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Fiber seems so, well, boring. It’s not one of the nutrition topics commonly placed among cutting-edge research. By definition, fiber is indigestible––can’t be absorbed into the body. So, it just stays in our gut to bulk up our stool. This is not to belittle the importance of bowel regularity. Constipation is a global issue. In the United States, for example, chronic idiopathic constipation impacts 5% of people. If even half of the adult U.S. population ate three more grams of fiber a day—a quarter cup (50 – 60g) of cooked beans or a bowl of oatmeal—we could relieve enough constipation to save billions of dollars in medical costs annually. But it’s not like we thought fiber really did anything. In fact, you can get the same laxative effect with indigestible plastic particles. Feeding people a couple spoonsful of sliced polyvinyl tubing – not recommended – ends up increasing their stool bulk, frequency, and softening the consistency. So, for decades fiber was viewed as a similarly inert, indigestible substance.
We’ve since moved beyond fiber’s classic function as an anti-constipation agent to recognizing its other beneficial effects, including the many ways it can facilitate weight loss. The first major review, “Dietary fiber and weight regulation,” based on a dozen interventional studies where people were randomized into higher or lower fiber diets, found that the additional consumption of 14 grams of fiber a day led to an average weight loss of four pounds (1.8 kg) over 3.8 months. That’s only about a pound (0.45 kg) a month, but the weight loss was greater among those with excess weight. Study participants with overweight or obesity lost triple the weight, compared to lean individuals. And 14 grams of fiber isn’t a lot. That would even take the average U.S. diet up to the recommended minimum average adequate daily fiber intake of 38 grams for men and 25 daily grams for women.
The increased fiber intake appeared to lead to about a 10% drop in daily calorie intake. Why would more fiber mean fewer calories? Well, conventionally, fiber is considered to have zero calories; so, it adds bulk to food without adding extra calories, resulting in a lower “energy-to-weight ratio” in the food. To illustrate, let’s compare a food to its fiber-depleted equivalent. Consider a bottle of cold-pressed apple juice. That’s basically just an apple with its fiber removed. You could drink a regular bottle of juice (about 15 ounces, 445 ml) in a matter of seconds. But to get the same number of calories, you’d have to eat nearly 5 cups (550 – 600 g) of apple slices.
First, you’d have to chew all those apple slices. Fiber-rich foods require more chewing, which slows down eating rate, which itself can improve satiety. That also allows for more secretion of saliva and stomach juices. Researchers spread a barium paste onto slices of different kinds of bread––talk about research that would not pass human safety review today––and found that on x-ray, the stomach shadow was larger after eating whole wheat bread compared to white.
Five grams of a highly viscous fiber, like what’s in apples and oats, can hold approximately a quart of water (945 ml) as it passes through the stomach and small intestine. So, that’s having an extra two pounds (0.9 kg) of zero-calorie food mass filling you up. If you feed people the same meal, one with natural fiber (whole-wheat pasta with pureed veggies and fruits) and the other meal composed of the same foods with the fiber-depleted (white bread with fruit and vegetable juices), but both meals have the same volume, the stomach empties faster with the fiber-depleted meal, about 45 minutes faster.
It’s not what we eat, but what we absorb. So, we can lose more weight on a high-fiber diet eating the exact same number of calories, because some of the calories get trapped and never make it into our system. Those on a standard American diet lose about 5% of the calories they eat in their waste. A higher-fiber diet can double that. It’s not just that the calories in high-fiber foods are less available to absorb; they trap calories across the board. Feed people whole-meal bread versus white bread, and stool analyses find that we flush more calories on the whole meal bread. Eat a Twinkie on a high-fiber diet and absorb fewer Twinkie calories. It’s like every calorie label you read gets instantly discounted on a high-fiber diet.
Most nutrition classes will teach that a gram of protein has four calories, a gram of fat has nine calories, and a gram of carbs has four. But that’s only on a typical low-fiber diet. On a higher-fiber diet, up around the average of those eating strictly plant-based diets, the effective calorie counts per gram drop from 4-9-4 for protein, fat, and carbs to around 3.5-8.7- 3.8. That may not seem like a lot, but if Americans just reached the minimum recommended fiber intake, that might decrease calorie absorption by more than 100 calories a day. That may be enough to prevent that average gradual annual weight gain most experience through middle age. Even a small change in daily calorie absorption could have long-term significance for weight management.
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