According to the Institute of Medicine, adults should be eating 20-35 grams per day. But why? What’s so important about fibre anyways and what does it do for us physiologically? And does it matter what kind of fibre we eat?
There are more than one type of fibre, But you won’t see it detailed on any nutritional label. We are told to focus on total fibre, regardless of where it came from. Although total fibre is important, getting enough of each type of fibre may be just as important in having a healthy, happy digestive system.
What is Fibre? And what does it do?
Fibre is similar to starches and sugars in that it’s mostly made of carbohydrates . The difference is that fibre refers to the carbohydrates that your body can’t digest, while starches and sugars are carbohydrates that humans easily digest.
Why would anyone want to eat something that their bodies can’t digest? It turns out that even though it isn’t something we can digest, fibre helps us to digest other foods and keeps our digestive system in good condition. Of course, the kind of fibre matters.
There are two main categories of fibre: soluble and insoluble.
Soluble fibre dissolves in water, while insoluble fibre, as the name suggests, doesn’t. The average fibre intake contains 75% insoluble fibre and 25% soluble fibre, each having its own special physiological impacts on digestion.
Insoluble fibre
Insoluble fibre is found in whole wheat foods, bran and nuts, as well as seeds. It’s what forms the mass that moves through your bowels, and while that might sound a little gross, it’s key to preventing conditions like diarrhea and constipation. Because it stays in a solid form, it helps compact the forming stool, which allows it to move more easily through the intestines. Insoluble fibre is also believed to act as a sponge, or scrub brush, cleaning the digestive system along the way by gently scraping the walls and snowballing any stray particles.
Soluble fibre
Soluble fibre, found in fruits, vegetable and flax seed, acts differently because it becomes gel-like in water, actually incorporating with liquids. Soluble fibre helps hydrate stool, allowing it to slip though the intestines smoothly, a key in preventing digestive disorders. But soluble fibre does so much more – often called “viscous fibre“, soluble fibre makes your digestive contents into a thick slurry, which slows down your digestive system, slowing the process of absorption in the small intestine.
The effect of this is that you feel fuller, longer, because of slower, steadier increases in blood glucose levels, and your body requires less food to attain the same absorption of energy and nutrients. One study found that adding one kind of soluble fibre to their subject’s diets reduced the their food intake by 11%!
But soluble fibre also helps regulate the levels of glucose in our blood more directly. Soluble fibre is often fermented during our digestive processes, producing compounds called short-chain fatty acids and gasses (FYI, this is where increases in flatulence can come from when eating beans – legumes are high in soluble fibre and fructo-oligo-saccharides).
Short-chain fatty acids have a number of physiological roles. These small fat molecules have been shown to directly influence insulin release from the pancreas and glycogen breakdown in the liver, leading to stable and healthy glucose levels. Short-chain fatty acids also influence the liver’s production of cholesterol. The effect of this is that increased fibre consumption lowers the circulating bad cholesterol in the blood. They’ve been found to promote the production of immune cells and antibodies, potentially boosting immune function. Furthermore, these little acids help regulate the pH (acidity) of the intestines and the colon, keeping it in the right range to promote nutrient absorption and discourage microbes from producing toxins and carcinogenic substances.
Fibre and Disease
The old adage “an apple a day keeps the doctor away” may be more true than we thought. Apples are great sources of fibre, and both soluble and insoluble fibres help reduce the risk of a variety of diseases. One of the most well understood is fibre’s connection to diabetes. Because fibre slows down your digestive system, glucose enters your bloodstream at slower, more stable rate, and this helps with managing Type 1 diabetes. But fibre can help prevent Type 2 diabetes, too.
Type 2 diabetes can be induced by our diets when our bodies are exposed to high blood glucose levels for a long period of time. This can happen because of consistent over-intake of sugars and carbohydrates, but it can also occur because our bodies are unable to produce enough insulin to lower rising blood sugar levels, or we become desensitized to insulin activity. By creating a slow, steady stream of glucose uptake, fibre helps our bodies avoid the sudden spikes glucose and insulin that, over time, can lead to Type 2 diabetes.
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Studies have shown that diets high in fibre reduce the risk of Type 2 diabetes significantly. For example, a meta-analysis of several large studies (totaling to over 700,000 people) found that eating an additional 2 servings of whole grains a day decreased the risk of Type 2 diabetes by 21%. Both soluble and insoluble fibre intake are correlated with reduced diabetes risk, though how the insoluble fibre is involved is less understood.
Fibre is also strongly linked to reducing risk of heart disease. This is likely due to its positive influence on blood cholesterol levels. A number of large, long-term studies (like this one, or this one) have found that people with high-fibre diets have up to a 40% lower risk of coronary heart disease.
Furthermore, a meta-analysis of seven large studies found that the risk of developing cardiovascular disease was 21% lower in people who ate 2.5 or more servings of whole grain foods a day compared with those who ate much less. Mostly, these studies suggest that soluble fibre is the key player in preventing cardiovascular problems. Because of the extensive, strong correlation between soluble fibre intake and lowered risk, soluble fibre is one of the very few things that the FDA officially recognizes as reducing the risk of heart disease.
Furthermore, a recent study has found that increasing soluble fibre intake, but not insoluble fibre intake, actually helps boost our immune systems. Mice were fed low-fat diets that either contained regular amounts of soluble or insoluble fibre for six weeks before being subjected to an agent that mimics a bacterial infection. When the two groups were compared, the soluble-fibre eating mice were only half as sick as their insoluble-eating counterparts, and they recovered 50% sooner. What made the soluble fibre so much better for the mice?
Scientists discovered that the soluble fibre-eating mice were producing higher levels of anti-inflammatory compounds, perhaps due to short-chain fatty acids. Immune cells in our bodies have to deal with both invaders like infections and self-caused problems like inflammation. The researchers believe that by reducing the inflammation response, ingesting soluble fibre altered the mice’s immune cells, making them switch from problematic inflammatory cells to anti-inflammatory cells that can deal with other problems, like the faked infection. While this research is preliminary, it’s impressive that the mice immune systems were altered by manageable amounts of fibre that could easily be eaten by people.
The one thing fibre intake doesn’t help much with, though, is the one disease it’s most often touted as a way of preventing: colon cancer. The truth is that the connection between dietary fibre and reduced risk of developing colon cancer is weak at best. While some smaller studies have linked high-fibre diets to lowered risk of colon cancer, larger ones, like a 16-year Harvard study of more than 80,000 nurses, have found no connection.
Take Home Message: EAT MORE Fibre!
The scientific evidence is unmistakable – fibre is vital for a healthy digestive system. More often than not, you’ll be told to simply eat more fibre, but from the evidence I’ve seen, you should be particularly eating more soluble fibre. And that isn’t the kind of fibre normally touted – often, you hear people push for whole grains, which are good sources of insoluble fibre, even though it’s soluble fibre that is much more strongly linked to fibre’s various health benefits. There’s another problem, too – according to nutrition labels, fibre is fibre. There is no distinction between soluble and insoluble fibre in packaged foods. Instead, you have to be your own nutrition expert, and know which fibre-filled foods are naturally high in soluble fibre. Here are a few good ones:
| Foodstuff | Serving Size | Total Fibre (g) | Soluble Fibre | Insoluble Fibre (g) |
|---|---|---|---|---|
| Grapefruit | 1/2 fruit | 1.3 | 0.9 | 0.4 |
| Squash, summer | 1/2 cup | 2.3 | 1.1 | 1.2 |
| Zucchini | 1/2 cup | 2.5 | 1.1 | 1.4 |
| Brown rice | 1/2 cup | 1.3 | 1.3 | 0 |
| Rolled Oats | 3/4 cup | 3 | 1.3 | 1.7 |
| Orange | 1 medium | 2 | 1.3 | 0.7 |
| Plums | 2 medium | 2.3 | 1.3 | 1 |
| Broccoli | 1 stalk | 2.7 | 1.3 | 1.4 |
| Carrots | 1 large | 2.9 | 1.3 | 1.6 |
| Tangerine | 1 medium | 1.6 | 1.4 | 0.4 |
| Peas | 1/2 cup | 5.2 | 2 | 3.2 |
| Pinto beans | 1/2 cup | 3 | 2.2 | 0.7 |
| Potatoes | 1 small | 3.8 | 2.2 | 1.6 |
| Apple | 1 small | 3.9 | 2.3 | 1.6 |
| Psyllium husk | 10g | 8 | 7.1 | 0.9 |
Of course, no matter what you should probably be eating more fibre. Studies have found that Americans eat 50% or less than the recommended intake every day! So while soluble fibre may be the best option, any fibre is a start!
What are the downsides to eating fibre? After all, it sounds so wonderful – there must be something bad about it, right? Well… there is the gas produced by soluble fibre fermentation, but that seems a small price to pay compared to the many benefits that fibre intake can have. Sorry to sound unbelievable, but no, in my opinion, there aren’t any real downsides. Actually, I’ll rephrase – there aren’t any downsides to eating the amount of fibre you get from the kinds of foods people eat, so long as you still get the necessary amounts of protein and other nutrients, too.
Switch to eating entirely fibre-rich grasses instead of the normal grains, fruits and veggies and you might not fare so well. The only thing I will recommend, though, is that if you plan on dramatically increasing your dietary fibre intake, do it somewhat slowly to allow your body to adjust to the rise in non-digestible material in your diet. This will help you prevent the only known clinical side effects of sudden, high-fibre intake – tummy trouble.