Meat and Fish

TL;DR. Processed meat is classified by the WHO's cancer agency as a Group 1 carcinogen and red meat as Group 2A, and both classifications describe strength of evidence, not size of risk: the absolute increase in colorectal cancer from 50 g of processed meat a day is around 18 percent relative, which is roughly one extra case per 100 people over a lifetime. Poultry and fish carry no comparable signal. Oily fish twice a week is one of the better-supported dietary recommendations there is, and the mercury caution applies to a specific short list of large predatory species, mainly in pregnancy.

1. What is in it

Per 100 g cooked:

What meat and fish supply that plants do not, or do less well:

• Vitamin B12, which is absent from plants entirely (Chapter 15).
• Haem iron, absorbed at 15 to 35 percent versus 2 to 20 percent for plant iron, and largely unaffected by tea, phytate, or calcium (Chapter 16).
• Zinc, in a more absorbable form than plant zinc.
• EPA and DHA, from oily fish, since plant ALA converts poorly (Chapter 13).
• Complete, highly digestible protein with DIAAS scores above 1.0.
• Creatine, carnosine, taurine, present only in animal foods; levels are lower in vegetarians and the practical significance is debated.

Liver is an extreme outlier in nutrient density: exceptional B12, vitamin A, iron, copper, and folate. It is also too rich in vitamin A for pregnancy: preformed retinol at these levels is teratogenic, and pregnant women are advised to avoid liver and liver products entirely (Chapter 15).

2. The processed and red meat classifications, explained properly

In 2015 the International Agency for Research on Cancer classified:

• Processed meat as Group 1: carcinogenic to humans.
• Red meat as Group 2A: probably carcinogenic to humans.

The headlines that followed ("bacon as bad as asbestos") were wrong, and the reason is worth understanding because it recurs throughout this book.

IARC groups describe how confident we are that something can cause cancer, not how much cancer it causes. Group 1 contains tobacco smoking, asbestos, plutonium, solar radiation, alcohol, and processed meat, because in all of them the evidence that they can cause cancer is strong. The magnitudes differ by orders of magnitude. This is the hazard versus risk distinction (Chapter 97).

The actual numbers:

• 50 g of processed meat a day (roughly two rashers of bacon or one hot dog) is associated with an 18 percent relative increase in colorectal cancer risk.
• Lifetime colorectal cancer risk is roughly 5 to 6 percent in the UK. An 18 percent relative increase raises that to about 6 to 7 percent: roughly one additional case per 100 people.
• Smoking raises lung cancer risk by roughly 2,000 percent.

Red meat, at 100 g a day, is associated with roughly a 17 percent relative increase in colorectal cancer, with weaker evidence.

The proposed mechanisms are reasonably well characterised:

• Haem iron catalyses the formation of N-nitroso compounds in the colon.
• Nitrites and nitrates added to cured meat react with amines to form nitrosamines. This is the same ion that is beneficial in vegetables, where vitamin C and polyphenols block nitrosation (Chapter 38). Context, not the ion.
• High-temperature cooking produces heterocyclic amines and polycyclic aromatic hydrocarbons (Chapter 91).
• What meat displaces: fibre, pulses, vegetables.

Beyond cancer, processed meat is consistently associated with higher cardiovascular disease and type 2 diabetes risk, and the association is stronger and more consistent than for unprocessed red meat. High sodium content is likely part of it.

Poultry and fish carry no comparable signal, and substitution analyses generally find replacing red and processed meat with poultry, fish, pulses, or nuts is associated with lower mortality.

A note on the dissent. A 2019 series in Annals of Internal Medicine concluded that adults could continue current red meat consumption, on the grounds that the evidence certainty is low. It caused a major controversy. The disagreement was largely about how to weigh low-certainty observational evidence when making population recommendations rather than about what the studies found. The associations themselves are not in serious dispute; their size and certainty are.

3. Fish: the case for, and the mercury caution

The case for oily fish is unusually good. Two portions a week, one of them oily, is recommended by most national bodies. Cohort data consistently associate fish consumption with lower cardiovascular mortality, and the effect appears larger for eating fish than for taking fish oil supplements, which have performed inconsistently in trials (Chapter 13).

Oily fish (salmon, mackerel, sardines, herring, trout, anchovies, fresh tuna) supply EPA and DHA. White fish (cod, haddock, plaice, pollock) supply protein, iodine, and selenium with very little fat.

Mercury accumulates up the food chain as methylmercury, which is neurotoxic and crosses the placenta. Concentration tracks size, lifespan, and trophic level.

Pregnancy specifics (UK guidance; others differ in detail): no more than two portions of oily fish a week; no more than four medium tins or two fresh steaks of tuna a week; avoid shark, swordfish, and marlin; avoid raw shellfish; cook fish thoroughly.

The balance of the evidence favours eating fish, including in pregnancy: several large cohort studies, including ALSPAC, found that maternal fish consumption was associated with better child neurodevelopmental outcomes, and that avoiding fish was worse than the mercury risk, provided the high-mercury species are avoided. The advice is to choose the right fish, not to avoid fish.

Other fish considerations:

• Farmed versus wild salmon: farmed is generally higher in total omega-3 (more fat overall) and higher in omega-6; contaminant levels in farmed salmon have fallen substantially since the much-cited 2004 study and are now generally comparable. Environmental concerns about sea lice, escapes, and feed sourcing are real and separate.
• Sustainability: roughly a third of assessed fish stocks are overfished. MSC certification and national seafood guides are imperfect and better than nothing.
• Small oily fish (sardines, anchovies, herring, mackerel) are the best answer on almost every axis: high omega-3, low mercury, cheap, sustainable, and tinned versions with bones supply substantial calcium.

4. Cooking, and what it creates

Covered fully in Chapter 91, summarised here:

• Heterocyclic amines (HCAs) form from amino acids and creatine at high temperatures, especially above 150 °C on direct heat.
• Polycyclic aromatic hydrocarbons (PAHs) form when fat drips onto flame and smoke deposits on the food.
• Both are highest in charred, blackened, well-done grilled and barbecued meat.

Practical reductions, all evidence-based:

• Marinate. Marinades, especially acidic ones and those containing herbs and spices (rosemary, thyme, garlic), reduce HCA formation substantially, in some studies by 50 to 90 percent.
• Microwave or par-cook first, then finish on high heat briefly.
• Turn frequently, which reduces surface temperature peaks.
• Avoid direct flame and dripping fat, or use a drip barrier.
• Cut off the charred bits.
• Lower-temperature methods (braising, stewing, poaching, sous vide) produce essentially none.

5. Practical guidance

• UK advice: no more than 70 g a day of red and processed meat on average (about 500 g a week cooked weight), with less processed meat being better.
• Processed meat is the category to cut first, before unprocessed red meat.
• Two portions of fish a week, one oily. Tinned sardines, mackerel, and anchovies are the cheapest and among the best options.
• Substitute rather than simply remove. The health and environmental benefit comes from what replaces the meat: pulses, fish, poultry, nuts, and whole grains, not refined carbohydrate.
• Eat liver occasionally if you like it for iron and B12, and not in pregnancy.
• Food safety: cook poultry and pork thoroughly (75 °C or steaming hot throughout), do not wash raw chicken (it aerosolises campylobacter over the kitchen), use separate boards, and refrigerate leftovers within two hours. Campylobacter from chicken is the most common cause of bacterial food poisoning in the UK.
• Whole cuts of beef and lamb can be served rare because contamination is on the surface; minced meat and poultry cannot, because grinding distributes surface bacteria throughout.

6. The environmental dimension

This is where the numbers are largest, and it belongs here honestly.

Greenhouse gas emissions per 100 g of protein, approximate, from Poore and Nemecek's 2018 Science analysis:

Ruminants dominate because of enteric methane fermentation and land use. Reducing beef and lamb frequency is by a wide margin the single most effective dietary change for greenhouse emissions, larger than local sourcing, packaging, or organic choices (Chapter 9).

The variation within categories is also enormous: the highest-impact beef producers emit around 12 times the lowest. So "how it was produced" matters, and it matters less than "which animal."

7. The bottom line

• Meat and fish supply B12, haem iron, zinc, and complete protein efficiently, and oily fish supplies EPA and DHA that plants effectively cannot.
• Processed meat is IARC Group 1 and red meat Group 2A, which describes evidence strength, not risk size. 50 g a day of processed meat raises lifetime colorectal cancer risk by roughly one case per 100 people. That is real and it is not asbestos.
• Cut processed meat first. Substitute with fish, poultry, pulses, and nuts rather than with refined carbohydrate.
• Two portions of fish a week, one oily. Small oily fish (sardines, anchovies, mackerel, herring) win on omega-3, price, mercury, and sustainability at once.
• Mercury advice targets a short list of large predators. In pregnancy, avoiding fish altogether appears worse than eating the right fish.
• Marinate, avoid charring, and do not cook over dripping fat. Do not wash raw chicken.
• Beef and lamb dominate the environmental footprint of a diet by a wide margin.

👉 Next: fermented foods.