Wednesday, February 20, 2013

The Weight Loss Benefits of Fiber

I recently attended Arizona State University's 8th Annual Building Healthy Lifestyles Conference. One of the speakers was Dr. S. Boyd Eaton, who co-authored a research article and book on paleolithic eating in 1985 and '88. One of his key points was fiber intake: our paleolithic ancestry ate close to 100 grams per day! Currently, daily fiber recommendations are 38 grams for men and 25 for women, and most people fall short of those intakes.

Dr. Eaton said the average paleolithic man was known to be very lean and fit, similar to the men who are second and third from the right in the illustration above. There are many factors that can be attributed to the physique disparities between paleolithic and modern men. Is a high fiber intake one of them?

I'll rephrase that question from a more practical standpoint: does adding fiber lead to your diet produce weight loss?

Fiber and Weight Loss

First and foremost, let's cover the basics. Fiber is a complex carbohydrate that mainly comes from the cell walls of plants. Unlike most other carbohydrates, fiber passes through the digestive system basically unscathed - humans lack the enzymes necessary to breakdown fiber. Therefore, fiber doesn't increase blood glucose like other carbohydrates (in fact, increased fiber intake improves blood glucose) [1].

Fiber is often described as dietary or functional. Dietary means the fiber is intact in food, and functional is extracted and manufactured in a processed food or supplement.

Now, let's get to the point: does adding fiber produce weight loss?

  • Women who were overweight (according to body mass index) and fell between 18-65 years old were placed on a high fiber or high protein diet for eight weeks, with fat and calorie intake even [2]. The high protein diet consumed 30% of calories from protein and 40% from carbohydrates, with an average daily fiber intake of 24 grams. The high fiber group consumed 20 and 50% of their calories from protein and carbohydrates, with a daily fiber average of 39 grams. The high protein diet lost averages of 8.8 lbs of fat and 5.4 cm from their waists, compared to 5.1 and 4.7. A conflicting factor in these results are the differences in protein versus non-fiber carbohydrates - was it the added protein, reduced carbohydrate total, or a combination of the two that led to a greater fat loss?
  • Obese women of the same age group were studied on one of two 10-week diets [3]. The high fiber, high protein (HPHF) diet featured an additional 20-25 grams of protein and an extra nine grams of fiber per day. The standard diet the same amount of total carbohydrates, but had a few extra grams of fat per day. The HPHF diet lost 3.3 lbs, including 1.3 lbs of abdominal fat, versus no changes in the standard diet.
  • A review of fiber research said five randomized control trials - studies that compare interventions with only one differing factor and have the ability to prove causation - showed diets higher in fiber produced about two more pounds of weight loss than other diets [1].
  • In terms of using fiber supplements, fiber added to diets that otherwise are similar in calories and macronutrients increased weight loss by 2.9 and 5.5 lbs after four and eight weeks [1]. These studies added an average of 7.5 grams of fiber per day.

Mechanisms, Foods, and Final Thoughts

Last March, I posted an article about my weight and all of the experiments that were used in route to a 30-pound loss. One of the last experiments was doubling my fiber intake. This change helped me lose three pounds when I was already pretty lean. A fellow graduate student who was also fit added about 10-15 grams per day and achieved a similar loss. 

Does increasing fiber intake lead to weight loss? Likely. If the fiber is added in place of other carbohydrates or fat, or if it is merely added as a supplement to what you are currently eating, the answer is yes. If it is added in place of protein, one of the studies above indicate that it won't produce weight loss.

Two factors likely explain why adding fiber decreases body weight. First, fiber increases satiety, which will decrease the total amount of food consumed [1]. Second, fiber reduces the blood glucose spike following a meal. Excess glucose is converted to fat, so fiber reduces or eliminates that outcome.

Some foods high in fiber are raspberries, pears, apples, sunflower seeds, almonds, artichokes, and broccoli. If you decide to add fiber to your diet with the goal of weight loss, substitute these for foods you currently eat instead of adding them to your current diet. 

If you decide to take a fiber supplement, start slowly: I added an extra four grams per day for a week, then increased to eight, and ultimately reached 12 after a few weeks. Water intake MUST increase to prevent gastrointestinal distress with added fiber.


Taylor, M.D. Ph.D said...

Great post, Sean. Did you come across any research that looked at the difference between soluble and insoluble fibers?

Dr. Sean Preuss said...

Hi Taylor,

Thanks for reading. I'm sorry for the late reply. References 2 and 3 measure soluble fiber intake. In both cases, soluble fiber was about twice as great in the higher fiber group. Does that help?