Construct validity asks whether a score actually measures the thing it claims to. Here the claim being tested is that the Planetary Health Diet Index captures a healthy diet, and the test is whether it tracks independent yardsticks of a good diet more closely than rival scores do.
Drawn from standard methodology, not from this paper.
The Planetary Health Diet Index was developed to measure adherence to the diet proposed by the EAT-Lancet Commissions. The authors note that its construct validity for accurately reflecting a healthy diet, as compared against other global dietary measures, remains to be evaluated.
Drawn from the stated aims of the paper.
Quantitative 24 hour recall and food record data from 152,004 non-pregnant people aged 15 and over, drawn from 45 surveys in 26 countries, were scored four ways: the Planetary Health Diet Index, a food group diversity score, and the positive and negative sub-metrics of the Global Diet Quality Score. Each was then tested against independent reference measures of a healthy diet, chiefly the mean adequacy ratio for nutrients and the proportion of energy coming from ultra-processed food.
The Planetary Health Diet Index performed as designed. A one standard deviation increase was associated with a higher mean adequacy ratio, by 5.49 percentage points (95 percent CI 3.65 to 7.33) in low income countries falling to 1.41 (1.23 to 1.58) in upper middle income countries, with less unprocessed red meat and a lower share of energy from ultra-processed food.
The simpler instruments beat it on nutrient adequacy. The same one standard deviation increase in the food group diversity score was associated with a mean adequacy ratio 11.2 percentage points higher (95 percent CI 9.13 to 13.3) in low income countries and 8.57 (8.42 to 8.72) in upper middle income countries, roughly double the index it was compared against. The pattern repeated for fruit and vegetable intake.
Adherence to the Planetary Health Diet Index was highest in low and lower middle income countries and lowest in upper middle and high income countries, which is the reverse of how diet quality usually sorts by national income.
| Participants | 152,004 across 45 surveys in 26 countries |
| Mean adequacy ratio per 1 SD of the Planetary Health Diet Index | 5.49 pp (95% CI 3.65 to 7.33) low income, to 1.41 (1.23 to 1.58) upper middle income |
| Mean adequacy ratio per 1 SD of the food group diversity score | 11.2 pp (95% CI 9.13 to 13.3) low income, to 8.57 (8.42 to 8.72) upper middle income |
| Appropriate fruit and vegetable intake, RR per 1 SD of food group diversity score | 2.78 (2.26 to 3.41) lower middle income, to 1.63 (1.22 to 2.19) high income |
| Unprocessed red meat per 1 SD of the Planetary Health Diet Index | minus 10.0 g/day (minus 11.4 to minus 8.73) in low income countries |
| Energy from ultra-processed food per 1 SD of the Planetary Health Diet Index | minus 2.47 pp (minus 3.77 to minus 1.18) in lower middle income countries |
Not available The full discussion is not openly available and the abstract offers no mechanism, so we do not know what the authors think caused this.
Drawn from no open full text; the paper is an index validation rather than a mechanistic study.
A sustainability oriented diet index does track nutrient adequacy in the right direction, but a much simpler count of how many food groups someone eats tracks it roughly twice as strongly, across dozens of countries.
People aged 15 and over in 45 surveys across 26 countries, excluding pregnant women.
For anyone choosing a diet quality metric for population monitoring, it argues that the added complexity of the newer index buys less on the health side than the simpler alternatives, particularly where undernutrition is still a concern.
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