Tuesday, January 26, 2010

Principles for front of pack scoring systems

Several front-of-pack or supermarket-shelf nutrition labeling programs use scoring systems to rate food products.

Examples include the Hannaford supermarket chain's Guiding Stars system ...
... and the NuVal system used by some other supermarket chains.
The computation of the scores seems fairly arbitrary to me. It might help to start with some basic mathematical principles. Here are three principles that I think a food scoring system should satisfy.

1) The score should not depend on the serving size. A major limitation of all such scoring systems is that our health really depends on how much of each food we eat, and in what combination. Unfortunately, the scores necessarily apply to each food as it sits on the supermarket shelf, not as we consume it. Because the creators of the score have no knowledge of how much we eat, the score should be treated as a description of the density of good and bad nutrients per unit of weight or food energy. The score should not depend on the serving size convention used or the number of servings in a package. I have seen some scoring systems that appear at first glance to be independent of the serving size, but on closer inspection have quirky limits on total daily nutrients per serving that contribute to the score.

2) The score should rate mixed foods in a consistent way. Take the example of a ham and cheese sandwich. Suppose the sandwich is, by weight or by calories, 40% bread, 30% meat, and 30% cheese. The score for a packaged ham sandwich in the supermarket should be the same as a weighted average of the scores for bread, meat, and cheese purchased separately:

SandwichScore = 40%*BreadScore+30%*HamScore+30%*CheeseScore.

Without this principle, the scoring system will be biased in favor of or against manufactured mixed foods in place of separate ingredients. I think a good scoring system would have no such prejudice.

3) The score should rate each good and bad nutrient independently. Suppose adding 20% more salt to a high-fiber food reduces the score by 20%. How much should adding 20% more salt change the score for a low-fiber food? Many of us would say the score should again change by 20%. Unfortunately, existing scoring systems may have strange interactions across the good and bad nutrients, so that the effect of one nutrient on the score depends on the value of other nutrients in a way that the authors probably did not intend.

Currently, the scoring systems seem to me to have an ad hoc quality that makes it difficult to take the quantitative scores very seriously. Broadly speaking, a scoring system may seem to work correctly in giving healthy foods a good score, and unhealthy foods a bad score, but consumers understood those broad outlines of their food options already even without a scoring system. The whole purpose of a scoring system is to add quantitative rigor to the information provided. They may have a long way to go.

In other recent reading on front-of-pack labeling, see the recent series by Timothy Lytton at the Fooducate blog. In general, Lytton suggests that FDA should not get into the business of developing its own front-of-pack system, but instead should just enforce existing rules against making implicit healthy claims for foods that fail to meet FDA's definition of "healthy." One exception to Lytton's hands-off recommendation is that he feels there may be a need for stronger regulation of the complex scoring systems.


Chris Wage said...

Am I being myopic or old-fashioned by thinking that these various score algorithms are a giant waste of time?

I used to joke with my friends counting calories that "the human body is not a bomb calorimeter" -- and likewise, diet choice can vary (with reason) from person to person for innumerable reasons.

Dumbing everything down to some sort of master "score" seems woefully misguided, at the expense of what really needs to happen -- education. If we can't take the (fairly minimal) time to educate ourselves about what to eat and why, it seems the cause is lost, whether or not everything is conveniently labelled with a cryptic number.

D. Watson said...

I have two concerns about your solutions. I talk about them more on my website, but basically I think the third point is just an implication of the second. If everything goes in linearly, then it's going in independently.

However, there is research that shows some nutrients interact to block absorption of each other. That would negate the linearity and independence assumptions. Also, cooking processes can affect ingredients differently, again confounding linearity so the final product would have different (usually less) nutrition than the ingredients.

We do need more consistency, and your proposals would be a step in the right direction. I'm not sure it's the ideal system, though.

Parke Wilde said...

Yes, perhaps some interactions across nutrients in the algorithm could be justified by real scientific understanding of nutritional interactions. But, that seems like a long-shot without known -- again -- the amount that people consume.

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Dr. David Katz said...

Dear Parke-

1) NuVal scores do NOT depend on serving size. Broccoli is highly nutritious, and scores 100. That is true of one florette, or three...

That said, since portion certainly does matter, the scores include all of the nutrient properties known to influence satiety and the number of calories it takes to feel full. Thus, as a general rule, eating higher up the NuVal scale means eating foods that lead to fullness on fewer calories. Evidence from such sources as the National Weight Control Registry suggest that this, in fact, is the only sustainable approach to portion control "Just eat les" has never proven particularly useful, as you doubtless know.

2) NuVal rates mixed foods in a consistent way. The score of a ham and cheese sandwich will be based on the proportional contributions of each of its ingredients to the calories in the total 'recipe.' Thus, the system can and does score any food, any recipe, or any meal in a consistent fashion. There is a more subtle conversation to have about the relative merits of 'weighting' by calories or number of servings, but this is probably not the place for that. In a derivative of the original NuVal algorithm designed for scoring total diet quality (and now in the testing phase), there is a clear advantage of weighting by number of servings.

3) NuVal rates each good and bad nutrient independently. That said, if you vary a single nutrient- say sodium- the impact on the final score will depend somewhat on what is in the algorithm for that to begin with. If, for example, you take an item like guacamole, which has many nutrient in its algorithm, and vary its salt content- the net effect on final score is apt to be different than if you added comparable salt to something much less nutritious, say a cheese doodle. The score is adjusted independently, but there is a simple mathematical and mechanical reality here: adding a single gallon of gasoline to an empty tank makes a more significant proportional difference than adding the final gallon to a tank that is nearly full.

I note, in addition, that each nutrient in NuVal is weighted based on the epidemiology literature based on the health conditions with which it is linked (were nutrients not associated with health outcomes, there would be no reason to care about them in the first place!), and specifically for the prevalence of the condition(s); the severity of the condition(s); and for the strength of association between the nutrient and the condition(s). Thus, trans fat gets a greater penalty than dietary cholesterol, for example. This principle is just as important as rating nutrients independently- rating them based on their actual, relative importance to health.

Happy to engage in further discussion, or refer you to more detailed print materials.

All best,
David L. Katz, MD, MPH, FACPM, FACP
Director, Prevention Research Center
Yale University School of Medicine
Principal Inventor, NuVal algorithm