Probiotics are touted for strengthening immunity and maintaining a healthy gut, but one of the less emphasized health benefits is their positive impact on nutrition. Athletes are prone to flirting with certain nutrient deficiencies because of exercise demands and – sometimes – less than optimal nutrient intake. Research suggests taking probiotics is an attractive dietary intervention to optimize nutritional intake.
Gut bacteria receive their nutrition for energy and growth from our intake of carbohydrate, protein and fat. While breaking down food, bacteria release different by-products that subsequently impact our health and metabolism – most importantly, short-chain fatty acids (SCFAs). Here we provide the evidence-based research on probiotics and how they impact our digestion and absorption of nutrients and how SCFAs enhance our health.
- Synthesize some B vitamins and vitamin K
- Increase absorption of calcium, iron and vitamin D
- Alleviate symptoms lactose intolerance
- Enhance dietary nitrate conversion to the vasodilator nitric oxide (e.g., beetroot juice)
- Increase antioxidant activity
- Lower cholesterol
Gut bacteria receive their nutrition for energy and growth from our intake of carbohydrate, protein and fat. While breaking down food, bacteria release different by-products that subsequently impact our health and metabolism – most importantly, short-chain fatty acids (SCFAs).
We must obtain most vitamins through our diet because we can’t make them, but deficiencies still occur because of inadequate food intake and unhealthful or unbalanced eating habits. Some vitamin-producing probiotic strains serve as a natural alternative to increase vitamin stores of some B vitamins and vitamin K.
B vitamins are found in many foods, but they can easily be destroyed during cooking and food processing. Vitamin K is found in two forms: phylloquinone (K1) – the main dietary form found in plants – and menaquinone (K2) – synthesized by beneficial gut bacteria and found in meat, dairy and fermented foods.
Riboflavin (Vitamin B2)
Riboflavin is essential in cellular metabolism, and it can be found in dairy products, meat and eggs. However, riboflavin deficiency still occurs, and exploiting certain bacterial strains that are riboflavin-producers is suggested for the fermented food industry.
A study explored the riboflavin-producing strains in different fermented milk products and found that Lactobacillus fermentum was efficient at producing riboflavin. Another study found that L. lactis produced riboflavin and was able to reduce the physiological consequences of riboflavin deficiency; however, this study was conducted in rats.
Folate (Vitamin B9)
Folate is involved in DNA replication and repair as well as synthesis of nucleotides, vitamins and some amino acids. Folate deficiency can lead to megaloblastic anemia (the number of red blood cells is lower than normal), neural tube defects (especially in pregnant women) and increased risk for heart disease because of elevated homocysteine levels. Certain strains of Bifidobacteria have demonstrated folate biosynthetic properties. The high level folate-producing strains include B. bifidum and B. longum.
Strozzi and Mogna conducted a pilot study using 23 healthy volunteers to evaluate the ability of three different Bifidobacteria strains to produce folate. The subjects were randomly assigned to one of three treatment groups with the specific probiotic strain at a dose of 5 x 10*9 colony forming units (CFU) per day. Fecal samples were collected from subjects for two consecutive days before treatment with probiotics and two consecutive days after 30-days of treatment. Subjects were instructed not to consume fermented dairy products containing Bifidobacteria – to prevent ingesting other sources containing Bifidobacteria. The study found that the three Bifidobacteria strains colonized the GI tract and produced significant amounts of folate.
Even though our intake of green plants provides vitamin K, a deficiency can result from an out of balance gut microbiome. This may result from pathogenic bacteria that overtake the good bacteria or after antibiotic usage. We can’t store vitamin K so we need to have the right amount of good bacteria to make vitamin K.
The amount of vitamin K produced by gut bacteria ranges from 10-50%. However, not all gut bacteria produce vitamin K. L. lactis has shown to produce high amounts of vitamin K, an amount that could even serve as a dietary supplement. Yet, the research on absorption and metabolism of bacterially produced vitamin K has not increased in the last two decades. We do know that maintaining a balanced gut microbiota will help ensure we’re getting vitamin K.
Increase Calcium Absorption
Calcium is one micronutrient that tends to be low among athletes. Athletes are at risk for developing calcium deficiency for many reasons. Calcium from food isn’t always absorbed, and the amount and bioavailability varies among foods. Some food components, such as oxalic acid (found in spinach) and phytates (i.e., an indigestible form of phosphorous – found in grains) can inhibit calcium absorption.
Also, exercise may increase calcium losses. A study found in nine male competitive cyclists that 10 weeks of intense endurance training increased urinary calcium excretion and lowered serum calcium levels, but these consequences were reversed after the tapering phase. Therefore, high intensity exercise may increase calcium excretion.
A study investigated calcium absorption and bioavailability from calcium-fortified soymilk containing seven strains of Lactobacillus, including L. acidophilus, L. plantarum L. casei and L. fermentum. The highest increase in calcium solubility after 24 hours came from L. acidophilus (89.3%) and L. casei (87.0%). The study concluded that some Lactobacillus species may improve calcium bioavailability.
Probiotics (such as the ones mentioned above) that have the enzyme phytase – which humans lack – may increase the bioavailability of calcium because they can reduce the amount of phytates that bind to calcium.
Increase Iron Absorption
Iron supports more than 180 biochemical reactions in the body, including the transport of oxygen. Ischemia – an inefficient supply of blood to an organ or part of the body – during excessive training results in an increased demand for iron. Iron uptake increases through intestinal absorption, but only if there’s adequate dietary iron intake. Intense training can result in an increase in hepcidin (i.e., a protein that is the main regulator for iron absorption), which can block iron absorption and result in iron deficiency.
Iron deficiency is prevalent among athletes and may affect physical performance. This is especially seen in women of reproductive age, because they have high iron requirements, and among those on plant-based diets because of their intake of non-heme iron (which has a lower bioavailability compared to heme iron found in animal-based foods).
Ferritin is the main biomarker to evaluate iron deficiency. For healthy female and male athletes >15 years old, the ferritin values are:
- Empty: ferritin values <15 mcg/l
- Low iron stores: 15-30 mcg/l
- Suggested cut-off: 30 mcg/l
Therapeutic approaches to increase iron levels include a higher intake of foods high in iron or iron supplementation. However, probiotics is a dietary factor that serves as a potential therapy to enhance iron absorption.
L. plantarum has demonstrated an ability to increase iron absorption by over 100% from oats that have low iron bioavailability (because of the high levels of the phytates).
A recent single blind cross-over study using 22 young, healthy women found L. plantarum increased iron absorption from an iron-fortified drink by 50%. At breakfast for four consecutive days, the women drank either a 200-ml fruit juice with 5 mg of iron alone or an iron-fortified fruit juice with L. plantarum. The probiotic-containing drink had 10*9 CFU in Trial 1 and 10*10 CFU in Trial 2.
- Trial 1: Mean iron absorption from the drink with 109 CFU of L. plantarum was 28.6% compared to 18.5% for the control drink
- Trial 2: Mean iron absorption from the drink with 1010 CFU of L. plantarum was 29.1% compared to 20.1% for the control drink
The average iron absorption was 28.8% with L. plantarum when the absorption values for all subjects were combined, which was significantly higher than the iron absorption of 19.3% with the control drink. The study found iron absorption was 50% higher with non-heme iron from a fruit drink with L. plantarum than a similar fruit drink without the probiotic.
Details of the mechanisms by which probiotics can increase iron absorption are lacking. It is suggested that some Lactobacillus strains can increase the bioavailability of iron because they have the phytase enzyme that can break down phytates during fermentation. Yet, L. plantarum has minimal phytase activity and the amount of phytates was low in the fruit drinks in the study. Enhanced iron absorption may be the result of the strain’s ability to colonize the gut and increase mucin (the main component of the mucus layer that lines the gut) excretion. It is suggested mucins can bind to iron and prevent iron removal. Also, mucin may increase iron uptake because of its effect on iron transport proteins that bring iron into cells. Finally, another possible mechanism may result from the decrease in gut pH because of Lactobacilli growth. This creates a more acidic environment to change the iron (i.e., ferric iron) to a more absorbable form (i.e., ferrous iron).
Enhance Vitamin D Absorption
Vitamin D is critical for bone health, maintaining phosphate and calcium homeostasis – and maybe for physical performance. Yet, vitamin D levels are very low, with the greatest effects based on geographic location (e.g., northern latitude), season of the year and skin color. Even fortified vitamin D milk is not sufficient enough to prevent vitamin D deficiency for all adults at every time of the year. It is suggested that athletes, compared to the general population, may be more susceptible to vitamin D deficiency, and possibly because of inflammation, muscle damage, increased immune activity and increased protein synthesis.
Before we delve into the research highlighting vitamin D-boosting probiotics, the importance of vitamin D for athletes requires emphasizing complications of vitamin D deficiency and the prevalence of vitamin D deficiency among athletes.
These precursors are transformed in the liver and kidneys to:
- 25-hydroxyvitamin D (25(OH)D): the inactive storage form
- 1,25-dihydroxyvitamin D (1,25(OH)2D): the biologically active form under tight regulation in the body
1,25-dihydroxyvitamin D interacts with vitamin D receptors found in every tissue of the body. This includes skeletal muscle, which suggests the important effect vitamin D may have on skeletal muscle.
The best indicator of vitamin D status is reduced blood levels of 25-hydroxyvitamin D [25(OH)D]. The three categories of vitamin D status based on serum 25(OH)D are:
- Risk of deficiency: <30 nmol/L
- Risk of inadequacy: 30-49 nmol/L
- Sufficiency: 50-125 nmol/L
Low levels of vitamin D are prevalent in the general population. A low level is also a risk factor for: osteoporosis, cardiovascular disease, type 2 diabetes, and cancer. Vitamin D deficiency is also associated with depression, cognitive decline and neurological complications. More specific to athletic performance, vitamin D deficiency also may break down muscle and result in muscle weakness.
A few studies have suggested that a lower vitamin D intake may be associated with microbiome changes.,
A 2015 systematic-review and meta-analysis of 23 studies and ~2,300 athletes found that 56% of athletes had inadequate vitamin D levels.
A recent cohort study assessed vitamin D levels in 80 professional NFL players (the Pittsburg Steelers) and its association with race, history of broken bones and staying on the team during the 2011 offseason.
- Mean vitamin D level was 27.4±11.7 ng/mL
- Significantly lower levels for black players, which was 84% of the team, (25.6±11.3 ng/mL) compared to white players (37.4±8.6 ng/mL)
- All athletes with vitamin D deficiency (<20 ng/mL) were black
- 91% of athletes with vitamin D insufficiency (20-32 ng/mL) were black
- 68.8% of the team had vitamin D levels lower than 32 ng/mL
The associated results:
- Vitamin D levels were much lower in players who had at least one bone fracture compared to players who had no fractures
- Players who were cut during the preseason as a result of injury or poor performance had significantly lower vitamin D levels compared to players who stayed for the regular season
Ultimately, athletes with vitamin D levels above 32 ng/mL played in more seasons than athletes with vitamin D deficiency. Vitamin D deficiency and insufficiency were prevalent among football players, especially black players. Those with a lower the vitamin D level had a higher risk for getting cut.
These results suggest the need to optimize vitamin D not just for NFL players, especially black players, but potentially all athletes. To address vitamin D deficiency, probiotics is an attractive nutritional intervention.
Jones, Martoni and Prakash published the first human data on probiotics increasing vitamin D absorption in humans. The randomized controlled trial found the probiotic strain L. reuteri may increase serum 25-hydroxyvitamin D by 25.5%.
The participants’ two-day dietary intake was measured at weeks 0 and 9 to evaluate calories, lipids, proteins, carbohydrates, vitamin A, retinol, carotenoids, vitamin E and vitamin D.
The study primarily measured the change in serum low-density lipoprotein-cholesterol (LDL-C) over the nine weeks. Fat-soluble vitamin analysis was conducted for weeks 0 and 9 following the intervention.
- L. reuteri increased serum 25-hydroxyvitamin D by 14.9 nmol/L (25.5%) – a significant difference compared to placebo, which was 17.1 nmol/L (22.4%)
- No differences in the absorption of other fat-soluble vitamins
Currently, this is the first study to show a probiotic supplement increased vitamin D levels. However, the mechanism is unclear. It may be the result of 1) an increase in vitamin D absorption or 2) greater synthesis of the vitamin D precursor. Yet, influencing the microbiome with the right probiotics signals a feasible approach to increase vitamin D.
Alleviate Symptoms of Lactose Intolerance
Dealing with lactose (i.e., the natural sugar in milk) intolerance as an athlete is frustrating, especially for those who want to use the fluid, protein and carbohydrate in milk or products with whey protein for recovery. Lactose intolerance results from a low amount of the lactose cleaving-enzyme β-galactosidase (i.e., lactase) in the mucosal layer of the small intestine. Secondary causes of lactose malabsorption result from other health complications (which can be reversible) including:
- Inflammation of the small intestine
- Protein-energy malnutrition (i.e., lack of protein)
Probiotics that have high β-galactosidase activity may help those who are lactose intolerant. This is why those who are lactose intolerant may be able to tolerate yogurt because the lactose is already partially broken down.
A randomized controlled trial investigated the effect of L. reuteri supplementation, compared to placebo, on symptoms of lactose intolerance in lactose intolerant people randomly assigned to one of three 20-subject treatment groups: tilactase (e.g., Lactaid) group, placebo group and L. reuteri group. Lactose maldigestion was assessed with the hydrogen breath test and GI distress symptoms. The study found that L. reuteri, compared to placebo, lowered the amount of excreted hydrogen and reduced GI symptoms following lactose intake.
Microbial β-galactosidase (also found in L. bulgaricus and Streptococcus thermophilus) may be an effective treatment to alleviate lactose intolerance symptom for lactose malabsorbers. In fact, the bacteria don’t need to be alive – just the cell walls need to be intact to protect the enzyme when it passes through the stomach. Lactose digestion may improve because the passage of lactose through the gut is delayed, which allows β-galactosidase more time to breakdown lactose.
Enhance the Conversion of Nitrate from Beetroot Juice to Nitric Oxide
Beetroot juice is the latest in functional foods for sport performance. The dietary nitrates in beetroot juice convert to nitric oxide (NO), a molecule that widens blood vessels and allows nutrients and oxygen to enter skeletal muscle. NO increases efficiency of oxygen use in muscles during strenuous exercise. The entero-salivary nitrate-nitrite-NO pathway (i.e., GI system) starts with conversion of dietary nitrate to nitrite in the mouth, then the stomach and finally the small intestine. Yet, the link in the conversion of nitrates to NO requires good bacteria throughout the entire GI tract.
Bacteria on the tongue that convert nitrate to NO (by using the enzyme nitrate reductase) can be destroyed by antibacterial mouthwash. Govoni et al. used seven healthy volunteers to investigate what impacts the ability of oral bacteria to metabolize inorganic nitrate to form nitrite and then bioactive NO. Notably, the bacteria in our mouth that convert nitrate to NO were destroyed by antibacterial mouthwash, which led to a lack of NO and no benefits from beetroot juice nitrates.
Beetroot juice (or other food containing dietary nitrate) is not the only way to generate NO. Our body can produce NO from the amino acid L-arginine and oxygen by using the enzyme nitric oxide synthase. This endogenous way of producing NO greatly impacts the GI tract.
To address this, a study investigated if specific gut bacteria, in humans and in rats, could produce NO. Fecal samples from eight healthy volunteers were used to measure the NO production by different bacteria. Lactobacilli and Bifidobacteria in human feces generated NO from nitrite, and a few of the bacteria strains generated NO from nitrate. Rats receiving Lactobacilli supplementation with nitrate present produced more NO in the gut than rats with no gut bacteria.
It’s suggested that some bacteria can capture NO by binding to specific proteins or by using nitrate reductase. Even though the study used rats and more human studies testing different bacterial strains are needed, the results suggest gut bacteria – not just oral bacteria – may have the capacity to generate NO.
Increase Antioxidant Activity
We use oxygen to harness energy from food, which produces reactive oxygen species (ROS) (i.e., free radicals) and can lead to oxidative stress (an imbalance between oxidant and antioxidant levels). ROS can damage lipids, proteins and nucleic acids in cells. To neutralize ROS, both antioxidants from food and certain enzymes in our body make up the biological antioxidant barrier.
Intense exercise generates a high amount of ROS, especially during exhaustive and long-lasting exercise. Subsequently, the intense exercise and increased oxygen consumption (which also leads to oxidative stress) results in athletes with greater amounts of ROS circulating in their body.
Probiotics may be antioxidant suppliers that can facilitate better recovery from oxidative stress. Some studies suggest that certain probiotic strains may provide antioxidant activity and reduce oxidative stress.
The first study to investigate the effect of probiotics on exercise-induced stress used L. rhamnosus and L. paracasei. Male athletes were assigned to either the probiotic group (12 men) that consumed a mix of the two strains (1:1, ~10*9 cells/day) or the control group (12 men) that didn’t receive supplementation. All athletes received a personalized diet and underwent the same intense exercise training for four weeks. Blood levels of reactive oxygen metabolites (ROMs), which measure oxidative stress, and biological antioxidant potential (BAP), which measures blood levels of antioxidants, were determined pre- and post-supplementation.
- Control group had much higher ROMs post-exercise compared to pre-exercise
- Probiotics group did not have a significant difference in ROMs levels pre- and post-exercise, which suggests that probiotics neutralized the exercise-induced ROMs
- Probiotics group had higher BAP levels post-exercise compared to pre-exercise
- Probiotics group had higher BAP levels post-exercise compared to control group
The data showed that intense exercise led to oxidative stress and probiotic supplementation increased antioxidant levels, which neutralized the ROS and oxidative stress. Notably, all athletes consumed antioxidants in their diet. Yet, because the gut microbiome regulates nutrient absorption, the higher BAP levels could be the result of greater antioxidant absorption by the probiotics.
Oxidative stress resistance measures the ability of bacteria to survive oxidative conditions because of their ability to combat ROS. The study found that L. rhamnosus and L. paracasei had high-oxidative stress resistance; thus, demonstrating their antioxidative capacity. The high level of antioxidant enzymes in the bacterial strains can target ROS in the GI tract.
Athletes with high cholesterol (hypercholesteromia) or athletes trying to optimize cholesterol levels have a reason to use probiotics, specifically L. reuteri. The high number of hypercholesteromic people who are unable to optimize their LDL-C still remains a concern and exploration of other cholesterol-lowing therapies are needed.
A randomized controlled trial found the cholesterol-lowering effects of L. reuteri. The 127 hypercholesteromic participants were randomly assigned to take the probiotic L. reuteri or placebo twice per day (at breakfast and dinner) for nine weeks. The L. reuteri capsules contained 2.9 x 10*9 CFU at baseline and 2.0 x 10*9 CFU at the end of the study. Blood samples were taken at six different visits to analyze serum levels of LDL-C, total cholesterol (TC), high-density lipoprotein-cholesterol (HDL-C) and non-HDL-cholesterol (non-HDL-C).
- Compared to placebo, L. reuteri capsules lowered:
- LDL-C by 11.64%
- TC by 9.14%
- Non-HDL-C by 11.30%
- Ratio of LDL-C/HDL-C by 13.39%
Finally, a recent meta-analysis by Guo et al. used data from 13 lipid-lowering probiotic trials (including L. acidophilus, L. plantarum, B. longum and B. lactis) and found that people with high, borderline high and normal cholesterol levels who received probiotics, compared to controls, had beneficial effects on total cholesterol and LDL cholesterol. The average net change was:
- TC: -6.40 mg/dL
- LDL-C: -4.90 mg/dL
How do Lactobacilli reduce cholesterol? Bile – which is made from cholesterol – helps digest fat. These gut microbiota can breakdown bile, which disrupts bile reabsorption and increases bile excretion. The lower amount of bile available for the body means more cholesterol is needed in the liver to make it.
Niemann-Pick C1-like 1 (NPC1L1) is considered the major protein for intestinal cholesterol absorption. NPC1L1 is highly expressed at the surface of the small intestine. Even though it hasn’t been confirmed in humans, NPC1L1 deficiency in mice has shown a considerable reduction in dietary cholesterol absorption.
Huang et al. found that the L. plantarum blocked cholesterol absorption by decreasing NPC1L1 after rats with high cholesterol were fed L. plantarum for four weeks at a dose of 10*9 CFU/day. Even though the study was conducted in rats, the results suggest the potential of L. plantarum for lowering cholesterol and the possible mechanism of how probiotics reduce cholesterol. The ability to decrease NPC1L1 expression was also found in L. rhamnosus. These results suggest that some strains of Lactobacillus may control cholesterol levels through NPC1L1.
The Effect on Energy Regulation: Probiotics Produce Short-Chain Fatty Acids
Some nutrient digestion occurs in the stomach, but most nutrient digestion and absorption occurs in the small intestine. Absorption of all fats, ~85% of carbohydrates and 66-95% of proteins occurs before the large intestine.
Non-digestible carbohydrates and protein in the colon account for 10-30% of total caloric intake. These indigestible nutrients wouldn’t be further absorbed and instead would be excreted if it weren’t for gut microbiota. Gut microbiota can produce many metabolites that regulate our health – specifically short-chain fatty acids (SCFAs), which have a positive impact.
Short-Chain Fatty Acids (SCFAs)
Anaerobic intestinal microbiota can produce SCFAs by fermenting non-digestible carbohydrate (e.g., soluble dietary fiber) and proteins (e.g., branched-chain amino acids). Many gut microbes can ferment unabsorbed carbohydrate because they have the enzymes required. SCFAs can:
- Shape the gut environment
- Impact the physiology of the colon
- Serve as energy sources for human cells (e.g., intestinal) and gut microbiota
- Interact with host-signaling mechanisms
SCFAs increase the acidity of the gut, which increases the absorption of certain nutrients (e.g., magnesium and calcium) and stops pathogenic microorganisms from invading.
The type and amount of SCFAs produced depends on our age, diet (e.g., availability of non-digestible carbohydrates), composition of gut microbiota, gut transit time and pH of the colon. The bacteria that produce SCFAs include:
- And others
In the gut, 90-95% of the SCFAs are: acetate, propionate and butyrate.
Acetate gives Bifidobacteria the ability to stop the microorganisms that cause disease in the gut. SCFAs provide energy for colon cells and nearby tissues. Acetate is the main SCFA in the blood and serves as an important energy source for peripheral tissues. Butyrate provides energy for intestinal cells and increases mucin production. The increase in mucin could possibly lower the amount of bad bacteria that attach to our gut and strengthen the gut barrier. These adaptations suggest SCFAs are critical in maintaining the function of our gut barrier.
Following absorption of SCFAs, we use SCFAs in different biosynthetic routes. Our gut cells use butyrate, and the remaining SCFAs go to the liver for further metabolism. In the liver, SCFAs integrate into different carbohydrate and lipid metabolic pathways:
- Propionate is either used in gluconeogenesis (i.e., generation of glucose from non-carbohydrate sources) or regulates cholesterol synthesis
- Acetate and butyrate is used in lipid and cholesterol synthesis
SCFAs have been associated with improving glucose tolerance by increasing the secretion of incretin hormone glucagon-like peptide (GLP)-1 and may increase the expression of vitamin D receptors on cells.
Probiotics: Linking Optimal Digestion and Absorption of Nutrients
Dysbiosis from a dysregulated or dysfunctional microbiome may lead to consequences involving inefficient nutrient use. Gut bacteria can improve our nutrient absorption, especially of vitamins and minerals. By-products from probiotic metabolism (e.g., SCFAs) demonstrate the indirect, positive health effect that probiotics can also lend.
Certain probiotics positively impact our nutrition because they have some metabolic tools we don’t have: the genes to make vitamins, the ability to shape the gut environment to increase absorption of certain nutrients and the enzymes to breakdown non-digestible macronutrients to metabolites that positively influence our health. Ultimately, probiotics are important players that bring together nutrition, gut health and human physiology.
by Katie Mark, MS
Katie Mark is currently a Master of Public Health candidate at Tufts University School of Medicine. She is a road cyclist working toward becoming a registered dietitian.