24.4 — What a Healthy Diet Actually Is

Nutrition science has a problem that physics does not.

You cannot randomise ten thousand people to eat one way for thirty years and check who dies. It would be unaffordable, unethical in places, and nobody would comply.

So most of what we know comes from observing populations over decades — which shows association and cannot cleanly establish cause, because people who eat more vegetables also tend to smoke less, exercise more and be wealthier.

And the short trials that can be randomised measure a blood marker over twelve weeks, not a heart attack over twenty years.

Which is why nutrition advice appears to change constantly, and why single studies reported as breakthroughs so often fail to replicate.

But some things have survived every method, every population and every decade of scrutiny. Those are worth knowing, and they are shorter than the arguments about them.

What is genuinely established

1. Eat plenty of vegetables and fruit.

The most consistent finding in all of nutrition. Around five portions a day is associated with substantially lower cardiovascular disease and mortality, with benefit continuing to around ten portions in the largest analyses.

2. Eat wholegrains rather than refined grains.

Consistently associated with lower cardiovascular disease, type 2 diabetes, colorectal cancer and mortality.

3. Eat legumes and nuts.

Around a handful of nuts daily is associated with meaningfully lower cardiovascular mortality, and legumes are among the most consistently beneficial foods studied.

4. Eat fish, particularly oily fish.

Around one to two portions a week.

5. Limit processed meat.

Classified by the WHO as a group 1 carcinogen — the same evidence category as tobacco, which describes how certain the link is, not how large the risk. Around 50 g a day is associated with an 18 percent increase in relative colorectal cancer risk, which in absolute terms is small but real.

Red meat is classified as probably carcinogenic, with weaker evidence.

6. Limit free sugars, particularly in drinks.

7. Limit salt. Reducing intake lowers blood pressure, and blood pressure drives stroke and heart disease (Chapter 18.2).

8. Limit ultra-processed foods (Chapter 24.3).

9. Replace saturated fat with unsaturated fat, not with refined carbohydrate.

10. Get enough fibre — 25 to 30 g a day.

11. Do not exceed your energy needs over time.

Every dietary pattern that has been shown to work contains most of that list. Which is the point: the patterns agree far more than their advocates do.

The patterns with real evidence

The Mediterranean diet

The best-evidenced dietary pattern in existence.

What it is: olive oil as the main fat; abundant vegetables, fruit, legumes, nuts and wholegrains; fish and seafood regularly; moderate dairy, mostly as yoghurt and cheese; little red and processed meat; and traditionally, wine with meals.

The evidence: the PREDIMED trial randomised over 7,000 people at high cardiovascular risk to a Mediterranean diet supplemented with either extra virgin olive oil or nuts, against a low-fat control.

It was stopped early because the Mediterranean groups had around 30 percent fewer major cardiovascular events.

The trial was retracted and republished after problems with randomisation at some sites were found, and the reanalysis reached the same conclusion. It remains the strongest randomised evidence for any diet.

Also associated with lower rates of type 2 diabetes, some cancers, depression and cognitive decline.

The DASH diet

Designed specifically to lower blood pressure.

Vegetables, fruit, wholegrains, low-fat dairy, lean protein, nuts and legumes; low in saturated fat, sugar and sodium.

The evidence: DASH lowered systolic blood pressure by around 6 mmHg in people with hypertension, and combining it with sodium restriction lowered it by around 11 mmHg — comparable to a single blood pressure medication.

Which is one of the clearest demonstrations that diet produces effects on the same scale as drugs.

Plant-based patterns

Vegetarian and vegan diets are associated with lower rates of heart disease, type 2 diabetes, obesity and some cancers.

They require attention to specific nutrients: B12 is non-negotiable; and iron, iodine, zinc, calcium, omega-3 and protein adequacy need thought (Chapter 24.2).

And the important qualification: a plant-based diet can be very unhealthy. Chips, white bread, sugar and refined snacks are all plant-based. Studies distinguishing healthful from unhealthful plant-based diets find opposite associations with disease. The plants have to be whole foods for the benefit to appear.

The Blue Zones

Regions with unusually high numbers of people living past 100: Okinawa, Sardinia, Ikaria, Nicoya, and Loma Linda.

What their diets share: mostly plants; legumes daily; wholegrains; nuts; little meat; little processed food; and moderate total intake.

And they share more than diet: daily natural physical activity, strong social ties, a sense of purpose, and low stress.

A caution worth including: some Blue Zone longevity claims have been challenged on the grounds of poor birth record-keeping and pension fraud in the relevant periods. The dietary and lifestyle observations remain interesting; the extreme longevity statistics deserve scepticism.

The contested questions, answered honestly

Low-carb versus low-fat for weight loss.

Trials comparing them, when protein and calories are matched, find similar average results with enormous individual variation.

The DIETFITS trial randomised over 600 people to healthy low-fat or healthy low-carb for a year and found no significant difference in average weight loss — and, notably, no ability to predict who would do better from genetics or insulin response.

The honest answer: the best diet is the one you can sustain, and adherence predicts outcome better than composition does.

Intermittent fasting.

Time-restricted eating, 5:2, and alternate-day approaches.

They work for weight loss to the extent that they reduce total calories. Trials comparing them against continuous calorie restriction, matched for calories, generally find similar results.

Some evidence suggests metabolic benefits beyond weight loss, and it is not yet conclusive.

Useful as a tool for people who find it easier to restrict a window than to moderate every meal. Not magic.

Breakfast.

"The most important meal of the day" was a marketing slogan. Trials of adding or removing breakfast find little effect on weight. Eat it if you want it.

Meal frequency.

The idea that frequent small meals boost metabolism is not supported. Total intake matters; frequency largely does not.

Eggs.

Removed from restricted lists in most guidelines (Chapter 24.1). An egg a day is fine for most people.

Coffee.

Consistently associated with lower all-cause mortality, and lower rates of type 2 diabetes, Parkinson's disease and liver disease. Three to four cups a day sits at the bottom of the mortality curve in most analyses.

Caveats: it affects sleep for hours (Chapter 20.11), it can raise blood pressure acutely, and unfiltered coffee raises cholesterol slightly because of compounds that filters remove.

Dairy.

More neutral than either its critics or defenders claim. Fermented dairy — yoghurt and cheese — is associated with neutral or favourable outcomes despite its saturated fat, which is one reason the food matters as much as the nutrient.

Gluten.

Genuinely harmful in coeliac disease, which affects around 1 percent (Chapter 9.3). Non-coeliac gluten sensitivity exists and is real for some people, though blinded challenge studies suggest that FODMAPs in wheat are the trigger for many.

For everyone else, avoiding gluten confers no benefit and frequently means eating more refined gluten-free products with less fibre.

Detoxes and cleanses.

Your liver and kidneys do this continuously and do not require assistance (Chapters 9.6, 10.3). No detox product has ever been shown to remove any specific toxin.

Alkaline diets.

Blood pH is tightly regulated between 7.35 and 7.45 and cannot be changed by food (Chapter 8.5). The premise is false. The diets recommend vegetables, which helps, for reasons unrelated to pH.

Superfoods.

No such category exists. The term is marketing and is restricted as a health claim in the EU. Blueberries are good. So are frozen peas.

Weight

Energy balance, honestly

Weight change follows energy in versus energy out. That is thermodynamics and it is not in dispute.

And it is a poor guide to what to do, because both sides of the equation adjust.

When you lose weight: basal metabolic rate falls, more than expected from the smaller body size; leptin drops, increasing hunger; ghrelin rises; and non-exercise movement decreases without anyone deciding to move less.

Which is why maintaining weight loss is harder than achieving it, and why "just eat less" fails so consistently as advice.

It is not a failure of willpower. It is a defended system responding as it evolved to.

What has evidence

Protein and fibre, for satiety.

Whole foods over ultra-processed — the controlled feeding trial in Chapter 24.3 is the key evidence.

Any dietary pattern you can maintain.

Physical activity — more for maintaining loss than for producing it, since exercise alone is a slow route to weight change.

Sleep — short sleep increases hunger hormones and appetite measurably (Chapter 24.7).

Self-monitoring, which is among the more consistent predictors of success.

Support and structure, including structured programmes.

And medication and surgery where appropriate (Chapter 22.8), which have changed what is achievable in obesity treatment substantially.

What does not

Extreme restriction, which reliably produces rebound.

Cutting out food groups without reason.

Detoxes, teas and supplements.

And BMI as an individual measure. It is useful for populations and poor for individuals — it does not distinguish muscle from fat, and it performs differently across ethnic groups, with lower thresholds appropriate for South Asian populations. Waist circumference and waist-to-height ratio are better guides to metabolic risk, and a waist under half your height is a reasonable target.

Hydration

Around 6 to 8 glasses a day is the usual advice, and the number has no strong evidence behind it.

Much of your intake comes from food — fruit, vegetables and cooked grains all contain a lot of water.

Tea, coffee and other drinks count. The idea that caffeine's mild diuretic effect makes coffee dehydrating does not hold up at ordinary intakes.

A better guide than counting glasses: pale straw-coloured urine. Dark urine means drink more.

And drinking excessive water is not harmless — it can dilute blood sodium dangerously, which has killed marathon runners and people at music festivals (Chapter 23.6).

Food safety at home

And this belongs in a practical nutrition chapter, because it prevents a great deal of illness.

Wash hands before handling food and after handling raw meat.

Separate raw and cooked — separate boards, and store raw meat on the bottom shelf so nothing drips onto ready-to-eat food.

Cook thoroughly. Poultry, mince, burgers, sausages and pork all the way through — whole cuts of beef and lamb can be pink inside because contamination is on the surface, whereas mincing distributes it throughout.

Chill promptly. Fridge at or below 5 degrees. Cool leftovers and refrigerate within 1 to 2 hours.

Do not reheat rice that has been left at room temperature — Bacillus cereus spores survive cooking and produce a toxin that reheating does not destroy. Cool rice quickly and refrigerate it.

Reheat thoroughly, once.

Respect use-by dates on high-risk foods (Chapter 24.3).

And people who are pregnant, very young, elderly or immunosuppressed should avoid unpasteurised dairy, raw or undercooked eggs and meat, and certain soft cheeses and pâtés — because listeria and toxoplasma are far more dangerous in those groups.

The whole thing in one paragraph

Eat mostly whole foods. Plenty of vegetables, fruit, legumes, wholegrains and nuts. Some fish. Some or no meat, and little processed meat. Unsaturated fats — olive oil, nuts, oily fish. Not much added sugar, not much salt, not much ultra-processed food. Enough protein, more as you age. Enough fibre. Drink water. Do not eat more energy than you use. And eat in a way you could still be eating in ten years, because a diet that ends is a diet that did not work.

That is essentially all of it, and it has not changed much in fifty years despite everything that has been written since.

What the next page fixes

Chapter 24.5 covers what exercise actually does to the body — the physiology of why movement is the closest thing to a universal medicine.