Whole grains are a lousy source of fiber

Eating more dietary fiber can reduce your risk of stroke. That’s the conclusion of a major meta-analysis to be published in the May issue of Stroke.

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
Bell Peppers312.16.77
Barley (hulled, not pearl)35417.34.89
Wheat flour, whole grain34010.73.15
Corn flour, whole grain3617.32.02
Grapes (seedless)690.91.30
Brown rice (long grain)3703.50.95

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.

Hi! I blog about food and health for Supplement SOS. I like green vegetables, long walks on the beach and triple-blind placebo-controlled intervention studies with large sample sizes. Liked this post? Follow on Google+, Twitter or via RSS or email me!.

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14 comments on “Whole grains are a lousy source of fiber
  1. Add to that that most people only look at the product label, not the ingredients/nutritional value. Many foods labeled as whole grain in fact have the same low amount of fiber as their non-whole grain counterparts.

    • Lena Zegher says:

      That’s a great point Ralph. This table is based around the absolute best-case scenario — that all “whole-grain” products use only 100% whole-grain ingredients. As you point out, that’s rarely the case.

      Another thing I’ve noticed anecdotally is that “multi-grain” seems to be a popular descriptor for grain products touted as healthy. That can also be misleading for people who interpret “mutli-grain” as an indication of healthiness where in fact it could simply mean made with refined wheat and refined corn flours.

  2. Alex Hatcher says:

    I really like your blog. the thoughts AND then with actual data is very useful.

  3. Matt Nelson says:

    Thanks for the article! Great read and very informative.

  4. Anonymous says:

    While I understand the reason for your comparison, I think it is fundamentally flawed. But is it any surprise that leafy vegetables do the highest on amount of fibre per calorie? When 80% of lettuce is water, of course it will itself have a low calorie per gram content. Whereas things like oats will have more calories per gram due to their carb content. As a result the way the list is ordered inherently biases in favour of your argument.

    I think the amount of fibre per gram is far more valuable because it standardises the amount, whereas you’d have to eat physically a lot more lettuce to get the same amount of fibre as say 100g of oats. And in that case you can see that whole grain flour and oats do the best for amount of fibre per gram.

    • Lena Zegher says:

      You *would* have to eat a lot more lettuce, tomatoes or oranges in terms of mass — but you also *can* eat a lot more lettuce, tomatoes or oranges because they have a lower calorie count per 100 grams.

      I understand your point of view, and I agree there is a limit on the mass of food it’s practical to eat per day. But there’s also a limit on the number of calories it’s practical (or desirable) to eat per day.

      It’s a question of which of these limits is more relevant to the individual person. From my perspective, people typically seem to have more trouble butting up against their daily calorie allowance than they do struggling to eat large masses of calorie-sparse foods. Therefore, fiber-per-calorie is, in my opinion, the more important metric.

      • Anonymous says:

        “you also *can* eat a lot more lettuce, tomatoes or oranges because they have a lower calorie count per 100 grams.”

        But are you really going to eat 10x as much lettuce (in grams) than the same amount of oats to get your fibre? I highly doubt it. I love eating a bit of iceberg lettuce as much as the next person, don’t get me wrong, but I can’t imagine anyone scoffing down that much lettuce in one go to meet your fibre demands. After all if there is a richer source of fibre then why wouldn’t you take that option?

        “people typically seem to have more trouble butting up against their daily calorie allowance than they do struggling to eat large masses of calorie-sparse foods.”

        This is a nice idea and all, but it’s not very realistic. The types of people that need to limit their caloric intake usually fall into two categories. Either they are responsible and can maintain a strict and healthy diet which allows them to lose weight (or maintain an appropriate weight) or they are irresponsible, eat too much and have problems with their diet. Your advice will be irrelevant to the former because they could adequately manage their caloric intake whether its oats or lettuce, they can stick to it. On the other hand with the latter I doubt they’ll find eating a ton of lettuce appealing. Moreover, as a medical student, I know that some people just plain cannot control their diet. I’ve seen diabetics with glucose levels of >12mM and even after plenty of counselling and advice from a dietician they cannot control their glucose levels, and the medication? they don’t even know what they’re taking much less remember to take it.

        I guess my point is that while eating lettuce certainly would give you more fibre per calorie, in a real world situation I doubt anyone who actually needs that advice will be taking it. They’d more likely eat a bit of lettuce, get annoyed about how bland it tastes and not eat any more. Thereby negatively affecting their fibre intake anyway.

  5. Sharon says:

    If you need fiber, you need beans and legumes. A half-cup of cooked black beans (approx 80 calories) has almost 10 grams of fiber.

    • Lena Zegher says:

      Thanks for the comment Sharon!

      I’m actually working on a broader series of tables that compares a wider variety of foods (including legumes) across a broader range of nutrients. I think it will be interesting to see how the different groups compare when looked at from a wider perspective.

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