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Are Powdered Greens & Megadoses a Good Idea?

“What do you think about powdered fruit and vegetable supplements?”

There's seemingly a short cut for everything, with fruits and vegetables being no exception. But are these powders a worthy short cut?

In this blog, I’m going to review the following:

  • The concept of bioavailability.

  • Nutrient examples of bioavailability.

  • Whether greens powders are worth the effort, considering bioavailability.

What is Bioavailability?

One area my brain goes to when athletes ask me about greens powders is the concept of bioavailability: How much of those nutrients that we’re holding in our hands as pills or read on a package’s nutrition facts panel can we absorb at the gut level and into the bloodstream (i.e., into circulation where it mostly matters)?

I like to think of bioavailability as a salary and the effect that state income tax has on one's salary. If your salary was $80,000 and income tax was 5%, you would take home $76,000 (assuming no federal taxes, deductions, benefits, etc.). However, if state taxes were 10%, your take-home money would decrease to $72,000. Depending on the state you live in, your "available" or "net" income differs compared to the salary you negotiated. In nutrition, the "bioavailability" will depend on many "taxes", like the presence of a nutrient deficiency, the type and combinations of foods/beverages consumed at a snack or meal, and even the dose of a nutrient consumed in one sitting.

A bar graph showing the different net salaries due to income tax.

Now consider you're in the milk aisle holding a carton of 2% cow's milk. You flip it over to view the nutrition facts panel. Within an 8-ounce (or 1 cup) serving, the panel shows 290 milligrams of calcium (or a daily value of ~20%). This is your untouched salary. However, once you read about calcium in this blog post, you’ll realize that, so long as your blood vitamin D levels are normalized, you’ll absorb 25-35%—or ~75-105 mg of the milk carton's listed 300 mg.

But this could change if the milk were consumed with certain nutrients. Consider blending that milk with strawberries to make a smoothie. Given the presence of oxalic acid (found in strawberries), the amount of calcium that can be absorbed drops, making the ~75-105 mg potentially even lower.

Those are food matrix examples of bioavailability in action.

Nutrition Facts panel for 1 cup (8 ounces) 2% milk.

That said, the Recommended Dietary Allowance (RDA), representing the "average daily dietary intake level that is sufficient to meet the nutrient requirements of nearly all [97-98%] individuals in a life stage and gender group", accounts for bioavailability in their recommendations, so don't overthink it.* For calcium, the RDA of 1,000 mg is what you can continue to use when setting a daily goal or when a registered dietitian evaluates one's intake.

What bioavailability tells us is that (1) the nutrition facts panel isn't a true reflection of what's truly absorbed (including calories) and (2) we don't eat nutrients, but combinations of food that further change the nutrient-of-interest's bioavailability.

*The Estimated Average Requirement (EAR) also accounts for this, to which the RDA is based off of.

Nutrient Example: Calcium

Daily Value (%DV): This is the percentage on the right-hand side of the Nutrition Facts panel.

1,300 mg. If the label states a serving contains 25% calcium, multiply by 13 to reach the milligram value (i.e., 25 x 13). Notice how this differs from the RDA below.

Daily requirement for adults 19-50 years of age (RDA):

​Tolerable Upper Intake Level for adults 19-50 years of age (UL):

2,500 mg, which is a combination of food and supplements.

How Well is Calcium Absorbed?

Many factors affect calcium’s absorption, including what you consume with the calcium in a meal and what your vitamin D blood levels are.

Vitamin D, sugars, sugar alcohols, and protein all improve calcium absorption. A glass of calcium-rich cow’s milk contains vitamin D, milk sugar (lactose), and protein, making it an optimal combination for calcium absorption. Cow's milk optimizes calcium's bioavailability.

When blood vitamin D levels (measured as 25-OH D) are normalized between ~30-40 ng/mL, calcium absorption at the intestinal level is maximized at a value of ~30%. Yet in the presence of a vitamin D deficiency, calcium absorption can drop to 10-15%. (paragraph source)

There are plenty of factors that reduce calcium absorption, including fiber, unabsorbed fatty acids remaining in the gut (i.e., the person could have steatorrhea), the excessive presence of zinc and magnesium, oxalic acid, and, to a small degree, phytic acid,

Oxalic acid is found in strawberries, spinach, rhubarb, Swiss chard, beets, celery, eggplant, greens, okra, squash, currants, blackberries, blueberries, gooseberries, pecans, peanuts, tea, Ovaltine, and cocoa. It's not as if you have to avoid oxalic acid, as the prior list of options are quite healthful. However, if intently trying to increase your calcium intake, spreading out high-calcium from high-oxalic options would be helpful.

Phytic acid and fiber inhibit calcium absorption to a smaller degree than oxalic acid. Most Americans don’t consume enough phytic acid for the reductions in calcium absorption to really matter. Plus, calcium bound to fiber tends to be released by bacteria in the colon (large intestine) and absorbed at that site anyways (absorption here is ~4-10%).

Overall, in the average adult, 25-30% of calcium consumed is absorbed into circulation, so long as the single dose is less than 500 mg. Plant sources of calcium disrupt that guideline, given some have great calcium bioavailability while others don't. For instance, 50% of the calcium in bok choy is absorbed whereas only 5-13% of the calcium in spinach is absorbed due to the presence of oxalic acid.

If Choosing a Supplement, What Kind Should I Choose?

Two ideal supplements are calcium carbonate and citrate, which are recommended for different reasons.

Calcium carbonate provides the most bang for your buck. Calcium carbonate is 40% calcium by weight, meaning if a supplement contains 100 mg of calcium carbonate, 40 mg would be elemental calcium (elemental being what we're interested in). Stomach acid promotes its absorption, which is why this form of calcium should be taken with food. The presence of food also reduces gut discomfort with supplementation, like abdominal pain, bloating, gas, and constipation.

Calcium citrate is helpful for those with stomachs less acidic than they should be. This often occurs in the Elderly or those taking antacids (for reflux) or proton-pump inhibitors that work to create a less acidic environment in the stomach. Unlike calcium carbonate, the citrate form does not need to be taken with food, but only provides 21% of elemental calcium.

Other forms of calcium supplements include the following, with the percentage in parentheses noting the compound’s elemental calcium value: Calcium acetate (23%), calcium lactate (14%), and calcium gluconate (9%).

Sport Example: Vitamins C and E

A 2014 study evaluated 54 young men consuming 1,000 milligrams vitamin C and 235 mg vitamin E supplements daily for 11 weeks, all of the while participating in an endurance training program. What researchers discovered was a hindered cellular adaptation to training.

For reference, that dose of vitamin C is ~11 times the daily needs for an adult male and vitamin E is ~16 times.

Greens powders examples:

How Does Bioavailability Play Into Greens Powders?

Given the above conversation on bioavailability, below is an excerpt of the Athletic Greens nutrition facts panel (note: this is not a nasty post about Athletic Greens. It simply seems to be the most popular one people ask me about).

Of the 21 vitamins and minerals listed with dose-specific quantities, 10 provide more than 100% of an adult’s daily needs.

  • Vitamin C: 420 mg (467% DV)

  • Zinc: 15mg (136% DV)

  • Vitamin E: 83 mg (553% DV)

  • Thiamin (Vitamin B1): 3mg (250% DV)

  • Riboflavin (Vitamin B2): 2mg (154% DV)

  • Niacin: 20 mg NE (125% DV)

  • Vitamin B6: 3mg (176% DV)

  • Folate: 680 mc DFE (170%)

  • Vitamin B12: 22mcg (917% DV)

  • Biotin (Vitamin B7): 330 mcg (1100% DV)

Are you absorbing all of these nutrients in full? No.

Are you promoting competition for absorption amongst nutrients? Probably. Per above, calcium absorption decreases in the presence of high doses of zinc and magnesium.

Could you be creating side effects? Yes. Unabsorbed vitamin C can pull additional water into the gut, leading to abdominal pain, cramps, nausea, and diarrhea.

Are you better off eating food? Yes.

What’s missing from these supplements is the fiber. Not only does fiber act as a source of prebiotics for gut bacteria, fiber helps us slow down our eating (both to chew through and for the GI tract to manage), send signals to the brain that we’re no longer hungry due to taking up space in the gut, and promotes regular bowel habits. Fiber also has a role in overall gut health, weight loss and management, heart health and blood cholesterol levels, and a reduction in cancer risk.

With these concentrated sources of nutrients, often labeled as "superfood" supplements, it's uncommon for there to be research on these combinations of nutrients and in their high doses. Research on the benefits of individual nutrients exists, but what about the mushroom powder, broccoli flower powder, rose hip, and licorice root all together (potentially) affecting each's bioavailability? Sometimes these products remind me more of a let's-throw-everything-trendy-into-a-supplement-and-see-what-happens-because-we-know-these-buzzwords-will-sell. You know?

Take-Home Message: Chew Your Fruits & Vegetables

Greens powders remind me a bit of that cookie breakfast cereal: we're told to eat breakfast, so we're going to walk the line and eat a bowl of cookies and pass it off for a healthful bowl of cereal. It's not the same thing. The entire package of fruits and vegetables offer more overall health benefits compared with a single scoop of powder. Plus, if a scoop of greens powder is part of a diet that skips breakfast, relies on energy drinks to manage fatigue, and binges on fried chicken due to extreme hunger at night, greens powders will not save a terrible diet. And an athlete's performance will surely suffer if the overall diet is poorly managed.

What about those who don't like fruits and vegetables? I've noticed that's uncommon. You as the dietitian need to dig. There's likely a handful they are willing to eat, you can counsel them on roasted or canned options to switch up the flavor and texture, toss them with sauces and stir into meals they already enjoy (e.g., pasta, egg scrambles, stir fry), and ultimately, teach the person how to prepare produce in tasty ways.

Further Reading

Is it "being vegan" or something else that can help vegan athletes improve their performance?

It's a hormone, not a vitamin.

Paulsen et al. (2014). J Physiol,592(8):1887-1901.

This is my go-to textbook for learning everything about a single nutrient.

This is one of my favourite nutrition websites. Every vitamin and mineral has its own page (and different versions for both the health professional and consumer) that outlines what it does, recommended daily intakes, common food sources, nutrient-drug interactions, symptoms of deficiencies, and more. Often I Google "ODS [insert nutrient]".


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