Does four straight days of fast food change gut microbes?

One trial measured what happens in the gut when burgers, fries, and a soft drink are eaten for several days in a row. A two-week high-fat, low-fiber diet shows where metabolites and the gut lining go when that pattern runs longer.

What the studies looked at

Chenghao Zhu and colleagues, in 2020, fed 10 healthy adults both diets in a crossover. Ages were 18–25 years, mean 22.1 (SD 2.33). Mean BMI was 24.39 (SD 3.71, range 21.2–32.9).

The order was randomized. Four days were fast food, four days were a Mediterranean diet, with a 4-day washout between them. The diets were matched to each person’s energy needs, and the study provided the food. Fast food was burgers, fries, and a soft drink from one named restaurant. The registration number is NCT03205254.

Usual energy intake averaged 2526 kcal/day (SD 951), and energy was matched across the two diets. Fat was 44.6% of energy on fast food (SD 4.0) and 33.2% on the Mediterranean diet (SD 2.5). Fiber was 12.9 g/day on fast food (SD 2.8) and 58.0 g/day on the Mediterranean diet (SD 8.6). Protein was 89.9 g and 105.0 g, and neither differed statistically from the usual 102.0 g (p > 0.05).

Results

Microbial diversity (alpha diversity) did not differ significantly between the diets (p = 0.576, n = 10). Distances between communities grouped by person rather than by diet. Who was eating outweighed the four-day menu.

After correction for multiple testing, two genus-level changes remained. Butyricicoccus fell after fast food and rose after the Mediterranean diet (adjusted p = 0.019; unadjusted p = 0.0001). Collinsella did the reverse (adjusted p = 0.028; unadjusted p = 0.0004). Before correction, a Lachnospiraceae genus fell on fast food, and Parabacteroides, Bilophila, and Escherichia/Shigella rose. The paper names the bile-tolerant species Bilophila wadsworthia. Genus-level Bilophila had an unadjusted p of 0.0347 and did not survive correction.

Plasma metabolites moved as well. The bacterial tryptophan products indole-3-lactic acid (adjusted p = 0.003), indole-3-propionic acid (adjusted p = 0.021), and indole-3-acetic acid (adjusted p = 0.021) fell after fast food and rose after the Mediterranean diet. Tryptophan itself rose after fast food (adjusted p = 0.014). Kynurenine was unchanged, and the kynurenine-to-tryptophan ratio fell after fast food (p = 0.005). Bile acids and TMAO showed no confirmed difference between the diets within four days.

People did not respond alike. Two participants had no detectable Bilophila at any of the four time points. Baseline Bilophila tracked usual saturated-fat intake (r = 0.708, p = 0.022).

When four days become two weeks

Stephen O’Keefe and colleagues, in 2015, did not serve a restaurant menu. For two supervised weeks they swapped a high-fat, low-fiber Western-style diet with a higher-fiber, lower-fat African-style diet. Mucosal proliferation and inflammation were measured in 20 African Americans and 12 rural South African participants. Butyrate-related measures were taken in 20 people in each group.

Switching onto the Western-style diet cut total butyrate in colonic contents by half. Switching onto the African-style diet raised it 2.5-fold. The proliferation marker Ki67 fell on the higher-fiber diet and rose on the high-fat, low-fiber diet. CD3+ lymphocytes and CD68+ macrophages moved in the same direction. Genus-level composition shifted only modestly. Body weight stayed within 2 kg.

Limits

The Zhu trial is 10 people, four days each, young adults, and calorie-matched. It is a pilot, and no power calculation was done. It did not find a collapse of diversity, and it did not find new disease. Two genera remained after correction. Bile acids that did not move in four days sit on a different clock from the weeks-to-months studies the authors cite.

The Western-style diet in the O’Keefe trial was a research menu: high in fat and low in fiber. It is not a burger-restaurant receipt, and the participants were two groups with different usual diets. The two-week butyrate and mucosal findings do not transfer onto four days of fast food as if they were the same measurement.

What this means for a log

In both trials, bacteria and metabolites tracked fiber and the mix of fat more closely than the calories that had been matched. The Zhu gap was 12.9 g of fiber a day versus 58.0 g. If a photo misses the drink, the fries, or the vegetables that were not there, that gap disappears from the record. The genus names from a four-day pilot are not a daily target.