Study analysis · The American journal of clinical nutrition · 2004
Your fat-free salad dressing might be making your salad nearly nutrient-free.
In 7 healthy adults, salads with fat-free dressing produced almost no carotenoid absorption, while adding 6 g or 28 g canola oil to the dressing significantly increased absorption of alpha-carotene, beta-carotene, and lycopene—though no long-term health outcomes were measured.
Overview
What the study found
The study in plain English — the bottom line, every takeaway we extracted, and what to do with them.
In simple terms
This study is like a fair test where the same people ate salads with different amounts of fat in random order. It shows that fat helps the body absorb certain nutrients from salad right after eating, but it cannot tell us if that leads to better health over years.
What’s the bottom line?
Scientists gave 7 people salads with dressing that had no fat, a little fat (6 g), or a lot of fat (28 g). They measured carotenoids from the vegetables in the blood.
How strong is this study?
The study was carefully controlled because each person compared all three dressings, but it only included 7 people and they knew which dressing they were eating. So the results are a good clue, but not final proof—larger studies with more people are needed.
40 / 100
- COI disclosure+40/40
- Data availabilitydata not shared
- Code availabilitycode not shared
57 / 100
- Randomization+20/20
- Blindingblinding unclear
- Control group+15/15
- Sample size (n=7)+0.7/20
- Follow-up+10/10
100 / 100
23 / 100
- P-values+15/15
- Effect sizeno effect size reported
- Confidence intervalsno confidence intervals
- Pre-registrationnot pre-registered
Each component is scored out of 100 and then capped by the study design — a case series cannot reach the ceiling a randomised trial can, however well it is reported.
Where it sits
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 554 / 100
Probability of being correct
Participants are randomly assigned to treatment or control groups, minimizing bias. The gold standard for testing whether an intervention causes an effect.
This design can establish causation. Randomized crossover design supports causal inference for the acute effect of added fat on carotenoid absorption in this controlled setting. However, very small sample (n=7), lack of blinding, no confidence intervals, and short-term acute outcome limit strength; cannot establish long-term health effects.
COI Unknown
Could not determine conflict of interest status
No conflict of interest or funding statement was provided in the available text; unable to assess potential conflicts.
The provided text is truncated and does not include a COI or funding section. No author affiliations or disclosures are visible. Further information is needed to assess conflicts.
Key takeaways
- 01
No fat: almost no carotenoids absorbed.
- 02
6 g fat: more absorbed than no fat (P<0.04).
- 03
28 g fat: even more absorbed than 6 g (P<0.02).
- 04
No absolute risk increase (like extra cases per 1,000 people) was reported because this study measured short-term carotenoid absorption, not disease outcomes.
- 05
It shows the amount of fat in dressing changes absorption, but not what that means for long-term health.
Surprising findings
- Fat-free salad dressing resulted in negligible carotenoid absorption.Many assume salad is automatically healthy, but fat-soluble nutrients need fat to be absorbed.
- Lycopene absorption was disproportionately low and most improved by more fat.Tomatoes are famous for lycopene, but from fresh cherry tomatoes it was poorly absorbed and highly fat-dependent.
- The 3-5 g fat threshold may be insufficient for intact vegetables.Previous research with purified carotenoids suggested only 3-5 g fat was needed; this challenges that for whole plant matrices.
Practical takeaways
If you eat a salad with fat-free dressing, add a fat source—oil, avocado, nuts, seeds, or full-fat dressing—to absorb more carotenoids.
Based on single meals in 7 people; not a long-term health study.
medium for absorption mechanism, low for clinical outcomes confidenceDon't fear all fat in salads; a modest amount (around 6 g) may help, and more (28 g) may further increase absorption.
28 g oil is about 250 calories; balance total calorie needs.
low-to-medium confidenceConsider cooking or processing vegetables (e.g., tomato paste) if you want lycopene; fresh cherry tomatoes had poor absorption even with fat.
Not directly tested here, but supported by the study discussion.
low confidenceWhy this study matters
Fat-free dressing = negligible carotenoid absorption
After 7 subjects ate a mixed salad with fat-free dressing, appearance of alpha-carotene, beta-carotene, and lycopene in plasma chylomicrons was negligible. The study concluded essentially no absorption occurred. This is an acute absorption biomarker, not a relative or absolute disease-risk effect.
People eat salads for nutrients, but without fat, key phytonutrients may pass through unabsorbed.
6 grams of fat flips the switch
Adding 6 g canola oil (reduced-fat dressing) increased carotenoid appearance versus 0 g (P<0.04 for each carotenoid). This is about 1.5 teaspoons of oil.
A tiny amount of fat can make a measurable difference in nutrient absorption.
28 grams absorbs more than 6 grams
Full-fat dressing with 28 g canola oil further increased absorption versus 6 g (P<0.02 for each carotenoid). 28 g is about 2 tablespoons of oil. Again, this is an acute absorption effect, not a relative risk of disease.
More fat may mean more nutrient absorption, challenging the idea that less fat is always better.
Lycopene is especially fat-dependent
Lycopene from cherry tomatoes was disproportionately low relative to its content and showed the most dramatic improvement when fat increased from 6 to 28 g.
Tomato nutrients may be harder to absorb without enough fat.
Old 3-5 g fat threshold may be too low for whole vegetables
Previous research with purified carotenoids suggested 3-5 g fat was enough; this study found the threshold exceeded 6 g for intact plant matrices. The authors wrote: 'Our data suggest that 3-5 g fat should be considered a minimal amount that may not be sufficient for optimal bioaccessibility of carotenoids within the intact structures of plants.'
It contradicts a common nutrition guideline and suggests food form matters.
Small, industry-funded, acute study
The trial had only 7 subjects, measured single meals over 12 hours, and was funded by Procter & Gamble, with two authors employed by P&G. No disease outcomes were reported. This is a low-to-moderate confidence study.
Important caveat before changing your diet based on one small trial.
Want the whole report?
Detailed mode opens the full scientific breakdown — every score component, the methodology, conflicts of interest, the evidence analysis behind each claim, and the raw study data.
Overview
What the study found
The study in plain English — the bottom line, every takeaway we extracted, and what to do with them.
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
Scientists gave 7 people salads with dressing that had no fat, a little fat (6 g), or a lot of fat (28 g). They measured carotenoids from the vegetables in the blood.
Research results
No fat: almost no carotenoids absorbed. 6 g fat: more absorbed than no fat (P<0.04). 28 g fat: even more absorbed than 6 g (P<0.02).
What this means - more context
No absolute risk increase (like extra cases per 1,000 people) was reported because this study measured short-term carotenoid absorption, not disease outcomes. It shows the amount of fat in dressing changes absorption, but not what that means for long-term health.
To quantify the appearance of carotenoids in plasma chylomicrons after subjects ingested fresh vegetable salads with fat-free, reduced-fat, or full-fat salad dressings.
In 7 healthy adults, mixed-vegetable salads with fat-free dressing produced negligible chylomicron carotenoids. Adding 6 g canola oil increased carotenoid appearance versus 0 g (P<0.04 for each), and 28 g increased it further versus 6 g (P<0.02 for each). No relative or absolute disease-risk effect was reported.
Methods Used
Randomized crossover trial: 7 subjects consumed 3 identical salads with dressings containing 0, 6, or 28 g canola oil, in random order, separated by ≥2-week washouts. Blood was collected hourly for 12 h; chylomicrons were isolated by ultracentrifugation and carotenoids measured by HPLC with coulometric array electrochemical detection.
Main Finding
Carotenoid absorption from a single mixed-vegetable salad was negligible with fat-free dressing; 6 g fat increased absorption versus 0 g (P<0.04 for alpha-carotene, beta-carotene, and lycopene), and 28 g fat increased absorption versus 6 g (P<0.02 for each). No relative or absolute disease-risk increase was reported; the study measured short-term carotenoid absorption, not clinical outcomes.
Confidence Level
Low-to-moderate: randomized crossover and sensitive detection, but very small n=7, acute single-meal outcomes, no long-term health outcomes, and industry funding. No retraction or correction noted in PubMed/Crossref.
Study Flags
Red Flags
- •Very small sample (n=7)
- •Acute single-meal outcomes, no long-term health outcomes
- •Industry-funded by Procter & Gamble
Surprising Findings
Fat-free salad dressing resulted in negligible carotenoid absorption.
Many assume salad is automatically healthy, but fat-soluble nutrients need fat to be absorbed.
Practical Takeaways
If you eat a salad with fat-free dressing, add a fat source—oil, avocado, nuts, seeds, or full-fat dressing—to absorb more carotenoids.
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 554 / 100
Probability of being correct
Participants are randomly assigned to treatment or control groups, minimizing bias. The gold standard for testing whether an intervention causes an effect.
Human RCT
Subject
Moderate probability
on the GRADE evidence scale
This study is like a fair test where the same people ate salads with different amounts of fat in random order. It shows that fat helps the body absorb certain nutrients from salad right after eating, but it cannot tell us if that leads to better health over years.
Strengths
- Randomized crossover design with each subject serving as own control
- Washout period of at least 2 weeks between conditions
- Controlled low-carotenoid diet before each test day
Weaknesses
- Very small sample size (n=7)
- No blinding of participants or outcome assessors reported
- No confidence intervals reported
Methodology
Evidence Keywords
Statistical Reporting
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
Scientists gave 7 people salads with dressing that had no fat, a little fat (6 g), or a lot of fat (28 g). They measured carotenoids from the vegetables in the blood.
Research results
No fat: almost no carotenoids absorbed. 6 g fat: more absorbed than no fat (P<0.04). 28 g fat: even more absorbed than 6 g (P<0.02).
What this means - more context
No absolute risk increase (like extra cases per 1,000 people) was reported because this study measured short-term carotenoid absorption, not disease outcomes. It shows the amount of fat in dressing changes absorption, but not what that means for long-term health.
To quantify the appearance of carotenoids in plasma chylomicrons after subjects ingested fresh vegetable salads with fat-free, reduced-fat, or full-fat salad dressings.
In 7 healthy adults, mixed-vegetable salads with fat-free dressing produced negligible chylomicron carotenoids. Adding 6 g canola oil increased carotenoid appearance versus 0 g (P<0.04 for each), and 28 g increased it further versus 6 g (P<0.02 for each). No relative or absolute disease-risk effect was reported.
Methods Used
Randomized crossover trial: 7 subjects consumed 3 identical salads with dressings containing 0, 6, or 28 g canola oil, in random order, separated by ≥2-week washouts. Blood was collected hourly for 12 h; chylomicrons were isolated by ultracentrifugation and carotenoids measured by HPLC with coulometric array electrochemical detection.
Main Finding
Carotenoid absorption from a single mixed-vegetable salad was negligible with fat-free dressing; 6 g fat increased absorption versus 0 g (P<0.04 for alpha-carotene, beta-carotene, and lycopene), and 28 g fat increased absorption versus 6 g (P<0.02 for each). No relative or absolute disease-risk increase was reported; the study measured short-term carotenoid absorption, not clinical outcomes.
Confidence Level
Low-to-moderate: randomized crossover and sensitive detection, but very small n=7, acute single-meal outcomes, no long-term health outcomes, and industry funding. No retraction or correction noted in PubMed/Crossref.
Study Flags
Red Flags
- •Very small sample (n=7)
- •Acute single-meal outcomes, no long-term health outcomes
- •Industry-funded by Procter & Gamble
Surprising Findings
Fat-free salad dressing resulted in negligible carotenoid absorption.
Many assume salad is automatically healthy, but fat-soluble nutrients need fat to be absorbed.
Practical Takeaways
If you eat a salad with fat-free dressing, add a fat source—oil, avocado, nuts, seeds, or full-fat dressing—to absorb more carotenoids.
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 554 / 100
Probability of being correct
Participants are randomly assigned to treatment or control groups, minimizing bias. The gold standard for testing whether an intervention causes an effect.
Human RCT
Subject
Moderate probability
on the GRADE evidence scale
This study is like a fair test where the same people ate salads with different amounts of fat in random order. It shows that fat helps the body absorb certain nutrients from salad right after eating, but it cannot tell us if that leads to better health over years.
Strengths
- Randomized crossover design with each subject serving as own control
- Washout period of at least 2 weeks between conditions
- Controlled low-carotenoid diet before each test day
Weaknesses
- Very small sample size (n=7)
- No blinding of participants or outcome assessors reported
- No confidence intervals reported
Methodology
Evidence Keywords
Statistical Reporting
Scoring
How strong is this study?
The study was carefully controlled because each person compared all three dressings, but it only included 7 people and they knew which dressing they were eating. So the results are a good clue, but not final proof—larger studies with more people are needed.
40 / 100
- COI disclosure+40/40
- Data availabilitydata not shared
- Code availabilitycode not shared
57 / 100
- Randomization+20/20
- Blindingblinding unclear
- Control group+15/15
- Sample size (n=7)+0.7/20
- Follow-up+10/10
100 / 100
23 / 100
- P-values+15/15
- Effect sizeno effect size reported
- Confidence intervalsno confidence intervals
- Pre-registrationnot pre-registered
Each component is scored out of 100 and then capped by the study design — a case series cannot reach the ceiling a randomised trial can, however well it is reported.
Where it sits
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 554 / 100
Probability of being correct
Participants are randomly assigned to treatment or control groups, minimizing bias. The gold standard for testing whether an intervention causes an effect.
This design can establish causation. Randomized crossover design supports causal inference for the acute effect of added fat on carotenoid absorption in this controlled setting. However, very small sample (n=7), lack of blinding, no confidence intervals, and short-term acute outcome limit strength; cannot establish long-term health effects.
COI Unknown
Could not determine conflict of interest status
No conflict of interest or funding statement was provided in the available text; unable to assess potential conflicts.
The provided text is truncated and does not include a COI or funding section. No author affiliations or disclosures are visible. Further information is needed to assess conflicts.
Standing
Who’s using this study?
The videos and claims on this site that lean on this study, and the researchers who wrote it.
1 video from Thomas DeLauer cite this study, drawing 1 claim from it.
- Very strong evidence
Randomized or controlled trials support this claim, alongside consistent supporting evidence.
Evidence
Authored by
7 researchersIf this is your work, this is how we attribute it on Fit Body Science. Melody June Brown is listed as the lead author.