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A salad is a great way of getting fiber.

Fiber is an essential part of a healthy diet, and is frequently misunderstood. Fiber does far more than prevent constipation, helping with weight loss and overall health. Fiber is broadly defined as edible food that cannot be digested in the small intestine, and while some fiber is completely indigestible, other types can be partly digested further down the digestive tract.

1 Weight Loss and Fiber

One of the key benefits of fiber is to help with weight loss and weight control. Fiber can help with weight loss in many ways:

  • Fiber can slow the emptying of the stomach, and delay the absorption of nutrients in the small intestine. This produces a feeling of fullness for longer[1] and fiber (30g/day) reduces hunger from low calorie diets[2].
  • Higher fiber intake is linked to long term weight control[3][4][5][6].
  • Fiber displaces the more energy dense components of food[7] and can reduce the absorption of fat and protein[8].
  • Calories from fiber that is fermented varies between 1.5 and 2.5 Calories/gram[9][8].
  • Fiber reduces the Glycemic Index of foods, which may be why fiber reduces subsequent hunger[7][10][11].
  • Fiber reduces the energy that is absorbed from other (non-fiber) food[12].
  • It's probable that the benefits of fiber are linked to consuming high fiber foods rather than fiber supplements.
  • The characteristics of the fiber (solubility, fermentability, viscosity) are important in determining the impact on weight[13].

2 Other Health Benefits

There are a number of other health benefits from increased fiber intake.

  • Some fiber is fermented in the colon by bacteria. This fermentation produces carbon dioxide, methane, hydrogen, and short-chain fatty acids. These short chain fatty acids are the preferred fuel for the cells in the colon[14] and if they are lacking they may cause Ulcerative colitis[15][16] and the colon cells to self-digest[17].
  • Some, but not all, types of fiber will reduce constipation. For instance, Guar Gum is highly fermentable and does not change stool consistency[18].
  • Some types of fiber can normalize cholesterol levels[19].
  • Cereal fiber provides substantial protection from Coronary Heart Disease[20][21][22]. (There is little evidence for other types of fiber[19].)
  • Fiber may provide protection against colorectal cancer, though the evidence is not conclusive[23][24]. In addition, fiber may protect against breast cancer[25].

3 Increasing Your Fiber Intake

Rapidly increasing your fiber intake can cause digestive pain, bloating and flatulence. These problems can be minimized or avoided by increasing your fiber intake gradually. It is also important to increase your fluid intake along with your fiber intake, as fiber will absorb several times its weight in water. The health benefits of fiber may not be achievable with fiber supplements.

4 Types of Fiber

There are several ways of classifying fiber[26]:

  • Fermentable and non-fermentable. While the fiber cannot be digested directly by humans, it can be fermented by bacteria that naturally live in the colon. This means that fermentable fiber helps keep your digestive bacteria healthy. These bacteria also tend to partly digest the fermentable fiber and the resulting short chain fatty acids are used by the digestive tract. Without soluble fiber, there are indications that the colon will be starved of this essential nutrition.
  • Viscous and non-viscous. Some fibers form a thick, viscous gel tends to slow digestion and reduce cholesterol levels. Viscus can also fibers also help more with constipation.
  • Soluble and insoluble. A soluble fiber will disperse in water, while insoluble fiber will not. While soluble and insoluble fibers tend to have different characteristics, you need to consider other aspects of a fiber to understand it's impact on your health. Most insoluble fibers are non-fermentable, but soluble fibers can either be fermentable or non-fermentable. Likewise, insoluble fiber is generally non-viscus while soluble can be either.

These characteristics interact in complex ways, so all characteristics have to be considered together[13]. Depending on the combination of characteristics, there are different health benefits, as shown below[27].

Soluble? Viscus? Fermentable? Examples Health impacts
No No No Wheat bran, Cellulose
  • Course particles can irritate the digestive tract, which may reduce constipation.
  • Finely ground particles such as whole-wheat bread, have little effect on health.
  • No cholesterol reduction.
  • No increase in flatulence.
  • No calorific value.
  • May encourage meal termination[13].
Yes No Yes Inulin, wheat dextrin, oligosaccharides, resistant starches
  • Dissolves completely in water.
  • Digestion by bacteria improves digestive health.
  • May rapidly increase flatulence.
  • No reduction in constipation or diarrhea.
  • No reduction in rate of digestion (does not reduce glycemic index).
  • Possible cholesterol reduction[28][29]
  • No clinically supported health benefits.
  • 1.5-2.5 Cal/g [9][8].
Yes Yes Yes Oats, Barley, Guar Gum, Pectin[30] (Apples, etc.)
  • Dissolves in water to form a thick gel (think of cooked oatmeal/ porridge).
  • Digestion by bacteria improves digestive health.
  • Slows nutrient absorption (lowering glycemic index.)
  • May increase flatulence.
  • Reduces Cholesterol.
  • No reduction in constipation or diarrhea.
  • 1.5-2.5 Cal/g [9][8]
Yes Yes No Psyllium (Metamucil), Methylcellulose (Citrucel)
  • Dissolves in water to form a thick gel (think of cooked oatmeal/ porridge).
  • No bacterial support.
  • Slows nutrient absorption (lowering glycemic index.)
  • No flatulence.
  • Reduces Cholesterol.
  • Reduces both constipation and diarrhea, normalizing stools.
  • No calorific value.
  • May reduce weight gain and encourage meal termination[13].

Here's the same information in an alternative layout.

Wheat bran, Cellulose Inulin, wheat dextrin, oligosaccharides, resistant starches Oats, Barley, Guar Gum Psyllium (Metamucil), Methylcellulose (Citrucel)
Improve digestive bacteria? No Yes Yes No
Slows nutrient absorption No No Yes Yes
Causes flatulence No Yes Yes No
Reduces Cholesterol No No Yes Yes
Reduces constipation? Only if coarse ground No No Yes
Normalizes stools? (Reduces both constipation and diarrhea) No No No Yes
Calories? No 1.5-2.5 Cal/g [9][8] 1.5-2.5 Cal/g [9][8] No

5 Sources of Fiber

Many good sources of fiber are also generally considered healthy foods, and there is a sample list below. In addition, it's worth noting that when pasta or rice is cooked and then left to go cold, some of the carbohydrate changes to form 'resistant starch', a form of fiber. A cup of cold pasta for instance, contains about 1.9 grams of resistant starch[31].

5.1 Legumes (beans)

Amount Fiber (grams)
Split peas, cooked 1 cup 16.3
Lentils, cooked 1 cup 15.6
Black beans, cooked 1 cup 15.0
Kidney beans, canned 1 cup 13.6
Lima beans, cooked 1 cup 13.2
Refried beans, canned 1 cup 12.2
Baked beans 1 cup 10.4

5.2 Grains

Amount Fiber (grams)
Spaghetti, whole-wheat, cooked 1 cup 6.2
Barley, pearled, cooked 1 cup 6.0
Bran flakes 3/4 cup 5.3
Oatmeal, quick, regular or instant, cooked 1 cup 4.0
Popcorn, air-popped 3 cups 3.5
Brown rice, cooked 1 cup 3.5

5.3 Vegetables

Amount Fiber (grams)
Artichoke hearts, cooked 1 cup 14.4
Peas, cooked 1 cup 8.8
Broccoli, boiled 1 cup 5.1
Sweet corn, cooked 1 cup 4.2
Brussels sprouts, cooked 1 cup 4.1
Potato, with skin, baked 1 medium 2.9
Tomato paste 1/4 cup 2.7
Carrot, raw 1 medium 1.7

5.4 Fruits


Fruits contain 0.5-1.5% Pectin by fresh weight.

Amount Fiber (grams)
Prunes, uncooked 1 cup, pitted 12.4
Pear, with skin 1 medium 5.5
Apple, with skin 1 medium 4.4
Strawberries (halves) 1 1/4 cup 3.8
Banana 1 medium 3.1
Orange 1 medium 3.1

5.5 Nuts

Nuts Amount Fiber (grams)
Almonds 1 ounce (23 kernels) 3.5
Pistachio nuts 1 ounce (49 kernels) 2.9
Pecans 1 ounce (19 halves) 2.7
Peanuts 1 ounce 2.4

6 Flatulence

Flatulence (gas) is a constant source of both humor and embarrassment. Unfortunately, this embarrassment causes people to avoid high fiber foods, even when they understand the benefits. The major components of intestinal gas are nitrogen, hydrogen, carbon dioxide and methane, all of which are odorless. The actual odor comes from tiny amounts of compounds such as ammonia, hydrogen sulfide, other sulfur compounds, short chain fatty acids, etc. Even though only trace amounts are produced, the human nose can detect them in concentrations as low as 1 part in 100 million[32]. The amount of gas produced is proportional to fiber intake, but the intensity of the odor is independent of fiber intake[32]. Other factors, such as beer intake, increases flatulence odor but not quantity[32]. It's well known that beans and other legumes increase flatulence[33], so try different sources of fiber to find foods that do not cause problems. Also, live bacteria, which are found in fermented foods such as yoghurt, sauerkraut, etc., may reduce flatulence in people with irritable bowel syndrome[34][35]. Products such as Beano help reduce flatulence[36], but there is no evidence to determine if Beano will undermine the benefits of fiber. This is a concern given that Beano works by converting some of the fiber to simple sugars[37].

7 References

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