From Muscle Fiber to Dietary Fiber: New Connections with Muscle Gain and Fat Loss

Explore the new connections between muscle gain and fat loss, while learning about the role of muscle fiber and dietary fiber in achieving your fitness goals.

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By Staff Writer

Published June 5, 2024.

a bowl of oatmeal topped with fruit and nuts

When we read the words “dietary fiber” on cereal box labels and in trendy fitness articles, we first think about how it affects our digestive health.  However, we found new research demonstrating how increasing your dietary fiber intake can help you increase muscle mass and decrease body fat. In this article, we will explain the differences between fiber’s two forms and look at research investigating fiber’s impact on three biomarkers InsideTracker improves: cholesterolblood glucose, and testosterone. Learn how InsideTracker can help you maximize your health by tracking your biomarkers during a research-based dietary intervention such as increased fiber intake.

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Soluble vs. Insoluble: There is a big difference in function

Dietary fiber is a carbohydrate we cannot digest because we lack the enzymes required to break it down. It comes in two forms: soluble and insoluble. Both differ by the extent they dissolve in water. Names of soluble fibers –or fibers that dissolve in water-include pectins, gums and other hemicelluloses [1]. Sources of soluble fiber include oat bran, nuts, seeds, beans, lentils and some fruits and vegetables. Soluble fiber absorbs water and mixes with food in the stomach. Then, because of its gel-like nature, it starts collecting food, sugar, fats, and cholesterol in the small intestine. Subsequently, this slows the absorption of these substances into the body, which gives a feeling of fullness and prevents a rapid increase in blood sugar. Soluble fiber finally carries these substances through our gastrointestinal (GI) tract.

Insoluble fiber - which doesn’t dissolve in water - moves through our digestive system intact and increases our stool bulk. This helps relieve constipation or irregular bowel movements (back to the claim of better digestive health). Example of insoluble fiber are cellulose hemicelluloses and lignin- both of which are found in plant cell walls. Sources of insoluble fiber include whole grains, vegetables, and wheat bran.

a picture of different foods that are in the picture

Even though both forms of dietary fiber are associated with better digestive health, let’s look at recent research examining what fiber can really do for us and how InsideTracker can help you evaluate the effects of your fiber intake.

Looking for an easy way to lower cholesterol? Increase fiber intake.

Some cereal boxes claim that their fiber content “may reduce the risk of heart disease.” The claim is based on the role of soluble fiber. Soluble fiber traps cholesterol in its sticky gel, which reduces the total amount of cholesterol in our blood that reaches our heart. Arteries become narrow and blocked when there is too much cholesterol in our blood and this slows down the blood flow to our heart. Soluble fiber helps prevent this potential blood flow blockage to our heart.

Going beyond the stickiness, soluble fiber’s entrapment of cholesterol causes another physiological response: a reduction in bile. Bile is a fluid produced in the liver comprised of water, electrolytes, bile acids, cholesterol and phospholipids. It aids the digestion and absorption of fats. In the intestines, soluble fiber reduces the amount of bile reabsorbed and increases the excretion of bile in the feces. To make up for the loss of bile, our liver synthesizes more bile salts. And this uses more cholesterol because bile is made up of cholesterol. This causes the liver to increase the amount of LDL receptors gathering LDL from our blood. Essentially, the more the more that liver produces bile salts then the more that LDL cholesterol is taken up from the blood.

meta-analysis of 67 controlled studies looked at the role of dietary fiber in lowering cholesterol.  The study found that 2 – 10 g/day of soluble fiber was associated with a small decrease in serum total cholesterol and LDL cholesterol. The researchers also observed the same decreases amongst different sources of fiber such as pectin, guar gum, or oat products.

It is important to note that depending on the individual, the decrease in cholesterol caused by soluble fiber can range from 4% to 18% [1]. The variations are based on the person and may be due to different amounts of fiber intake, changes in body weight, and amount of exercise.

This research suggests that increasing dietary fiber is a safe and practical intervention in reducing cholesterol. Because the amount cholesterol decreases varies between individuals, the personalized approach InsideTracker provides can help you evaluate your cholesterol levels if you decide to implement this dietary strategy. Even better, InsideTracker will provide you with the nutrition information regarding what foods to eat such as focus foods and other research-proven options that work

Summary: Soluble fiber directly lowers cholesterol by binding to it and indirectly lowers cholesterol when soluble fiber excretes bile.

Fiber can quit your food cravings by stabilizing your blood sugar

Carbohydrates, especially sugars, can raise blood glucose.  However, carbohydrates such as fiber blunt the blood sugar spike by slowing down the absorption of sugar from the intestine.  A randomized clinical trial used healthy subjects to test the effects of two dietary interventions. One diet was higher in fiber (30g total fiber and 4g soluble fiber) than the other diet (10.4g total fiber and 1.97g soluble fiber). The group with the higher fiber experienced a significant change (-12.3%) in fasting glucose levels from baseline. The group with the 10.4g total fiber had an approximately 2% increase in fasting glucose levels. Because of their findings, the researchers concluded that even a modest increase in soluble fiber intake might improve glucose levels.

a poster describing the benefits of glucose

Another study found that increasing dietary fiber intake, especially soluble fiber, can help improve blood sugar levels in subjects with type II diabetes. The randomized, crossover study design used 13 subjects with type II diabetes and each subject followed each of the two diets for six weeks. The first diet for the subjects followed the American Diabetes Association recommendations, which contained a moderate amount of fiber (24g total: 8g soluble, 16g insoluble). Then, the subjects followed a high fiber diet (50g total: 25g soluble, 25g insoluble) that contained foods that were not fortified with fiber (foods that have fiber added to them). The outcomes of the dietary interventions were compared on the basis of glycemic control (an individual’s baseline level of blood sugar). The high-fiber intervention decreased the average daily postprandial (post-meal) and 24-hour blood glucose levels. Thus, the researchers concluded that the high-fiber diet significantly improved glycemic control in those with type II diabetes.

The researchers also highlighted the feasibility of consuming a high amount of dietary soluble fiber in unfortified foods. Also, the study reported an excellent compliance with the high-fiber diet, which suggests that this dietary strategy is sustainable [2]. And while side effects can be a common complaint when increasing fiber consumption, subjects of the intervention reported very few side effects.

Tracking your blood glucose level is simple, easy, and affordable in the plans InsideTracker offers. You can also monitor your progress as you make these dietary changes. 

Summary: Increasing dietary fiber consumption may help stabilize blood sugar, which controls appetite.

Connecting fiber with fat loss

The mechanism for lowering blood glucose is thought to be the result of the gel-like properties of soluble fiber [3]. Soluble fiber aids in lowering blood glucose levels because it slows the rate at which food leaves our stomach. This increases satiety (feeling of fullness) and helps explain why those who increase dietary fiber consumption to healthy levels might see weight loss too.

Studies with controlled energy intake have mentioned that increased fiber consumption can increase satiety and decrease hunger [4]. In fact, a review reported that the average values for published studies using an ad libitum (eat freely) energy intake approach, suggest that an additional 14g of fiber per day for more than 2 days was associated with a 10% decrease in energy intake and a weight loss of 1.9 kg over 3.8 months of the dietary intervention. With an average dietary intake of only 15 g/day in the United States,[5] this could be a feasible and easy strategy to help people reach and maintain healthier body weight.

Another study found for every 10g increase in soluble fiber intake there was a 3.7% decrease in visceral fat – deep abdominal fat between our organs. The five-year longitudinal study used African Americans and Hispanic Americans, which are high-risk populations. Both at the start of the study and after five years, the subjects underwent a physical exam, a questionnaire on lifestyle issues and a CT scan, which accurately measures subcutaneous (below the skin) and visceral fat. Despite the decrease in visceral fat, there was no association between soluble fiber and subcutaneous fat. The researchers noted soluble fiber and increased exercise may reduce visceral fat and the study demonstrates how soluble fiber affects weight through abdominal fat deposits. The higher satiety and increased duration between meals might likely explain the decrease in fat [6].

Summary: Increasing soluble fiber intake may decrease visceral fat and yield a healthier body weight.

a list of high fiber foods including beans, avocado, chickpea

Will combining high fiber and high protein positively affect SHBG and testosterone?

Sex hormone-binding globulin (SHBG) is a protein that binds to and transports testosterone, as well as other sex hormones. Approximately 65% of serum testosterone is bound to SHBG [7]. When testosterone, an anabolic hormone found in men and women that stimulates cellular growth is bound to SHBG, its function is inhibited. An increase in SHBG decreases the amount of free testosterone in the blood and can lead to a reduction in muscle growth amongst other anabolic effects [8].

What stimulates the release of SHBG? Here’s what we know. First, in the case of significant weight loss or those who have lower body fat, the liver increases the release of SHBG, which was previously suggested as independent of diet composition especially in women [9]. Yet, studies are now suggesting that diet may be a factor in the concentration of circulating SHBG, even though there are conflicting results.

Where does fiber come in? A study found that dietary fiber positively correlated with SHBG levels. The study used data from the Massachusetts Male Aging Study (MMAS), which involved 1552 men between 40 and 70 years old. After controlling for age and testosterone, the study found that fiber intake and SHBG were positively correlated meaning that as the consumption of fiber increased so did the levels of SHBG. This means that there is less free testosterone and therefore lower muscle and bone growth.

This contrasts with another study that found a negative correlation between fiber and SHBG and testosterone. The controlled-feeding study used 43 healthy men between the ages of 19 and 56 years old. The men were randomly assigned to either a low-fat, high-fiber diet or high-fat, low-fiber diet for 10 weeks. Following a two-week break, the men switched to the opposite diet. On the high-fat, low-fiber diet, SHBG bound to testosterone averaged 15% higher. The difference in SHBG bound to testosterone between the two diets was found to be significant. However, the small sample size in comparison to Longscope et al. (2000) remains in question.

According to Longscope et al. (2000) who found the positive correlation, the daily caloric intake was twice as much in the MMAS study and both study designs and analyses were different. To note, the study also found that the lower the protein intake, the higher amount of SHBG. Yet, how protein controls SHBG is not clear [10].

Given these conflicting findings, let’s recap the overall picture. An increase in dietary fiber intake may increase SHBG levels, which then reduces free testosterone levels. However, an increase in protein intake decreases the amount of SHBG. It is important to note that with age, testosterone levels decrease and SHBG increases [11]. This suggests that increasing protein intake could help increase free testosterone levels, regardless of age. However, further studies need to confirm this.

How do you know if adding fiber will affect your sex hormones? InsideTracker’s Ultimate Package can help you track your SHBG and testosterone levels and make sure they are where they should be, especially for maximizing muscle gain.  

Summary: An increase in fiber intake may increase levels of SHBG and decrease levels of free testosterone. However, a diet high in fiber combined with high protein and low fat may decrease SHBG levels and ultimately increase levels of free testosterone.

Adding fiber: Feasible and effective

Unlike many other dietary interventions, increasing your fiber intake is fairly simple and feasible. There are two strategies to effectively add fiber so that you don’t end up with gastrointestinal side effects some people experience. First, it’s important to add fiber gradually to avoid gas and intestinal discomfort.  Second, drinking more water will help move fiber through our GI tract. Dietary Reference Intakes recommend consuming 14g of fiber for every 1,000 calories; this is approximately 25g for adult women and 38g for adult men. But there’s no need to worry whether you should incorporate more soluble or insoluble fiber. The nutritionally balanced plan from InsideTracker will provide foods containing a mixture of both and will give you’re the proper amount based upon your internal biochemistry.

Knowing about and implementing dietary advice are two steps toward improving your health. InsideTracker provides the final step: a personalized tool to make sure it is working for you. In addition to monitoring your health, InsideTracker will provide recommendations for sources of dietary fiber and quantify how much you should eat to optimize your health.    

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[1] Aller, R., de Luis A.D., Izaola, O., La Calle, F., del Olmo, L., Fernandez, L., Arranz, T., Hernandez, J.M.G. Effect of soluble fiber intake in lipid and glucose levels in healthy subjects: A randomized clinical trial. Diabetes Res Clin Pr 2004; 65: 7–11.

[2] Chandalia, M., Garg, A., Lutjohann, D., Von Bergman, K., Grundy, S.M., Brinkley, L.J. Beneficial effects of high dietary fiber intake in patients with type 2 diabetes mellitus. New Engl J Med 2000; 342: 1392-8.

[3] Weickert, M.O., Pfeiffer, A.F.H. Metabolic effects of dietary fiber consumption and prevention of diabetes. J Nutr 2008; 138(3): 439–442.

[4] Position of the American Dietetic Association: Health Implications of Dietary Fiber. J Am Diet Assoc 2008; 108(10): 1716-1731.

[5] Gray, A., Jackson, D.N., McKinlay, J.B. The relation between dominance, anger, and hormones in normally aging men: results from the Massachusetts Male Aging Study. Psychosomat Med 1991; 53: 375–38Position of the American Dietetic Association: Health Implications of Dietary Fiber. J Am Diet Assoc 2008; 108(10): 1716-1Howarth, N.C., Saltzman, E., Roberts, S.B. Dietary fiber and weight regulation. Nutr Rev 2001; 59: 129–39.

[6] Hairston, K.G., Vitolins, M.Z., Norris, J.M., Anderson, A.M., Hanley, A.J., Wagenknecht, L.E. Lifestyle factors and 5-year abdominal fat accumulation in a minority cohort: the IRAS Family Study. Obesity 2012; 20: 421–427.

[7] Allen, N.E., Appleby, P.N., Davey, G.K., Key, T.J. Lifestyle and nutritional determinants of bioavailable androgens and related hormones in British men. Cancer Cause Control. 2002; 13: 353–363.

[8] Vermeulen, A., Kaufman, J.M., Giagulli, V.A. Influence of some biological indexes on sex hormone-binding globulin and androgen levels in aging or obese males. J Clin Endocrinol Metab 1996; 81: 1821–1826.

[9] Morisset, A.S., Blouin, K., and Tchernof, A. Impact of diet and adiposity on circulating levels of sex hormone-binding globulin and androgens. Nutr Rev 2008; 66: 506–516.

[10] Longcope, C., Feldman, H.A., McKinlay, J.B., Araujo, A.B. Diet and sex hormone-binding globulin. J Clin Endocrinol Metab 2000; 85: 293–296.6

[11] Gray, A., Jackson, D.N., McKinlay, J.B. The relation between dominance, anger, and hormones in normally aging men: results from the Massachusetts Male Aging Study. Psychosomat Med 1991; 53: 375–385.