss similar to the widely applied, but more intense, AHA dietary guidelines.
Another way of saying this is that a high-fiber diet was nearly as effective as the much more complex AHA diet in this group of obese volunteers with MetSyn, even though the high-fiber diet didn't include calorie restriction!
Realistically, the degree of weight loss in both groups was modest*. These are typical results for this type of diet study. However, it is encouraging that neither group experienced weight rebound over a one-year period, suggesting that the loss may be sustainable.
The high-fiber diet group increased its intake of high-fiber foods, not simply dietary fiber. Food is complex, so we can't actually attribute the benefits to dietary fiber specifically. However, we can say that the advice to eat a high-fiber diet leads to modest weight loss and metabolic improvement, and is much easier to implement than the AHA diet.
This study adds to a growing body of evidence suggesting that high-fiber foods may play an important role in body weight and health.
*The calorie target for the AHA diet had the stated goal of achieving "a weekly weight loss of 0.5 to 0.9 kg [1.1 to 2 lbs]". LOL! The total weight loss over 52 weeks was 2.7 kg-- only 7 percent of the target loss (assuming 0.7 kg/wk loss). This really highlights the ineffectiveness of this type of calorie guidance for weight loss, at least for most people. I think a better approach for most people is to design a food environment, diet, and lifestyle that naturally leads to a lower calorie intake without requiring calorie counting. Eating high-fiber foods seems to help.
This experiment seems like it's testing intentionality or "conscious eating" more than anything to do with fiber. If they are simply taking people who eat according to no rules, and then place some kind of rules or restrictions on their eating habits, it isn't a total surprise to see modest weight loss, right?
It would have been nice if there was perhaps a third control group that was simply told to "eat healthier," with no references to high fiber, and see what efect that had. But I imagine adherence to that, over a year, would be pretty horrible.
The reduction in TNF-alpha was interesting, and it would be fun to speculate whether that was a prebiotic effect, but it might be more simply explained by a reduction in adipose tissue (which I assume there was).
1) How can the impact of high-fiber be assessed if this is not the only variable controlled for? The 2 diets seem to be completely different
2) How can the impact of high-fiber be assessed if (many of) the participants did not adhere to the diets?
I don't have access to the full article, but get the impression that the research question cannot be answered.
If anything, the AHA followers lost more weight, and got less diabetes.
How do we know the fiber group consumed more calories than the AHA group? Stephan - it seems that your post is implying that this is true.
Seems to me that the criteria for comparison is not great enough to show any meaningful difference. I think a high-fibre, lower carb diet, would work wonders for most people. Through elimination or reduction of starchy carbs (bread, potatoes, rice, pasta, beans) a lot of unnecessary calories can be eliminated. Whole raw or slightly cooked vegetables and raw fruits are the way to go.
Given the cost to metabolize fiber -- a calorie of fiber < calorie of standard carbohydrate, right?
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