fitness · nutrition

The continuing carb-troversy: was Goldilocks right?

Prominent among the Questions Of Our Time is this one: just how many/much carbs should I have in my diet? Is it best to eat low-carb, like the Atkins Diet? Or stick to kinder/gentler/higher-fiber carbs like the South Beach Diet? Or close-to-no carbs like the Whole 30 Diet? What about higher carb diets? Don’t people use those during high-intensity athletic activity? Or is there some happy medium, some just-right amount of carbs that will optimize on my health, longevity, weight, complexion, etc.? Just where is Goldilocks when we need her?

Goldilocks enjoying some soup in a medium bowl, hopefully with quinoa instead of white rice or pasta if she knows what's good for her... :-)
Goldilocks enjoying some soup in a medium bowl, hopefully with quinoa instead of white rice or pasta if she knows what’s good for her… 🙂

A new study in the Lancet is now taking sides with Goldilocks, publishing a study saying that a moderate carb diet is in fact the “just right” amount for optimum longevity. Here’s a summary from the Live Science news website:

The study, which involved more than 15,000 Americans who were tracked for a quarter of a century, found that those who ate a low-carb diet (with less than 40 percent of daily calories coming from carbs) or a high-carb diet (with more than 70 percent of daily calories coming from carbs) were more likely to die during the study period, compared with those who ate a moderate-carb diet, with about 50 to 55 percent of their calories coming from carbs.

For example, based on the findings, the researchers estimated that from age 50, people who consumed a moderate-carb diet would have a life expectancy that was about four years longer than those who consumed a very low-carb diet (with an average life expectancy of 83 years for moderate-carb eaters versus 79 years for very low-carb eaters).

Another of their key findings was when they investigated different types of low-carb diets:  ones with a lot of animal-sourced foods, versus plant-sourced foods. Here’s what the Live Science article has to say about the study:

…the analysis also found that, with low-carb diets, what mattered was the source of proteins and fats. Diets that involved replacing carbs with proteins and fats from animal sources, including beef, lamb, pork, chicken and cheese, were linked with a greater risk of death. In contrast, diets that involved replacing carbs with proteins and fats from plant sources, such as vegetables, legumes and nuts, were tied to a lower risk of death.

So is the take-away message that we should all aim for moderate (50ish%) of carb intake in our diets to maximize lifespan?

Not exactly.  As I say all the time in blog posts, science is complicated. Just ask these women– they can explain it to us.

African women scientists in a lab, going over procedures.
African women scientists in a lab, going over procedures.

You might be wondering– what about higher-carb diets? They aren’t prominent these days in the popular diet industry, but they are an option and serve a number of nutritional purposes. Also, lots of cultures outside of North America and Europe eat high-carb diets Does the study show that those lead to earlier death, too?

Well,  it depends partly on where you live and what you eat. The primary study participants used in the Lancet article were from the US. For them, the results were a U-shaped curve, which means that the highest risk was at either end— low carb or high carb.  Here it is:

U-shaped risk curve of carb dietary intake, with highest risks at low and high end of card intake (<40%, >70%).
U-shaped risk curve of carb dietary intake, with highest risks at low and high end of card intake (<40%, >70%).

But take a look at the right side of the U– it doesn’t rise as high as the left side does.  That means that the riskiness of high-carb diets in this study is not as big as the riskiness of the low-carb diets. (By riskiness, I mean all-cause mortality risk, but you get the idea here). So, their results suggest that higher-carb diets aren’t as risky (I know, that word again, but humor me) as the low-carb diets.

Later in the Lancet article there is another set of U-shaped curves, with the second one being an amalgamation of many other studies that these researchers analyzed.  These other studies were done in many parts of the world, including places where people eat lots of higher-carb diets with foods like white rice or other simple (vs. complex) carbs. There we see a greater rise in mortality risk as carb intake increases. Complexity enters again, this time at the level of the carbs themselves.  People who eat higher-carb diets but with more complex carbs may (this is still a bit tentative) may mitigate their mortality risk of a higher-carb diet.

Finally, there’s been a good bit of critique and response, of which you readers might be interested in two points:

Point one: There are SO MANY features of our dietary intake that interact with each other, our environment, our genetics and microbiome, etc., that it’s very hard to separate out and identify the effects of one variable on health and mortality. One commentary (in the Lancet) said this:

…this approach should be complemented by large and long-term clinical trials investigating the effects of different dietary patterns (constructed from information about the effects of individual nutrients and foods), because the effect of individual nutrients is likely to be modest.

So we aren’t going to get solid clinical recommendations for people based on the effects of shifting carbs alone.

Point two: The researchers’ findings only tracked mortality risk shifts.  They didn’t track them to any biomarkers or clinically observable features that we could identify that would help in offering medical advice to people. Of course, they weren’t trying to do the latter, but without the latter, the information isn’t so helpful for real-life medicine.

The commentary concluded with this:

When coherent information emerges from different approaches and is replicated, this will form a sound basis for robust public health recommendations.

Or, in other words, “yeah yeah.  Get back to me when you have something that will be useful for clinicians moving forward in trying to identify health issues connected to dietary carb intake, or a more complex but robust view on these interactions with health.”

Here’s the final-final point.  Let’s suppose that Goldilocks is right– moderation in carbs is the just-right path. What do we mean by “moderate intake”? 40–70% is a very big range, and loads of other factors can make a big difference in effects on health, mortality, etc.

If we’re supposed to take nutritional advice really seriously, need more than “just-right” to go on.  Luckily, other studies are working on exactly that. Stay tuned for the latest…






3 thoughts on “The continuing carb-troversy: was Goldilocks right?

  1. What I love, and want to be right, is the message of moderation. Also plant protein. All that butter cheese and bacon just can’t be good for a person.

  2. There is no doubt that I as an individual have a better chance of living longer on lower carbs, as that is how I control my diabetes and keep my A1c under 6. I wonder if the difference they saw in people who got protein from animal sources would hold up if they looked at people who ate grass fed/pastured meat. The small group variations may be much more important than this overall result.

    1. Good question– for the primary study they used self-reports (which likely didn’t track this), and also the original food reports are from 1987–89, when there was much less access to grass fed/pastured meat (outside of farm communities). Low and lowish carb diets work well for a lot of subgroups for a lot of medical purposes, so mileage varies hugely between subgroups. But your point also makes clear that there are lots of easily identifiable features of people and groups which help identify what sort of diets help them with various medical and health goals. One plate does not fit all here… 🙂

Comments are closed.