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Reference topic 11: The Hijacked Reward System

Why we crave sugar: dopamine, reward and modern food

ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.67 sources5 graded recommendationsTopic hub: FoodTopic hub: People and loneliness

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In September 1981, near Dongara in Western Australia, two insect scientists watched jewel beetles (Julodimorpha bakewelli) cruising 1-2 m above the ground, "apparently males seeking the large flightless females." Twice, a male dropped onto a discarded beer bottle and tried to mate with it.1

The bottle was a "stubbie," a short brown beer bottle. Darryl Gwynne and David Rentz found beetles on other stubbies too, including ones they laid out themselves. "Once on the bottles the beetles did not leave unless displaced by us," they reported. One male was under attack from ants "biting at the soft portions of his everted genitalia." Nearby lay a dead male covered with ants.1

Mismatch card

The system

The reward system (dopamine-driven wanting, opioid and endocannabinoid liking) and the appetite it steers.

What it expects

Sugar that costs effort or comes wrapped in fruit, ethanol at a fraction of a percent, and food that takes time to chew.

What it gets

Ultra-processed food supplying 55% of American calories, and alcohol in concentrated drinks.44

What happens in the gap

Faster eating and about 500 kcal a day of extra intake in a controlled trial; 2.6 million alcohol-attributable deaths a year.4031

The handrail

Design the food environment: whole-food defaults, distance from snacks, smaller portions, home cooking, slower meals, less alcohol. (●●○)

Strength of evidence

●●○ Small, short trials agree that ultra-processed diets raise intake, but the reward-system mechanism is unproven and long-term evidence is mostly observational.

What this topic covers

  • ·Eggs Too Big to Sit On
  • ·Wanting Is Not Liking
  • ·The Surprise Signal
  • ·Sugar Came Wrapped
  • ·The Drunken Monkey
  • ·Engineering the Bliss Point
  • ·Twenty People in Bethesda

The practical version is in the book: Chapter 5, Eat.

The handrail

Topic 40 holds the full food program. Here, work on the environment, not on willpower:

If eating feels out of control, see a clinician: binge eating disorder is treatable. Rigid food rules can become a compulsion of their own (Topic 43).

Each recommendation carries the strength of the evidence behind it. How the grades work.

  1. Add friction

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Keep hyper-palatable snacks out of sight and reach, and make whole foods the easy option at home and at work. Moving food farther away reduced how much people ate (low-certainty evidence).61 A mostly healthier range of options raised the odds of a healthier pick 3.8-fold across income groups, in mostly online studies.62 A hospital cafeteria redesign cut calories per purchase 6.2% over two years.63 ●●○ for changing what is on offer; ●○○ for distance alone

  2. Swap whole meals

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Start with the meals you eat most often. On a metabolic ward, people ate about 500 kcal a day less on minimally processed menus than on ultra-processed ones. At home, over eight weeks, they lost about twice as much weight.4043 ●●○ Such meals can take more time and money: in Hall's trial their ingredients cost about 40% more.3940

  3. Cook more often

    EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.

    Eating home-cooked main meals more than five times a week, versus fewer than three, was associated with 28% lower odds of overweight in 11,396 UK adults.64 ●○○ (a snapshot, not a trial)

  4. Shrink portions and slow down

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Smaller portions, packages and plates reduce intake, modeled at 144-228 kcal a day if sustained across the whole diet.65 Eating more slowly lowers intake without leaving people hungrier over the next few hours.66 ●●○

  5. Drink water, and treat alcohol as optional

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Sweetened drinks are among the ultra-processed foods most consistently linked to harm,5152 and with alcohol, less is better and none is fine.32 ●●○ If you have been drinking heavily for a long time, do not stop abruptly without medical advice, because withdrawal can be life-threatening. Approved medicines and behavioral treatments exist.67

The skeptic's corner

The case against this topic, from the research itself.

"Ultra-processed" is a blunt category. French food specialists sorting the same foods into NOVA groups agreed poorly.46 Critics call the definition too broad to guide choices, and its links with obesity likely confounded: people who eat a lot of it differ in other ways too.47 A US advisory committee graded the evidence tying ultra-processed diets to body fat as "Limited."48 Yet the 2025-2030 US dietary guidelines told Americans to "Avoid highly processed" foods that are salty or sweet.49

Most of the health evidence is weak. An umbrella review (a review of earlier pooled analyses) linked ultra-processed food to 32 of 45 health outcomes. But it rated the evidence low or very low in quality for all but four.50 Over three decades of US follow-up, the highest quarter of intake carried only 4% higher mortality and no excess cancer or cardiovascular deaths. Processed meat products showed the strongest links, and sweetened drinks were also implicated.51 In one large European cohort, ultra-processed breads, cereals and plant-based alternatives were not linked to cancer plus heart and metabolic disease.52 The label lumps soda with sandwich bread. The trials are small and short, and independent experts noted "significant order-effects" in the English trial: its result depended partly on which diet came first.53

"Food addiction" is contested too, and psychiatry's standard diagnostic manual does not list it. The widely quoted figure of 14% of adults pools hundreds of studies. All used a self-report questionnaire that applies the criteria for addiction to alcohol and other drugs to eating.54 A national survey of older Americans found 12.4%.55 Some researchers prefer "eating addiction," judging the evidence for an addictive substance in food poor.56

The dopamine story has weakened too. In a 2025 brain-scan study of 50 adults, much larger than earlier ones, people drank a fatty, sugary ultra-processed milkshake. Scans starting 30 minutes later found no significant dopamine release in the striatum, a hub of the reward system.57 An earlier, smaller study, scanning people as they sipped, had reported release both at first taste and after swallowing.58 Even dopamine's prediction-error account is disputed: one study in mice challenged it, and another found a signature it had long predicted.5960

As for the drunken monkey, the chimpanzee measurements answer early doubters who said wild primates do not eat fermented fruit.2628 But the ancient enzyme shows only that apes adapted to break down ethanol, not that they evolved to crave it.

Terms this topic defines

Related topics

Sources cited above

  1. 1.Gwynne, D.T., and Rentz, D.C.F. (1983). "Beetles on the bottle: male buprestids mistake stubbies for females (Coleoptera)." Journal of the Australian Entomological Society, 22(1), 79-80. https://doi.org/10.1111/j.1440-6055.1983.tb01846.x (observations on 12 and 13 September 1981 at a site 12 km southeast of Dongara; the stubbie is a 370 mL bottle; two more stubbies were found with beetles on them, and beetles found two of four stubbies laid on open ground within 30 minutes; six males in all were seen mounting bottles; the authors wrote that "rows of regularly spaced, small tubercles around the base of the bottles reflect light in a similar way to punctations on the elytra")
  2. 2.Improbable Research (2011). Ig Nobel Prize winners (2011, Biology). https://improbable.com/ig/winners/
  3. 46.Braesco, V., Souchon, I., Sauvant, P., Haurogné, T., et al. (2022). "Ultra-processed foods: how functional is the NOVA system?" European Journal of Clinical Nutrition, 76(9), 1245-1253. https://doi.org/10.1038/s41430-022-01099-1 (Fleiss' kappa 0.32 to 0.34, where 1 is perfect agreement)
  4. 47.Gibney, M.J., Forde, C.G., Mullally, D., and Gibney, E.R. (2017). "Ultra-processed foods in human health: a critical appraisal." American Journal of Clinical Nutrition, 106(3), 717-724. https://doi.org/10.3945/ajcn.117.160440; and Astrup, A., and Monteiro, C.A. (2022). "Does the concept of 'ultra-processed foods' help inform dietary guidelines, beyond conventional classification systems? Debate consensus." American Journal of Clinical Nutrition, 116(6), 1489-1491. https://doi.org/10.1093/ajcn/nqac230
  5. 48.2025 Dietary Guidelines Advisory Committee and Nutrition Evidence Systematic Review team (2024). Dietary Patterns with Ultra-Processed Foods and Growth, Body Composition, and Risk of Obesity: A Systematic Review. USDA. NCBI Bookshelf NBK611035. https://www.ncbi.nlm.nih.gov/books/NBK611035/
  6. 49.US Department of Health and Human Services and US Department of Agriculture (2026). Dietary Guidelines for Americans, 2025-2030 (released 7 January 2026). https://cdn.realfood.gov/DGA.pdf
  7. 50.Lane, M.M., Gamage, E., Du, S., Ashtree, D.N., et al. (2024). "Ultra-processed food exposure and adverse health outcomes: umbrella review of epidemiological meta-analyses." BMJ, 384, e077310. https://doi.org/10.1136/bmj-2023-077310 (GRADE: 22 analyses low quality, 19 very low, 4 moderate). (32 of 45 outcomes associated; evidence low or very low quality in 41 of 45 analyses)
  8. 51.Fang, Z., Rossato, S.L., Hang, D., Khandpur, N., et al. (2024). "Association of ultra-processed food consumption with all cause and cause specific mortality: population based cohort study." BMJ, 385, e078476. https://doi.org/10.1136/bmj-2023-078476 (74,563 women and 39,501 men; median follow-up 34 and 31 years).
  9. 52.Cordova, R., Viallon, V., Fontvieille, E., Peruchet-Noray, L., et al. (2023). "Consumption of ultra-processed foods and risk of multimorbidity of cancer and cardiometabolic diseases: a multinational cohort study." Lancet Regional Health - Europe, 35, 100771. https://doi.org/10.1016/j.lanepe.2023.100771
  10. 53.Science Media Centre (2025). Expert reaction to Dicken et al. (2025), 4 August 2025, including comments from Nerys Astbury (University of Oxford) and Kevin McConway (Open University). https://www.sciencemediacentre.org/expert-reaction-to-randomised-crossover-trial-of-ultra-vs-minimally-processed-foods-that-follow-healthy-diet-guidelines-and-weight-loss/
  11. 54.Gearhardt, A.N., Bueno, N.B., DiFeliceantonio, A.G., Roberto, C.A., et al. (2023). "Social, clinical, and policy implications of ultra-processed food addiction." BMJ, 383, e075354. https://doi.org/10.1136/bmj-2023-075354 (summarizing 281 studies from 36 countries measured with the Yale Food Addiction Scale, which assesses the 11 DSM-5 criteria for substance use disorder; the article notes that food addiction is not a DSM-5 diagnosis).
  12. 55.Loch, L.K., Kirch, M., Singer, D.C., Solway, E., et al. (2026). "Ultra-processed food addiction in a nationally representative sample of older adults in the USA." Addiction, 121(3), 510-521. https://doi.org/10.1111/add.70186 (nationally representative; Americans aged 50 to 80)
  13. 56.Hebebrand, J., Albayrak, Ö., Adan, R., Antel, J., et al. (2014). "'Eating addiction', rather than 'food addiction', better captures addictive-like eating behavior." Neuroscience and Biobehavioral Reviews, 47, 295-306. https://doi.org/10.1016/j.neubiorev.2014.08.016
  14. 57.Darcey, V.L., Guo, J., Chi, M., Chung, S.T., et al. (2025). "Brain dopamine responses to ultra-processed milkshakes are highly variable and not significantly related to adiposity in humans." Cell Metabolism, 37(3), 616-628.e5. https://doi.org/10.1016/j.cmet.2025.02.002 (50 adults; PET began 30 minutes after the milkshake. The authors' preprint describes the earlier study as 10 adults who sipped milkshakes during the scan).
  15. 58.Thanarajah, S.E., Backes, H., DiFeliceantonio, A.G., Albus, K., et al. (2019). "Food intake recruits orosensory and post-ingestive dopaminergic circuits to affect eating desire in humans." Cell Metabolism, 29(3), 695-706.e4. https://doi.org/10.1016/j.cmet.2018.12.006
  16. 59.Jeong, H., Taylor, A., Floeder, J.R., Lohmann, M., et al. (2022). "Mesolimbic dopamine release conveys causal associations." Science, 378(6626), eabq6740. https://doi.org/10.1126/science.abq6740
  17. 60.Amo, R., Matias, S., Yamanaka, A., Tanaka, K.F., et al. (2022). "A gradual temporal shift of dopamine responses mirrors the progression of temporal difference error in machine learning." Nature Neuroscience, 25(8), 1082-1092. https://doi.org/10.1038/s41593-022-01109-2
  18. 26.Sanders, R. (2025). "In the wild, chimps likely ingest the equivalent of several alcoholic drinks every day." Berkeley News, University of California, Berkeley, 17 September 2025. https://news.berkeley.edu/2025/09/17/in-the-wild-chimps-likely-ingest-the-equivalent-of-several-alcoholic-drinks-every-day/ (news release on Maro et al. 2025; it also describes Dudley's 2014 book The Drunken Monkey and the skepticism the hypothesis first met). (many scientists, primatologists especially, first doubted the hypothesis)
  19. 28.Maro, A., Sandel, A.A., Blaiore, B.Z.A., Wittig, R.M., et al. (2025). "Ethanol ingestion via frugivory in wild chimpanzees." Science Advances, 11(38), eadw1665. https://doi.org/10.1126/sciadv.adw1665 (an estimated 14 g of ethanol a day)
  20. 61.Hollands, G.J., Carter, P., Anwer, S., King, S.E., et al. (2019). "Altering the availability or proximity of food, alcohol, and tobacco products to change their selection and consumption." Cochrane Database of Systematic Reviews, 9(9), CD012573. https://doi.org/10.1002/14651858.CD012573.pub3
  21. 62.Pechey, R., Hollands, G.J., Reynolds, J.P., Jebb, S.A., et al. (2022). "Is altering the availability of healthier vs. less-healthy options effective across socioeconomic groups? A mega-analysis." International Journal of Behavioral Nutrition and Physical Activity, 19(1), 88. https://doi.org/10.1186/s12966-022-01315-y (six studies, four online and two in laboratories; 7,375 participants).
  22. 63.Thorndike, A.N., Gelsomin, E.D., McCurley, J.L., and Levy, D.E. (2019). "Calories purchased by hospital employees after implementation of a cafeteria traffic light-labeling and choice architecture program." JAMA Network Open, 2(7), e196789. https://doi.org/10.1001/jamanetworkopen.2019.6789
  23. 40.Hall, K.D., Ayuketah, A., Brychta, R., Cai, H., et al. (2019). "Ultra-processed diets cause excess calorie intake and weight gain: an inpatient randomized controlled trial of ad libitum food intake." Cell Metabolism, 30(1), 67-77.e3. https://doi.org/10.1016/j.cmet.2019.05.008 (10 men and 10 women, 28 days each; ultra-processed meals eaten about 17 kcal a minute faster; a drive to meet protein needs could explain at most about half the intake gap; ingredients cost $106 a week against $151 for 2,000 kcal a day)
  24. 43.Dicken, S.J., Jassil, F.C., Brown, A., Kalis, M., et al. (2025). "Ultraprocessed or minimally processed diets following healthy dietary guidelines on weight and cardiometabolic health: a randomized, crossover trial." Nature Medicine, 31(10), 3297-3308. https://doi.org/10.1038/s41591-025-03842-0 (the UPDATE trial; weight fell 2.06% on the minimally processed diet and 1.05% on the ultra-processed one)
  25. 39.National Institutes of Health (2019). "NIH study finds heavily processed foods cause overeating and weight gain." News release, 16 May 2019. https://www.nih.gov/news-events/news-releases/nih-study-finds-heavily-processed-foods-cause-overeating-weight-gain
  26. 64.Mills, S., Brown, H., Wrieden, W., White, M., et al. (2017). "Frequency of eating home cooked meals and potential benefits for diet and health: cross-sectional analysis of a population-based cohort study." International Journal of Behavioral Nutrition and Physical Activity, 14(1), 109. https://doi.org/10.1186/s12966-017-0567-y
  27. 65.Hollands, G.J., Shemilt, I., Marteau, T.M., Jebb, S.A., et al. (2015). "Portion, package or tableware size for changing selection and consumption of food, alcohol and tobacco." Cochrane Database of Systematic Reviews, 2015(9), CD011045. https://doi.org/10.1002/14651858.CD011045.pub2
  28. 66.Robinson, E., Almiron-Roig, E., Rutters, F., de Graaf, C., et al. (2014). "A systematic review and meta-analysis examining the effect of eating rate on energy intake and hunger." American Journal of Clinical Nutrition, 100(1), 123-151. https://doi.org/10.3945/ajcn.113.081745
  29. 32.Zhao, J., Stockwell, T., Naimi, T., Churchill, S., et al. (2023). "Association between daily alcohol intake and risk of all-cause mortality: a systematic review and meta-analyses." JAMA Network Open, 6(3), e236185. https://doi.org/10.1001/jamanetworkopen.2023.6185 (also adjusted for study quality)
  30. 67.National Institute on Alcohol Abuse and Alcoholism. Understanding Alcohol Use Disorder. https://www.niaaa.nih.gov/publications/brochures-and-fact-sheets/understanding-alcohol-use-disorder
  31. 44.Williams, A.M., Couch, C.A., Emmerich, S.D., and Ogburn, D.F. (2025). Ultra-processed Food Consumption in Youth and Adults: United States, August 2021-August 2023. NCHS Data Brief No. 536. National Center for Health Statistics. https://www.cdc.gov/nchs/products/databriefs/db536.htm (2021-2023: 55.0% of calories for Americans aged 1 and over, 53.0% for adults, 61.9% for ages 1 to 18)
  32. 31.World Health Organization (2024). "Over 3 million annual deaths due to alcohol and drug use, majority among men." News release, 25 June 2024. https://www.who.int/news/item/25-06-2024-over-3-million-annual-deaths-due-to-alcohol-and-drug-use-majority-among-men (the 2.6 million deaths were 4.7% of all deaths)

This topic carries 67 notes in the research manuscript; the 32 above are those behind the material on this page. Full source register.