Obesity rates took off precisely when low-fat guidelines were adopted

The 1980 US Dietary Guidelines — the first time the government told Americans to eat less of something rather than enough of everything — recommended cutting saturated fat and cholesterol. Americans complied. Steak gave way to pasta, butter to margarine, eggs to muesli. The result was not improved health. Obesity, which had been creeping up slowly for decades, suddenly accelerated.

In the UK, calorific intake actually fell after the equivalent guidelines were adopted in 1983. Exercise levels rose over the following two decades. Neither fact is compatible with the "too many calories, too little exercise" explanation for the obesity explosion that followed.

The substitution was predictable. Energy comes from fat, carbohydrate, and protein; protein stays roughly stable regardless of diet. A low-fat diet is effectively a high-carbohydrate diet. The most versatile and palatable carbohydrate is sugar — the very substance Yudkin had already flagged. Food manufacturers filled the low-fat gap with sugar: low-fat yoghurts bulked up with it, cakes infused with transfats. The Lancet had warned of this risk in 1974: "The cure should not be worse than the disease."

Source claim: US and UK obesity rates were flat for decades, then spiked in the early 1980s — precisely coinciding with the adoption of low-fat dietary guidelines that drove high-carbohydrate, high-sugar substitution.