Fast food: what the evidence can and cannot tell you

Almost everything known about fast food and health comes from watching what people eat and counting who falls ill later. Pooling 9 studies with 169771 people, the highest fast food intake went with 55 percent higher odds of non-alcoholic fatty liver disease (OR 1.55). Odds and risk are two different things, and because fatty liver is common, that figure runs ahead of the real rise in risk. Across 17 studies in women, the highest intake of fast food and other ultra-processed food went with 25 percent higher odds of breast cancer. That number carries a catch worth reading twice. The link showed up in the case-control studies and vanished in the cohorts, which are the stronger half of the evidence. Pooled data also link fast food to 2.41 times the odds of ulcerative colitis and 2.65 times the odds of Crohn disease. Both of those pooled intervals are wide, and their lower edges, 1.07 and 1.23, sit close to no link at all. All of that is observational. It shows that people who eat the most fast food are also more often ill, and it cannot show which came first. The one randomised feeding trial on this page never served anyone fast food. It gave 18 boys aged 11 to 14 a diet rich in the browning compounds typical of fast food and snacks, for two weeks, and nitrogen digestibility fell 6 percent. The sodium figure we carry, a median 471.06 mg per 100 g, was measured on 20 dishes in Lima in 2019, and it is the only measurement we have.

The numbers, per 100 g

How much is actually in it

• Measured by atomic absorption in Lima, fast food carried a median 471.06 mg of sodium per 100 g, close to street food at 492.36 mg. — 20 frequently consumed away-from-home foods in three areas of Metropolitan Lima, Peru, collected in 2019 (n = 20 preparations) Evidence C source
  Median sodium 471.06 mg/100 g in fast food (IQR 115.31 to 695.18), 492.36 mg/100 g in street food, 65 percent of foods classified high in sodium under Peruvian rules

What a real portion looks like

• When a fast-food combo defaulted to low-calorie sides, people ordered 337 kcal less than when they picked every item themselves, in a hypothetical order placed on a screen rather than a meal anyone ate. — 377 US adults in a randomised scenario-based drive-through experiment, July 2020 (n = 377) Evidence B source
  -337 kcal (SE 19) for optimal defaults and +132 kcal (SE 20) for traditional high-calorie defaults, both P < 0.001
• Of five menu label designs tested on 6210 US adults ordering hypothetically on a screen, only the traffic-light label moved the healthfulness of what they ordered, and it moved the score from 49.25 to 49.93 points. — 6210 US adults, nationally representative online panel, April 2025, ordering from two fast-food menus (n = 6210) Evidence B source
  Nutrient Profile Index 49.93 versus 49.25 points in the burger restaurant (P < 0.001), no effect in the sandwich restaurant

What your body absorbs

• Two weeks of a diet rich in Maillard reaction products, the browning compounds typical of fast food and snacks, lowered nitrogen digestibility by 6 percent in adolescent boys. — 18 healthy adolescent males aged 11 to 14 years, 2-period crossover, 2 weeks per diet with a 40-day washout (n = 18) Evidence B source
  47 percent higher faecal nitrogen excretion, 12 percent lower apparent nitrogen absorption and 6 percent lower nitrogen digestibility

What it does to your health

• Pooling 9 observational studies with 169771 people, the highest fast food intake went with 55 percent higher odds (OR 1.55) of non-alcoholic fatty liver disease. — 169771 participants in 9 observational studies, adults, searched to February 2025 (n = 169771) Evidence C source
  OR 1.55 (95% CI 1.51 to 1.59) for NAFLD and OR 1.37 (95% CI 1.27 to 1.49) for obesity
• Across 17 observational studies, women in the highest category of fast food and ultra-processed food intake had 25 percent higher odds of breast cancer, a link that held in the case-control studies and did not hold in the cohorts. — women in 17 observational studies, case-control and cohort, searched to May 2025 (n = 17 studies) Evidence C source
  OR 1.25 (95% CI 1.09 to 1.43)
• Pooled epidemiological data link fast food intake to 2.41 times the odds of ulcerative colitis and 2.65 times the odds of Crohn disease. — pooled epidemiological studies plus a hospital-based case-control study in Riyadh with 158 ulcerative colitis, 244 Crohn disease and 395 control participants (n = 797 in the case-control arm) Evidence C source
  Pooled OR 2.41 (95% CI 1.07 to 5.45) for ulcerative colitis and 2.65 (95% CI 1.23 to 5.70) for Crohn disease
• Across 17 studies with 159885 adults, junk food intake, a bundle of ultra-processed foods, fast foods, snacks and sugary drinks in which fast food is not counted on its own, went with 16 percent higher odds of developing mental health problems in the cohort data. — 159885 adults in 17 studies, 7 longitudinal, 9 cross-sectional and 1 case-control, searched to July 2023 (n = 159885) Evidence C source
  OR 1.16 (95% CI 1.07 to 1.24) in cohorts and OR 1.15 (95% CI 1.06 to 1.23) in cross-sectional data

Safety, limits and interactions

• In a meta-analysis of 22 cross-sectional studies with 18487 people in Saudi Arabia, fast food was listed among the common risk factors for gastroesophageal reflux disease, at 43 percent, and a frequency counted among cases is not a relative risk. — 18487 participants in 22 cross-sectional studies in Saudi Arabia, searched to December 2024 (n = 18487) Evidence C source
  Pooled GERD prevalence 33 percent, fast food listed among the common risk factors at 43 percent

Who this works differently for

• Across 65 prospective studies with 831798 women, 38 of them pooled quantitatively, fast food eaten before conception was one of five dietary elements positively associated with gestational diabetes (P < 0.05), and the review reports no separate effect size for fast food. — 831798 participants in 65 prospective studies, 38 pooled quantitatively, searched to August 2021 (n = 831798) Evidence C source
  Fried food, fast food, red and processed meat, heme iron and a low-carbohydrate pattern all positively associated with gestational diabetes (P < 0.05), with no separate effect size reported for fast food

What is still unknown

• Counting fast-food outlets within 1500 m of home over 13 years showed no link with C-reactive protein, interleukin-6 or adiponectin in 1350 US nurses. — US women in the Nurses Health Study II with stored blood and addresses, samples in 1999 and 2011, CRP n=1350, IL-6 n=809, adiponectin n=836 (n = 1350) Evidence C source
  No association, for example CRP beta 0.00 (95% CI 0.01 to 0.01), and no effect modification by neighbourhood socioeconomic status

The bottom line

If you want one thing to act on, look at what happens when you take the combo instead of building the order yourself. In a scenario experiment with 377 US adults, a combo put together the traditional way pushed the order 132 kcal up compared with picking each item. The bigger number in that study, 337 kcal less, came from combos the restaurant had already rebuilt around low-calorie sides. That one is the chain making the choice for you, and it is not something you can do at the counter. Both figures are calories ordered on a screen, not calories anyone ate. Labels did much less. Of five menu label designs tested on 6210 adults, only the traffic-light label moved anything in what people ordered, and it moved a meal healthfulness score from 49.25 to 49.93 points. That is close to nothing, and it is useful to know before you trust a label to do the work for you. Two gaps here are wide enough to say out loud. We hold nothing on what cooking or preparation does, because the USDA retention tables have no entry for this category and no measured samples exist, so the spread between one kitchen and another is unmeasured on our side. We also hold nothing on the common myths, because the American and European health claim registers are indexed by substance and never name fast food at all. The question most people actually want answered, who in particular should stay away, we cannot answer properly either. The only safety evidence we have counts complaints rather than risk. In 22 cross-sectional studies with 18487 people in Saudi Arabia, fast food was listed among the common risk factors for reflux, at 43 percent. A frequency counted among people who already have reflux says nothing about who will develop it. Pregnancy has a little more behind it, and less than it first looks. Across 65 prospective studies with 831798 women, 38 of them pooled quantitatively, fast food eaten before conception was one of five dietary elements positively associated with gestational diabetes (P < 0.05), and the review gives no separate number for fast food on its own. Mood sits on the same kind of evidence: across 17 studies with 159885 adults, junk food, a bundle of ultra-processed foods, fast foods, snacks and sugary drinks in which fast food is not counted on its own, went with 16 percent higher odds of developing mental health problems in the cohort data. One finding points the other way and belongs on this page. Counting fast-food outlets within 1500 m of home over 13 years showed no link with C-reactive protein, interleukin-6 or adiponectin in 1350 US nurses. Living close to the food is a different thing from eating it, and a result of nothing is still a result.

Sources

1. ffd-r5-08 · cross-sectional
   The median sodium content in street food products was 492.36 mg/100g (IQR: 83.93 - 918.78), 471.37 mg/100 g in artisanal food (IQR: 76.04 - 765.39) and 471.06 mg/100 g in fast food (IQR: 115.31 - 695.18).
   Rev Peru Med Exp Salud Publica, 2023 · checked 2026-09-23
2. ffd-r5-06 · RCT
   Compared with the choice combo meals, optimal combo meals reduced calories ordered by consumers (-337 kcal, standard error = 19, P < .001), while traditional combos increased them (+132 kcal, standard error = 20, P < .001).
   J Acad Nutr Diet, 2023 · checked 2026-09-23
3. ffd-r5-07 · RCT
   Only the TLL was effective at promoting more healthful food choices in the burger restaurant compared to the control label (mean [standard error] NPI score: 49.93 [0.18] points vs 49.25 [0.18] points; P < .001).
   JAMA Health Forum, 2026 · checked 2026-09-23
4. ffd-r5-05 · RCT
   Compared with consumption of the WD, consumption of the BD resulted in 47% higher fecal nitrogen fecal excretion (P = 0.002), 12% lower apparent nitrogen absorption (P = 0.000), and a 6% lower nitrogen digestibility (P = 0.000).
   Am J Clin Nutr, 2006 · checked 2026-09-23
5. ffd-r5-01 · systematic review and meta-analysis of observational studies
   A higher consumption of fast food was significantly associated with a 55% increased risk of NAFLD (OR = 1.55, 95% CI: 1.51-1.59, p < 0.001, I2 = 15.6%).
   Front Public Health, 2025 · checked 2026-09-23
6. ffd-r5-02 · systematic review and meta-analysis of observational studies
   The pooled analysis of 17 observational studies showed a significant association between the highest FFs and UPFs consumption and increased breast cancer risk (OR 1.25, 95% CI [1.09-1.43], p = 0.001).
   Glob Health Res Policy, 2025 · checked 2026-09-23
7. ffd-r5-03 · case-control study and meta-analysis
   the meta-analysis revealed higher odds of fast food intake associated with UC and CD, with pooled odds ratios (95% CIs) of 2.41 (1.07, 5.45) and 2.65 (1.23, 5.70), respectively.
   Nutrients, 2025 · checked 2026-09-23
8. ffd-r5-04 · systematic review and meta-analysis of observational studies
   pooling results of cohort studies showed that junk food consumption is associated with a 16% increment in the odds of developing mental health problems (OR = 1.16, 95% CI: 1.07 to 1.24)
   BMC Psychiatry, 2024 · checked 2026-09-23
9. ffd-r5-11 · systematic review and meta-analysis of cross-sectional studies
   Common risk factors were observed such as smoking (44%), eating fast food (43%), spicy food (34%), NSAID use (31%), and drinking tea or coffee (32%).
   Saudi Med J, 2025 · checked 2026-09-23
10. ffd-r5-09 · systematic review and meta-analysis of prospective studies
    pre-pregnancy intake of fried food, fast food, red and processed meat, heme iron and a low-carbohydrate dietary pattern was positively associated with the risk of gestational diabetes mellitus (GDM) (all P < 0.05)
    Crit Rev Food Sci Nutr, 2023 · checked 2026-09-23
11. ffd-r5-10 · longitudinal
    In conclusion, we found no evidence for longitudinal associations between FFOs count and inflammatory markers in this study.
    Nutr Metab Cardiovasc Dis, 2026 · checked 2026-09-23