The Health Effects of “Processed Foods” and Why Nutrition is More Important than the Amount of “Processing”

In the endless debates about the health effects of various approach to feeding our canine and feline companions, the subject of “processed foods” or “ultra-processed foods” comes up often. Generally, the argument is made that traditional commercial pet foods, including canned but most especially extruded dry foods (aka kibble), are “ultra-processed” and are functionally equivalent to potato chips and sliced lunch meat. Since the evidence is pretty consistent that convenience foods, packaged snack foods, and most “fast-foods” are associated with increased health risks in humans, the conclusion is that these traditional commercial foods must be unhealthy for our pets.

This is a superficially logical argument, but it is also convenient for proponents of various unscientific approaches to animal health because it fits nicely with the “appeal to nature fallacy” many of them are committed to. That is the idea that something which appears “natural” to them is healthier than something which seems artificial or unnatural.

This is, of course, an arbitrary and nearly meaningless aesthetic judgment, not a rational or scientific categorization. However, since the health risks of ultra-processed foods for humans, and the apparent equivalence of traditional pet diets with such foods, reinforce an existing bias, this is taken as a strong argument for alternative diets that present the appearance of being more “natural: and more like the less processed “whole foods” widely recommended for humans. 

In these discussions, I have tried many times to point out a clear and gaping hole in this reasoning—just because commercial pet foods come in a bag or a can, doesn’t mean they are all the same or that they are equivalent to human snack foods. Potato chips, lunch meats, and fast foods are intentionally designed to be appealing, in terms of appearance, taste, sensory experience (e.g. mouth feel and smell), and low cost. They are not meant to be nutritionally substantive or “healthy” in any way.

This is quite different from pet foods, which are deliberately formulated to be nutritionally appropriate and to support long-term health. The “processing” is not the real problem with “processed foods,” it is the excesses of salt, calories, and harmful fats and the absence of micronutrients, fiber, and other dietary components that make such foods a health risk.

A recent research article and accompanying editorial make this point quite clearly.

Fang Z, Rossato SL, Hang D, Khandpur N, Wang K, Lo CH, Willett WC, Giovannucci EL, Song M. Association of ultra-processed food consumption with all cause and cause specific mortality: population based cohort study. BMJ. 2024 May 8;385:e078476. 

This study evaluated diet and health outcomes in tens of thousands of North American healthcare workers followed for over thirty years. Here are some key findings:

  • People with consumption of ultra-processed foods (UPF) in the highest 25% had an increase in all-cause mortality of 4% compared with people in the lowest 25% of UPF consumption. This is an absolute difference of 64 deaths per 100,000 person years (1472 compared with 1536 deaths per 100,000 person years).The worst diet led to a real, but very small difference in mortality rate compared to the best diet.
  • This difference was a little larger when considering non-cancer and non-cardiovascular causes of death specifically: 9%
  • There was no difference in the risk of death from cancer or cardiovascular disease between the highest and lowest consumption of UPF.

All of this supports the existing evidence that UPF represent a health risk, but it also puts this in perspective. The risk is relatively small, much smaller than other known risk factors such as smoking, drinking excessively, or driving without a seat belt.

Even more telling, “this association was no longer apparent after overall diet quality was taken into account.” This means that there was no detectable difference between those who ate a lot or only a little UPF when the overall quality of the diet was accounted for. Again, the problem with UPF is not the “processing” but the unhealthy nutritional composition, and people who ate a generally healthy diet did not significantly increased their risk of dying if they included some UPF. 

This point is further reinforced by the finding that the mortality risk difference was greater when alcohol was included as a UPF, and it was less when whole-grain bread products were included as a UPF. The health risks were affected by the nutritional quality and the other risk factors associated with a food (such as the toxic nature of ethyl alcohol) regardless of whether both items were equally “highly processed.”

The specific types of food most closely associated with increased mortality risks were meat products and sweetened beverages (though, interestingly, drinks sweetened with sugar had a slightly stronger negative effect that artificially sweetened, again undermining the “natural is better” argument). 

The authors state their conclusion based quite clearly, and it reflects the point I have been making for years now:

Our data together suggest that dietary quality has a predominant influence on long term health, whereas the additional effect of food processing is likely to be limited.

Because the nutritional composition and overall diet quality is what matters, not the level of processing, the implications of this for diet recommendations is clear:

Limiting total ultra-processed food consumption may not have a substantial influence on premature death, whereas reducing consumption of certain ultra-processed food subgroups (for example, processed meat) can be beneficial.

The accompanying editorial expand son this point:

Not all ultra-processed food needs to be universally restricted and…careful deliberation is needed when considering whether to include recommendations about ultra-processed food in dietary guidelines. In countries where affordable, mass produced packaged wholegrain products such as breads are a recommended dietary staple and a major source of fibre, adding a sweeping statement in dietary guidelines about avoiding ultra-processed foods is not helpful.

As veterinary professionals and pet owners, these results reinforce the point that we should not obsess about the “processing” involved in the making of the pet foods we use. While inclusion of some fresh or whole foods may well turn out to have some health benefits, it is far more important that making sure our pets are eating complete and balanced diets that meet their nutritional needs, regardless of the form they take.

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2 Responses to The Health Effects of “Processed Foods” and Why Nutrition is More Important than the Amount of “Processing”

  1. A skeptic cat lady says:

    Dr. McKenzie, I spent two hours on your website looking for your opinion on high-meat vs “regular amount” meat food for cats. As cats are obligate carnivores, some people feed them all meat diets (homemade and commercial). I see you are against raw food, not against the grain but is there evidence that high meat and only meat is better for cats? Also, what is the evidence that overweight cats should be fed high-meat diets (even assuming they are nutritionally balanced)? Especially, as you are interested in aging, high-protein diets are not recommended for healthy longevity in humans.

  2. skeptvet says:

    You raise several issues that are commonly problems in making choices about nutrition for our pets.

    The first is that we and our pets need nutrients, not ingredients. Saying a diet is “high-meat” tells me almost nothing about its nutritional content or potential health effects. If a diet has adequate amounts of specific amino acids and nitrogen or if it has excesses or deficiencies requires a more detailed understanding of the nutritional composition- what kind of meat, what cuts, how processed, what other ingredients, what supplements, by weight or as a % of metabolizable energy, etc. It also requires understanding the age, lifestyle, health status, and other variables associated with the cat. So I can’t really give an opinion on “high-meat” diets in general because it is a pretty meaningless classification. I will say, though, that diets advertised as such are likely to be problematic because the advertising strategies uses on an unreliable and unscientific way of characterizing the diet, which doesn’t suggest that company takes a very informed or serious approach to nutrition.

    An “all-meat” diet usually means a diet of exclusively muscle tissue and organ meats. This is absolutely a deficient, unbalanced and unhealthy diet for cats if efforts aren’t made to provide nutritional adequacy and balance in some way (usually through supplementation), so I definitely wouldn’t recommend such diets. Clinical cases of cats with serious health problems have been reported on such diets.

    As for the impact of protein on aging, that too is a quite complex subject. While restriction of total protein, and even specific amino acids, has been shown to extend lifespan in some strains of inbred lab mice, it is also likely that older people and animals required higher overall protein levels, or more highly digestible protein sources, to reduce the development of serious age-associated problems such as sarcopenia. The optimal type and amount of protein for cats of different ages, much less for individual cats, isn’t known. And, again, the “meat content” of the diet on the ingredient list or advertising isn’t really how we should evaluate this anyway.

    Overall, I think it’s reasonable to suspect cats need a higher protein content in the diet, and a different range of specific amino acids, than dogs or humans, but how much of what is not certain, and the source of that protein is likely a lot less important.

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