Soluble Fiber's Role in Improving Blood Glucose Regulation
A few weeks ago, Martina from the KetoDiet App website wrote an excellent blog post about counting net carbs vs. total carbs for those following a carbohydrate-restricted lifestyle. In the article, she correctly states that insoluble fiber passes through your system without being absorbed, so it should be subtracted from the total carbohydrate count. In addition, Martina explains that, unlike insoluble fiber, soluble fiber does not pass through the colon but rather is fermented into short-chain fatty acids (SCFAs) by our gut bacteria.
The principal SCFAs are acetate, butyrate, and propionate. Of these, propionate is the only one that can be converted into glucose via gluconeogenesis (1). But would this have any significant effect on postmeal blood glucose given that most fiber is insoluble and only 15-40% of any soluble fiber will be converted to propionate (2)? Take brussels sprouts, for instance, one of the few foods that contains more soluble than insoluble fiber. A 1-cup serving has about 4 grams of soluble fiber. Less than 2 grams will be converted into propionate, and the amount of glucose produced from it via hepatic gluconeogenesis would be quite small, so serum blood glucose levels wouldn't be affected much.
More importantly, as Martina pointed out, De Vadder, et al, recently reported the results of a study on mice suggesting that SCFAs promote intestinal gluconeogenesis (IGN), with propionate acting as substrate and butyrate promoting the expression of genes necessary to induce this process. Compared with control mice fed their normal diet, mice who were fed diets enriched with SCFAs and fructo-oligosaccharides (FOS) experienced lower fasting serum blood glucose levels, improved glucose tolerance, and a significant reduction in the enzyme responsible for hepatic gluconeogenesis (3). Essentially, propionate is converted to glucose and used within the intestine, decreasing the amount of glucose produced by the liver, resulting in the above-mentioned benefits. Yes, this was an animal study, but it provides an intriguing explanation for the improvement in blood glucose levels seen in human studies examining the role of dietary fiber (4, 5).
Blood glucose response to fiber-rich food may differ among some people with Type 1 diabetes, whose blood glucose levels can reportedly rise as a result of higher food volume in the stomach or other stimuli, regardless of digestible carbohydrate intake. Dr. Bernstein and others with T1 have suggested such a response (what he calls The Chinese restaurant effect), but I know of no studies confirming this and would appreciate references if anyone has them.
Health Benefits of Fiber and SCFAs
We've known about the benefits of fiber on colon health for decades. Although it's believed these are due in part to insoluble fiber's ability to reduce intestinal transit time so potentially carcinogenic substances in our food spend as little time as possible in our GI tract, more recent research suggests that fiber's chemoprotective effects may be due primarily to the effects of one of the SCFAs, butyrate (6,7), which provides energy and nourishment for the cells of the colon.
In addition, higher fiber intakes may reduce levels of C-reactive protein (CRP) and other inflammatory markers, improve our immune response, and protect the gut from harmful microorganisms that can make us ill (8). In essence, fiber's fermentation to SCFAs helps keep our gut well nourished and working the way it should.
Increasing High-Fiber Carbohydrate Intake May Be Helpful for Weight Loss
Let's go back to restricting intake to less than 20 grams of total carbohydrates in order to lose weight. When carbohydrate intake is this low, there's a limited number of foods that can be eaten: meat, cheese, fish, eggs, butter, cream, coconut oil, olive oil, and small amounts of greens and other very-low-carb vegetables. Yes, carbohydrate intake may be well below 20 grams of total carbs daily, but depending on portions consumed, calorie/energy intake may be too high to promote weight loss. A serving of bullet-proof coffee contains 440 calories, zero carbs, zero protein, and zero fiber.
Aside from fiber's beneficial effects on overall health outlined above, consuming at least a moderate amount of fiber can facilitate weight loss, and both insoluble and soluble fiber can be helpful in this regard.
Insoluble fiber passes through the body without being absorbed. It adds volume to meals, but zero calories and zero carbohydrates. In fact, you could almost say that insoluble fiber provides negative calories, in that it may lower the amount of energy derived from fat and protein when all three are consumed at a meal (9). Most foods are higher in insoluble than soluble fiber, with a few exceptions (Please refer to Martina's fiber chart in the blog post linked to above).
Soluble fiber does contribute calories/energy to the body, but no carbohydrates. The chief benefits provided by soluble fiber are due to its fermentation to the SCFAs acetate, butyrate, and propionate, which have been shown to promote satiety, reduce intake, and and decrease body fat (3,10-11).
Carbohydrate and Fiber: What's Optimal?
Like carbohydrates, fiber is another substance where across-the-board recommendations can't be made, but I think it's best to aim for at least 20 grams daily, with 1/3 or more from soluble fiber. If you're limiting yourself to less than 20 grams of total carbohydrate, the maximum amount of fiber you can possibly get is 18 grams, and that's only if the carbs come from foods that are more than 90% fiber, such as chia seeds, flaxseed, or avocado. Generally speaking, those who consume diets containing less than 20 grams of total carbohydrate end up with 10 or fewer grams of fiber. Some people reportedly consume less than 5 grams on a consistent basis.
The goal of carbohydrate restriction is to keep blood glucose and insulin levels low, and this can be accomplished without reducing carbs to near-zero levels. I consume around 30-45 grams of fiber daily, and my net carb intake is around 35-50 grams, so my fiber to nonfiber carb ratio is roughly 1:1. This works well for me, but some people do better with 20-30 grams of fiber and slightly lower net carb intake. People with certain GI disorders* may require restriction of certain types and amount of fiber.
How do you get to 30 grams of fiber yet still maintain net carb intake of less than 40 grams? Here's one of the sample menus with recipes that will be included in my upcoming book, The Low Carb Dietitian's Guide to Health and Beauty.
Sample Low-Carb, High-Fiber Menu
Cinnamon Flaxseed Pudding*
1 cup blackberries
Coffee or tea with 2 Tbsp half and half
1/2 oz dark chocolate (at least 85% cocoa)
Water, tea, or other sugar-free beverage
3 celery stalks with 1 Tbsp almond butter
Chocolate Avocado Pudding*
Water, tea, or other sugar-free beverage
Protein: 88 grams
Total Carbohydrates: 68 grams
Fiber: 31 grams
Insoluble Fiber: 20 grams
Soluble Fiber: 11 grams
Net Carbohydrates: 37 grams
Fat: 108 grams
Cinnamon Flaxseed Pudding
Number of Servings: 1
½ cup cottage cheese
2 Tbsp ground flaxseed
½ tsp cinnamon
3 Tbsp chopped toasted pecans
Stevia or other sweetener, if desired
Combine all ingredients in small bowl.
Number of Servings: 1
4 oz fresh mozzarella cheese, sliced into ¼-inch rounds
1 large vine-ripened tomatoes, sliced ¼-inch thick
1/2 cup fresh basil leaves
Coarse sea salt, to taste
2 Tbsp cup extra-virgin olive oil
On a plate, alternate mozzarella slice, then tomato slice, followed by basil leaf, and repeat sequence, overlapping each item slightly. Sprinkle salt and drizzle olive oil over top.
Number of Servings: 4
2 Tbsp coconut oil
1 cup chopped onion
2 tsp sea salt
2 tsp chopped garlic
2 tsp ground ginger
2 tsp coriander
1 tsp turmeric
1 tsp chili powder
4 cups broccoli florets
2 cups snow peas
2 cups mushrooms
½ cup coconut milk
1 lb chicken breast, cut into bite-sized pieces
Heat oil in a wok or large saucepan over medium-high heat. Add the onion, then cook and stir until browned. Mix in garlic and spices. Add broccoli, pea pods, and mushrooms. Cook and stir for 1-2 minutes.
Add coconut milk and chicken. Reduce heat to medium and cook for 7 to 8 minutes, until chicken is no longer pink. Stir and remove from heat. Serve immediately.
Chocolate Avocado Pudding
Number of servings: 4
2 medium very ripe avocados
½ cup unsweetened coconut milk
½ cup unsweetened cocoa powder
1 cup erythritol or other granulated sugar substitute equivalent to sweetness of ¾ cup sugar
2 tsp vanilla extract
Cut avocado in half, remove pit, and scrape flesh into food processor or blender. Add coconut milk, cocoa powder, sweetener, and vanilla extract. Process until ingredients are well combined and there are no lumps of avocado. Divide into four dishes and refrigerate at least 30 minutes or until ready to serve.
Roasted Brussels Sprouts Recipe
And for anyone who was hoping for a brussels sprouts recipe after seeing the photo and reading about their high soluble fiber content, please check out this delicious side dish, Roasted Brussels Sprouts with Pecans, from Kalyn's Kitchen.
A Balanced Low Carb Diet: Eat Plenty of Plants and Animals
If you're consuming less than 20 grams of carbohydrate daily and achieving your weight loss goals, I'm very happy for you. But I would consider adding at least a few grams of high-fiber carbohydrates -- such as vegetables, berries nuts, and dark chocolate -- to optimize overall health and increase your likelihood of long-term success. And if your weight loss has stalled, I definitely recommend increasing your intake of these foods and perhaps decreasing fat and increasing protein intake, depending what you're doing currently. A sustainable low-carb lifestyle consists of a balance of nutritious animal and plant foods, based on personal tolerance, preferences, and goals.
* * *
*For those with small-intestinal bacterial overgrowth (SIBO) or other conditions requiring a low-reside diet, such as acute diverticulitis or other inflammatory bowl conditions, the recommendations for fiber intake provided in this article may be contraindicated. Consult your healthcare provider, who can provide guidance or refer you to a dietitian for recommendations.
1.Mithieux G, et al. Intestinal glucose metabolism revisited. Diabetes Res Clin Pract. 2014 Sep;105(3):295-301
2. Bergman EN, et al. Energy contributions of volatile fatty acids from the gastrointestinal tract in various species. Physiol Rev. 1990 Apr;70(2):567-90
3. De Vadder F, et al. Microbiota-generated metabolites promote metabolic benefits via gut-brain neural circuits. Cell. 2014 Jan 16;156(1-2):84-96
4. Post, RE, et al. Dietary Fiber for the Treatment of Type 2 Diabetes Mellitus. J Am Board Fam Med Jan-Feb; 25:16-23, 2012
5. Aller R, et al. Effect of soluble fiber on lipid and glucose intake in healthy subjects: a randomized clinical trial. Diabetes Res Clin Pract. 2004; 65(1): 7-11
6. Cho Y, et al. Colon cancer cell apoptosis is induced by combined exposure to the n-3 fatty acid docosahexaenoic acid and butyrate through promoter methylation. Exp Biol Med (Maywood) 2014;239(3):302-310
7. Fung KY, et al. A review of the potential mechanisms for the lowering of colorectal oncogenesis by butyrate. Br J Nutr. 2012 Sep; 108(5):820-31
8. Levison ME, et al. Effect of Colon Flora and Short-Chain Fatty Acids on Growth In Vitro of Pseudomonas aeruginosa and Enterobacteriaceae. Infect Immun. Jul 1973; 8(1): 30–35
9. Baer DJ et al. Dietary fiber decreases the metabolizable energy content and nutrient digestibility of mixed diets fed to humans. J Nutr. 1997 Apr;127(4):579-86
10.Darzi J, et al. Do SCFA have a role in appetite regulation? Proc Nutr Soc. 2011 Feb;70(1):119-28
11. Lin HV, et al. Butyrate and propionate protect against diet-induced obesity and regulate gut hormones via free fatty acid receptor 3-independent mechanisms. PLoS One. 2012;7(4):e35240