Finns who ate closer to the 'planetary health diet' had slightly smaller waists, and a blood test offers a reason why. The average score was under 4 out of 13, and over seven years the diet did not predict who gained weight

The planetary health diet was drawn up in 2019 by a commission of scientists asking what ten billion people could eat without exhausting the land and water that feed them. Their answer was a plate heavy on whole grains, vegetables, fruit, beans and nuts, with small amounts of meat and dairy. Since then a second question has followed it around: is the diet that is kind to the planet also kind to the waistline? Several studies say people who eat closer to it weigh less. A paper from the Finnish Institute for Health and Welfare, published in the European Journal of Nutrition on 7 October 2026, tries to say why.

The team took two national health surveys, FinHealth 2017 and a 2007 study called DILGOM, and pooled 9,225 adults with an average age of 54. Each had filled in a questionnaire on what they ate over the past year, been weighed and measured, and given blood. The diet was scored out of 13: one point each for enough whole grains, vegetables, fruit and berries, legumes, and nuts and seeds; one each for keeping potatoes, dairy, red and processed meat, poultry, eggs, fish and sugar under a limit; and one for a high ratio of unsaturated to saturated fat. The blood went through a machine that reads 158 substances at once, mostly fats and the particles that carry them, plus some amino acids.

First, the diet and the tape measure. Each extra point on the score went with a waist about 0.4 cm smaller in men and 0.5 cm smaller in women, after allowing for age, exercise, smoking and cholesterol medicines. For scale, the average waist in the study was 98 cm for men and 88 cm for women. Women with higher scores also weighed a little less; in men the link with weight and BMI did not hold up.

Then the blood. Of the 158 substances, seven tracked the diet score in both sexes: acetate, which gut bacteria make when they ferment fibre, and six measures of the kind of fat in the blood, such as the share that is polyunsaturated or saturated. When the researchers put these into the sums one at a time, the link between diet and waist shrank. The share of polyunsaturated fat appeared to account for 36% of it in men and 47% in women, linoleic acid's share for 31% and 50%. Acetate accounted for much less, 8% and 4%. Their conclusion is that better fat quality and more fibre may be part of how such a diet keeps waists smaller.

It is a reasonable idea, and the paper is frank that it is only suggested. We would add a few things a reader should know before repeating it. The first is how far these Finns were from the diet. The average score was 3.4 out of 13 for men and 4.1 for women. Men ate 162 g of red and processed meat a day against a limit of 28 g, 15 g of legumes against a target of 75 g, and 4 g of nuts against 25 g. So this is not a study of people on the planetary health diet. It compares people who meet three of its targets with people who meet five.

The second is the result that is easy to miss. Part of the group was measured again seven years later, and the diet score had no relationship at all with who gained or lost weight or waist in between. The estimates sit at zero. Everything the paper builds on comes from a single day: the diet, the waist and the blood were all recorded together. On one day's data nobody can tell which came first, and the authors say plainly that a larger waist can itself change what is in the blood.

The third is our own reading, not the authors'. One of the 13 points is awarded for the ratio of unsaturated to saturated fat in the diet. The blood measures that explained the most are the ratio of polyunsaturated and saturated fats in the blood. Blood fatty acids are a well-known mirror of the fats a person eats, which the paper itself notes. So when the blood measure 'explains' half the link, it may be showing the diet score a second time, more accurately than a questionnaire can, and not a new step between plate and waist. The measures also overlap heavily with each other, so the percentages cannot be added up.

The fourth is the repeat. The team ran everything again in 10,242 other Finns from two older surveys, which is good practice. There, the diet was linked to waist in women only, three substances carried the link, and each accounted for 5% to 8% of it. One of them, the omega-3 share, pointed the other way from the first group: by the paper's description it went with a larger waist. The authors put the weaker repeat down to slightly lower diet scores in those surveys. The abstract also says seven substances carried the link in both sexes, while the paper's own table shows the omega-3 share falling short of significance in men.

None of this makes whole grains, beans, nuts and vegetable oils a bad idea, and the case for them does not rest on this study. What this paper adds is modest: among adults eating a fairly ordinary Finnish diet, those a point or two closer to the planetary pattern had waists up to a centimetre smaller, and their blood showed the fats you would expect. Whether eating that way keeps weight off over time is the question the seven-year data were there to answer, and in this study they did not.