The authors analyzed 8 large prospective cohort studies. Their results showed a pooled estimate of 7% reduction in relative risk of first-time stroke for each additional 7 grams of fiber added to the diet. (The authors point out that data is sparse for diets containing more than 25 grams of fiber per day, so extrapolation of the protective effect above this amount may be premature.)
However, the study lacked the statistical power to reliably discern the effects of soluble vs. insoluble fiber, or the effects of fiber from different dietary sources.
In short, the only message the authors fully endorse is the paper’s conclusion:
Higher total dietary fiber intake is significantly associated with lower risk of first stroke. Overall, findings support dietary recommendations to increase intake of total dietary fiber. However, a paucity of data on fiber from different foods precludes conclusions regarding the association between fiber type and stroke.
Encouraging stuff. And truthfully, if that were the end of the story, I probably wouldn’t have been tempted to write about it. What snagged my attention in the first place was an interview with the paper’s co-author, Victoria Burley, published on the U.S. Department of Health and Human Services Healthfinder.gov website, particularly this snippet:
“Increasing your fiber intake doesn’t necessarily mean wholesale change to your diet,” Burley stressed. “It might just mean switching from white bread to whole-meal, or from corn flakes to bran flakes.”
The Healthfinder article mentioned whole-wheat pasta as a good source of dietary fiber. A video on MedlinePlus also recommended a serving of whole-wheat pasta as an easy way to increase fiber intake by 7 grams.
In fairness to both Ms. Burley and the various US health agencies, both articles also mentioned fruits and vegetables as potentially valuable sources of dietary fiber. Furthermore, academics and government bodies have a responsibility to provide actionable health advice that’s as easy as possible for the public to follow.
But as a humble blogger, I have no such restrictions, and my attention was snared — how do whole grains compare to fresh produce when it comes to fiber content?
First, to ward off any accusations of cherry-picking, I decided to use the five most commonly consumed (non-starchy) vegetables and fruits in the comparison. The best source I know for this data is the USDA Economic Research Service Food Availability (Per Capita) Data System. Using the retail weight metric, the five most commonly consumed vegetables in the US are onions, tomatoes, lettuce, bell peppers and carrots (in that order). For fruits, it’s bananas, melons, apples, oranges and grapes.
Next, we have to decide which metric to use for fiber. There are various limits to what a person can eat — price, quantity, personal preferences, allergies and so forth. But in my opinion, the primary limit on diet is calories. Each person has a set caloric budget to spend in order to maintain (or reach) a given weight. For me, knowing the quantity of nutrient X per 100 grams is not particularly relevant if I can eat three times as much of source A than source B per day. What I’m really interested in is the quantity of the nutrient per calorie.
Accordingly, here’s a table of exactly that for five most commonly consumed fruits and vegetables in the US, along with five of the most popular grains. All nutritional values are taken from release 25 of the National Nutrient Database for Standard Reference from the USDA Agricultural Research Service.Fiber per calorie for vegetables, fruits and grains
Calories per 100g
Fiber per 100g
Fiber per 100 calories
|Barley (hulled, not pearl)||354||17.3||4.89|
|Wheat flour, whole grain||340||10.7||3.15|
|Corn flour, whole grain||361||7.3||2.02|
|Brown rice (long grain)||370||3.5||0.95|
Out of the 15 foods for fiber per calorie, the top 4 are vegetables. In fact, out of all the grains, only the relatively uncommon hulled-not-pearl barley prevents a whitewash, narrowly beating out onions. For the fruit, apples and oranges make it into the top half of the board, while bananas, melons and grapes fare relatively badly. But the real losers are the grains – 3 out of the bottom 5, and all but one in the lower half of the table. The biggest surprise for me was brown rice — I had expected it to perform around average, at least compared to the other grains, but it comes dead last.
The real strength of a table like this is that it lets you see how much of your calorie budget you’d have to dedicate to reaching a certain amount of fiber intake. For example, to add the additional 7 grams of fiber the meta-analysis focuses on, you’d need 80 calories of lettuce but around 740 calories of brown rice. For a person consuming 1500 calories a day, that’s 5% of their calorie budget vs. 50%. 50% is a massive commitment to reach a single nutritional goal.
In much of the messaging I get from governmental health bodies, whole grains are virtually synonymous with fiber. But from this perspective (and particularly for corn and rice) they’re a pretty lousy source.