Wednesday 8 April 2015

The weird belief that people follow dietary guidelines

A study was published in Nutrition last week in support of the strange fantasy that Americans became obese because they slavishly followed government dietary guidelines. This is the premise of Nina Teicholz's recent book The Big Fat Surprise (see here for my review). Teicholz is thanked for her "support" at the end of the new study.

The revisionist history is that, from the 1970s, the US government told its citizens to abstain from fats and stuff their faces with carbohydrates. Americans dutifully complied and became obese as a result, because fats are wonderful and carbs are dreadful.

The evidence for this hypothesis comes from dietary data from the US National Health and Nutrition Examination Study (NHANES) which the Nutrition study cites:

Consumption of fats has dropped from 45% to 34% with a corresponding increase in carbohydrate consumption from 39% to 51% of total caloric intake. In addition, from 1971 to 2011, average weight and body mass index have increased dramatically, with the percentage of overweight or obese Americans increasing from 42% in 1971 to 66% in 2011.

For Teicholz and her supporters, this is proof that fat consumption has fallen by 25 per cent since 1971. But this is like saying that the British population has declined because the UK has a smaller share of the world's population today than it did fifty year ago. It is, to be blunt, a lie. What actually happened is Americans kept eating the same amount of fat but started eating more of other foods, mainly carbohydrates, thereby increasing their overall calorie intake which, combined with more sedentary lifestyles, led to world-beating levels of obesity.

The inconvenient fact that the amount of fat consumed has not fallen for over forty years is acknowledged in the study itself:

Since 1971, the shift in macronutrient share from fat to carbohydrate is primarily due to an increase in absolute consumption of carbohydrate as opposed to a change in total fat consumption.

... Daily fat consumption fell to 83 g in 1971, and remained at approximately the same level through 2011. In contrast, carbohydrate consumption, although basically at from 1965 to 1971 in terms of total calories, has risen to 278 g/d since 1965, an increase of 30.6%.

It is true that the US government recommended that dietary fat should make up less than 35 per cent of energy consumed, but it should go without saying that they wanted this to happen by people reducing the amount of fat they ate, not by increasing the amount of everything else they ate.

Americans abided by neither the letter nor the spirit of the guidelines, and yet the authors make the silly claim that...

These patterns and changes in consumption are consistent with the hypothesis that Americans on average adhered to the government dietary recommendations regarding fat, saturated fat, cholesterol, and carbohydrate.

You can read the USDA dietary guidelines for 1980 here. They are perfectly sensible, but it is a stretch to claim that Americans have been faithfully following them.

For a start, it says "use less of all sugars" and "eat less of foods containing these sugars", something that Americans conspicuously failed to do over the next twenty years. The rise in sugar consumption—which USDA plainly did not endorse—is one of the major causes of the rise in carbohydrate consumption.

USDA explicitly told people not to eat more simple carbohydrates like sugar because "they provide calories but little else in the way of nutrients." Insofar as it told people to eat more complex carbohydrates, it said "If you limit your fat intake, you should increase your calories from carbohydrates to supply your body's energy needs." USDA recommended that Americans "substitute starches for fats and sugars". That's substitute, not complement.

On the subject of "maintaining an ideal weight", USDA recommended that Americans "increase physical activity", "eat less fat and fatty foods" and "eat less sugar and sweets". Americans did none of this. They reduced physical activity, ate the same amount of fatty foods and ate more sugar. Unsurprisingly, they did not maintain an ideal weight.

Insofar as Americans were aware of the guidelines at all, they clearly did not "dutifully" follow them. The only aspect of the guidelines which they came close to meeting, albeit by accident, was a reduction in fat consumption as a percentage of total calorie consumption, but since this was entirely an artifact of increased calorie consumption, it is not what the USDA desired or recommended.

In sum, American nutritional guidelines were neither extreme nor dangerous (as the revisionists would have you believe), but even if they had been, it wouldn't have made any difference since Americans ignored them.

(See Carbsane for more of the problems with this study.)

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