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Quiz: Probiotics, Prebiotics, and "Psychobiotics"

Test your understanding of Chapter 16. Choose the best answer, then click Show Answer to check.

Question 1

According to the ISAPP scientific definition, which of these is required for a product to be called a probiotic?

  1. It must contain at least 50 billion colony-forming units.
  2. It must contain live microorganisms, in adequate amounts, with a shown health benefit.
  3. It must be approved by the FDA as a drug.
  4. It must contain fiber that feeds gut bacteria.
Show Answer

The correct answer is B. The ISAPP (2014) definition is "live microorganisms that, when given in adequate amounts, confer a health benefit on the host." Option D describes a prebiotic, which is food for microbes rather than live microbes. No specific CFU number or FDA approval is part of the definition.

Concept Tested: Probiotic Bloom's Level: Remember

Question 2

A product contains heat-killed bacterial cells and their parts, and a health benefit has been shown. Under the ISAPP definitions, what is it?

  1. A probiotic
  2. A prebiotic
  3. A synbiotic
  4. A postbiotic
Show Answer

The correct answer is D. A postbiotic is "a preparation of inanimate microorganisms and/or their components that confers a health benefit on the host." It can't be a probiotic, because probiotics must be alive. A synbiotic combines live microbes with a substance microbes use.

Concept Tested: Postbiotic Bloom's Level: Remember

Question 3

Under the 1994 Dietary Supplement Health and Education Act (DSHEA), which statement is true about probiotics sold as dietary supplements in the US?

  1. The FDA does not approve them for safety or effectiveness before they are sold.
  2. The FDA tests every bottle before it reaches stores.
  3. They must show benefit in two clinical trials before sale.
  4. They are allowed to claim they cure diseases if the company believes it.
Show Answer

The correct answer is A. Under DSHEA, the FDA does not have the authority to approve dietary supplements before they are marketed; companies are responsible for safety and labeling, and the FDA can act after a product is on the market. Option D is wrong: supplements may make structure-function claims but cannot claim to treat, cure, or prevent a disease.

Concept Tested: Dietary Supplement Regulation Bloom's Level: Remember

Question 4

Why can't the results of a trial on one Lactobacillus strain be assumed to apply to a different Lactobacillus strain?

  1. Because only one Lactobacillus strain can survive in the human gut
  2. Because the FDA approves each strain separately before sale
  3. Because probiotic effects are strain specific and also depend on the condition being treated
  4. Because Lactobacillus strains all have the same effect, so testing a second one is unnecessary
Show Answer

The correct answer is C. A 2018 review of 228 trials found that probiotic efficacy depends on both the strain and the disease, and NCCIH says one strain's benefit doesn't prove another works. Option B is wrong because most probiotics are supplements that the FDA doesn't approve before sale.

Concept Tested: Strain Specificity Bloom's Level: Understand

Question 5

A probiotic label lists "20 billion CFU at time of manufacture." Why might this overstate what you actually get?

  1. CFU counts dead bacteria as well as live ones.
  2. Many strains lose live cells during storage, so fewer may be alive by the time you take it.
  3. The FDA requires labels to round CFU numbers up.
  4. CFU measures the weight of the capsule, not the number of microbes.
Show Answer

The correct answer is B. ISAPP's label guide explains that most strains lose live cells over their shelf life, so a count at manufacture can be much higher than at the end of shelf life. Option A is backwards: CFU counts only microbes alive enough to form colonies.

Concept Tested: Colony-Forming Unit Bloom's Level: Understand

Question 6

In a 2011 study, the strain L. rhamnosus JB-1 reduced anxiety-like behavior in mice, but in a 2017 trial of 29 healthy men it did nothing measurable. What does this pair of studies best illustrate?

  1. A replication failure across species, showing a mouse result may not carry over to people
  2. Proof that the mouse study was faked
  3. Proof that probiotics can never affect the brain
  4. Publication bias, because the human study was never published
Show Answer

The correct answer is A. When a later study tests the same idea and doesn't find the original result, that's a replication failure. It doesn't prove fraud or that probiotics can never matter; the human study was small and used healthy men, not people with anxiety. Option D is wrong because the human study was published.

Concept Tested: Replication Failure Bloom's Level: Understand

Question 7

A meta-analysis of probiotic trials found improvement on one depression questionnaire but not on three others. A news story says, "Probiotics proven to reduce depression!" What is the best response?

  1. Accept it, because any significant result counts as proof.
  2. Reject probiotics completely, because three scales showed no difference.
  3. Ignore the questionnaires and look only at the number of participants.
  4. Be cautious: with several outcomes, one positive result could be chance, so the evidence is mixed.
Show Answer

The correct answer is D. When many outcomes are measured, the chance that at least one looks significant by luck rises, which is why trials name a primary outcome in advance. The Rahmannia 2024 review's authors urged cautious interpretation. Option B overcorrects: mixed results mean uncertainty, not proof of no effect.

Concept Tested: Primary Outcome Bloom's Level: Apply

Question 8

Sixteen-year-old Malia is getting chemotherapy, and a relative wants her to start a store-bought probiotic. Based on the chapter, what is the most accurate advice?

  1. It is safe, because probiotics are natural.
  2. Her oncology team should review the exact product first, because people with weakened immune systems face higher risk from live microbes.
  3. She should take a double dose to make sure enough bacteria survive.
  4. Probiotics are only risky for adults over 65.
Show Answer

The correct answer is B. NCCIH says the risk of harm from probiotics is greater in people with severe illness or weakened immune systems, which can include people on chemotherapy. Option A is the "natural means safe" myth, and the book never recommends doses.

Concept Tested: Probiotic Safety Bloom's Level: Apply

Question 9

In a 2008 study, published journal articles made about 94% of antidepressant trials look positive, but FDA records showed about 51% were positive. What explains the gap?

  1. Publication bias: negative trials were often unpublished or reported as if positive.
  2. The FDA made errors in judging the trials.
  3. The drugs became more effective after the trials ended.
  4. Journals only publish trials with more than 1,000 people.
Show Answer

The correct answer is A. Turner and colleagues found that 22 negative or questionable trials were never published and 11 were published in a way that conveyed a positive result, which made the drugs' effect look about 32% bigger overall. That pattern is publication bias. Nothing in the study suggests FDA errors.

Concept Tested: Publication Bias Bloom's Level: Analyze

Question 10

Three meta-analyses of probiotics (and related treatments) for depression report effect sizes of about 0.24, 0.26, and 0.96. Which explanation from the chapter best accounts for the difference?

  1. One of the reviews must have made a math mistake.
  2. Effect sizes always triple when more trials are added.
  3. Larger effect sizes always come from the best-designed reviews.
  4. The reviews included different trials, people, strains, and lengths of treatment, and the trials disagreed a lot.
Show Answer

The correct answer is D. The reviews pooled different sets of trials: for example, Asad 2025 included only people with a diagnosis and reported high heterogeneity, while Liu 2019 also included people without a diagnosis (and found a bigger effect in clinical samples). Strain specificity means "probiotics" can mean different products in each review. Option C is wrong because a bigger effect size does not mean better design.

Concept Tested: Systematic Review Bloom's Level: Analyze