. 2022 Dec 19;14(2):215–237. doi: 10.1016/j.advnut.2022.10.005

TABLE 3.

Characteristics and gut microbiota outcomes of included comparisons by subgroups¹.

Study (year)	Comparison diet 1	Comparison diet 2	Study type	Diet length	Dietary control	Alpha diversity	Phylum	Class	Order	Family	Genus	Species	Other	Major differences in gut microbiota outcomes	Effects? (p<0.05)
Primary comparisons: meat vs. nonmeat
Lang et al. 2018 [87]	Red meat², ³	Nonmeat², ³	CO, Yes	4 wk	F	N	N	NR	NR	NR	Y	—	—	3 OTUs differed when high and low amounts of saturated fat were combined	Yes
Lang et al. 2018 [87]	White meat², ³	Nonmeat², ³	CO, Yes	4 wk	F	N	N	NR	NR	NR	Y	—	—	1 OTU differed when high and low amounts of saturated fat were combined	Yes
van Faassen et al. 1987 [85]	Western omnivorous², ³	Vegetarian², ³	CO, NS	20 d	F	—	—	N	—	N	N	—	N², ⁴	No difference	No
van Faassen et al. 1987 [85]	Western omnivorous², ³	Vegan², ³	CO, NS	20 d	F	—	—	N	—	N	Y	—	N², ⁴	Omnivorous diet ↑ counts of genus Lactobacilli	Yes¹⁴
Kohnert et al. 2021 [91]	Omnivorous⁴, ⁵	Vegan⁴, ⁵	PL	4 wk	C	N², ⁴	NR	NR	NR	NR	Y⁴, ⁵	Y⁴, ⁵	—	Differences in several genera and species (Table 4)	Yes
Kahleova et al. 2020 [89]	Habitual omnivorous⁴, ⁵	Vegan⁴, ⁵	PL	16 wk	C	Y	Y⁴, ⁵	N	N	Y⁴, ⁵	Y⁴, ⁵	Y⁴, ⁵	—	Differences in alpha diversity and bacterial abundance (Table 4)	Yes
Pagliai et al. 2020 [74]	Mediterranean⁴, ⁵	Vegetarian⁴, ⁵	CO¹⁰, No	12 wk	C	Y⁴, ⁵	N⁴, ⁵	Y⁴, ⁵	Y⁴, ⁵	Y⁴, ⁵	Y⁴, ⁵	—	—	Differences in alpha diversity and bacterial abundance from phylum to genus levels (Table 4)	Yes
Total % of significant out of measured (%) – meat vs. nonmeat						40	25	25	50	50	86	100	N/A	N/A	86
Proportion of significant out of measured (counts) – meat vs. nonmeat						2/5	1/4	1/4	1/2	2/4	6/7	2/2	N/A	N/A	6/7
Primary comparisons: higher vs. lower meat within omnivorous habitual/basal diets
Hess et al. 2018 [84]	Red meat⁴, ⁵	Mushroom⁴, ⁵	CO, Yes	10 d	P2	—	Y	NR	NR	NR	Y	—	—	Red meat ↓ phylum Bacteroidetes and ↑ phylum Firmicutes	Yes
Foerster et al. 2014 [86]	Red meat⁴, ⁵	Whole grain⁴, ⁵	CO, Yes	3 wk	P2	—	—	—	—	—	—	Y	Y¹³	Whole grain ↑ DGGE band number and species Collinsella aerofaciens; red meat ↓ species Clostridium sp	Yes
Crimarco et al. 2020 [88]	Animal-based meat², ⁶	Plant-based meat², ⁶	CO, No	8 wk	P1	—	NR	NR	NR	NR	Y⁴, ⁵	Y⁴, ⁵	—	Differences in bacterial abundance (Table 4)	Yes
McKenna et al. 2021 [90]	Higher red meat², ⁴, ⁷	Lower red meat², ⁴, ⁷	PL¹¹	10 wk	P2	N	NR	NR	Y², ⁴	Y², ⁴	Y², ⁴	—	—	Differences in bacterial abundance (Table 4)	Yes
Windey et al. 2012 [73]	Higher meat², ⁸	Lower meat², ⁸	CO, No	2 wk	C	—	—	—	—	—	—	—	N13	No difference in DGGE band numbers	No
Total % of significant out of measured (%) – higher vs. lower meat						0	100	0	100	100	—	100	N/A	N/A	80
Proportion of significant out of measured (counts) – higher vs. lower meat						0/1	1/1	0	1/1	1/1	3/3	2/2	N/A	N/A	4/5
Total % of significant out of measured (%) – primary comparisons						33	40	25	67	60	90	100	N/A	N/A	83
Proportion of significant out of measured (counts) – primary comparisons						2/6	2/5	1/4	2/3	3/5	9/10	4/4	N/A	N/A	10/12
Secondary comparisons: meat subtypes
Lang et al. 2018 [87]	Red meat², ³	White meat², ³	CO, Yes	4 wk	F	N	N	NR	NR	NR	Y	—	—	Several OTUs differed at low or high amounts of saturated fat, but no difference when both amounts were combined.	Yes
Schmedes et al. 2019 [71]	Lean seafood⁴, ⁵	Non-seafood meat⁴, ⁵	CO, Yes	4 wk	P1	—	N	NR	NR	NR	Y	—	—	Non-seafood diet ↓ genus group Clostridium cluster IV	Yes
Bratlie et al. 2021 [93]	Cod2	Nonfish habitual Meat²	PL	8 wk	P2	—	NR	NR	Y	NR	NR	NR	—	Cod/Salmon ↓ order-level bacterial counts of Bacteroidales, Clostridiales, and Selenomonadales	Yes
Bratlie et al. 2021 [93]	Salmon²	Nonfish habitual Meat²	PL	8 wk	P2	—	NR	NR	Y	NR	NR	NR	—		Yes
Meslier et al. 2020 [92]	Mediterranean (higher fish)², ⁴, ⁹	Habitual omnivorous (higher nonfish meat)², ⁴, ⁹	PL	8 wk	P1	—	NR	NR	NR	NR	NR	Y², ⁴	—	Differences in beta diversity and several species (Table 4)	Yes
Total % of significant out of measured (%) – secondary comparisons						0	0	0	100	0	100	100	N/A	N/A	100
Proportion of significant out of measured (counts) – secondary comparisons						0/1	0/2	0	2/2	0	2/2	1/1	N/A	N/A	5/5
Secondary comparison: vegetarian vs. vegan
van Faassen et al. 1987 [85]	Vegetarian², ⁴, ⁹	Vegan², ⁴, ⁹	CO, NS	20 d	F	—	—	N	—	N	Y	—	N¹²	Vegetarian diet ↑ counts of genus Enterococci and Lactobacilli	Yes
Total % of significant out of measured (%) – all comparisons						29	29	20	80	50	92	100	N/A	N/A	89
Proportion of significant out of measured (counts) - all comparisons						2/7	2/7	1/5	4/5	3/6	12/13	5/5	N/A	N/A	16/18
Secondary comparison not available: unprocessed vs. processed meat

All vegetarian diets are lacto-ovo vegetarian that included dairy and eggs. Vegan diet did not contain any animal-based products. Meat, if not specified, refers to mixed meat subtypes. Category “Study type” includes CO and PL RCTs, as well as the washout period for crossover RCTs (Yes for existence, No for no washout, and NS for not specified or not reported). Category “Length” is the length of intervention of each study arm (d, day; wk, week). Category “Dietary control” includes full control (F) that provided all study foods; partial control that provided some but not all study foods, either with dietary guidance on basal diet (P1) or with uncontrolled habitual diet (P2) and counseling only (C) that did not provide any study foods except for dietary guidance. Categories “Alpha diversity” through “Other”: bacterial abundance at taxonomic levels such as phylum, class, order, family, genus, and species, or in other forms of measurement (e.g., OTU, operational taxonomic unit). Y, significant changes or differences were observed; N, no significant change or difference was observed; NR, outcomes were evaluable using methods reported by the study authors, but no data were reported at the specific taxonomic level; Em dash (-), outcomes and data likely were not measured based on study description, and thus were not reported for the selected comparisons; N/A, not applicable. Results were considered significant only when the authors of the included studies reported the statistical significance of the outcome measures. Category “Effects”: overall effects of diet on gut microbiota. Summary of Table 3: Among the primary comparisons, 83% of the comparisons showed significant effects of meat intake on the gut microbiota (Figure 2B). Although global bacterial composition measured using alpha diversity metrics was reported in 50% of primary comparisons, only 33% of them showed significant effects of meat intake on the gut microbiota composition. In contrast, among 83% of primary comparisons that reported bacterial abundances at the genus level, 90% of them showed significant effects. Only 4 of the 12 primary comparisons reported outcomes at the species level, but all 4 comparisons suggested significant effects. A similar hierarchical pattern of findings was observed among the secondary comparisons. Two of 2 comparisons (100%) at the genus level and 1 of 1 comparison at the species level showed significant effects, but zero of 1 comparison showed effects for alpha diversity metrics (Figure 2C).

Difference in post-intervention values between groups.

Each diet was consumed with high vs. low amounts of saturated fat.

⁴

Within-group pre–post changes.

⁵

Difference in between-group changes.

⁶

Pre- and post-intervention samples combined.

⁷

Comparisons were made among baseline vs. 1-wk intervention vs. post-intervention within and between the groups.

⁸

Samples were collected during the diet intervention period without a clearly specified timepoint.

⁹

Comparisons were made among baseline vs. 4-wk intervention vs. 8-wk intervention within and between the groups.

¹⁰

Cointervention with energy restriction.

¹¹

Cointervention with resistance training.

¹²

Counts of total aerobes and anaerobes.

¹³

Denaturing gradient gel electrophoresis (DGGE) bandclass counts.

¹⁴

Results from this study were obtained via the cultivation-dependent method, which could potentially miss capturing significant gut microbial outcomes compared with more advanced techniques.