CrossFit | Sugar and Sight

Sugar and Sight

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ByCrossFitJanuary 15, 2020

In a now famous paper, Dr. Loren Cordain et al. share their research into how the eyesight of hunter-gatherer groups compares to industrialized populations. Myopia, the most common eye problem worldwide, affects 40% of the U.S. population. In hunter-gatherer groups, rates are typically only 0-2%, and cases are less severe.

In a natural experiment, researchers observed changes in eyesight among indigenous populations as they adopted a Western style of living. This includes the addition of foods containing sugar and highly refined grains, which do not exist in the natural environment. Rates of myopia skyrocketed post-Westernization, reaching levels typical of industrialized populations within a generation. The high prevalence of myopia found in children is not seen in their grandparents who grew up on a native diet.

Myopia, commonly called nearsightedness, results from the over-lengthening of the eyeball or excessive curvature of the cornea. Due to these defects, light rays do not focus properly on the retina, which leads to blurred vision. Excessive near work, such as reading, is widely believed to cause myopia. To see if this is true, the researchers compared populations in outlying areas where people were literate but maintained their traditional diet with populations including urban children who were literate and had access to Western food products. Myopia rates were seven times higher among the urban children (2.9% vs. 21.7%). This supports the hypothesis that diet is a factor in the development of myopia. Cordain et al. share their findings regarding carbohydrate consumption in these populations:

In [a] study of 229 hunter-gatherer societies … although refined cereals and sugars were rarely if ever consumed by groups living in their traditional manner, these foods quickly became dietary staples following western contact. Schaefer (1971; 1977) has shown that, in two Eskimo groups undergoing western acculturation, the per capita consumption of sugar in all forms increased from 11.8 kg in 1959 to 47.4 kg in 1967. The same groups’ per capita consumption of cereals and flour products increased from 71.0 kg in 1959 to 80.0 kg in 1967. Prior to western contact, neither of these carbohydrates was ever consumed (Stefansson 1919).

In the U.S., rates of myopia have increased from 25% in the 1970s to 40% in 2000 (1). This coincides with the steady increase of carbohydrates in the standard American diet over the same time period.

The proposed mechanism behind the dietary contribution to myopia is hyperinsulinemia. Excessive insulin triggers an increase in free IGF-1, a hormone that regulates tissue growth. Insulin has been shown in animal models to promote elongation of the eyeball, whereas insulin’s antagonist, glucagon, has been shown to promote hyperopia (shortening of the eyeball) (2).

Researchers have also linked sugar to other eye conditions, such as glaucoma, cataracts, diabetic retinopathy, and macular degeneration. Carbohydrates bind to proteins and fats in a process called glycation. This process happens non-enzymatically in the body, which is to say haphazardly and disastrously, like pouring sugar into a gas tank. Advanced glycation end products (AGEs) have been shown to accumulate in the eye. This accumulation can lead to cloudiness and brown spots in the lens of the eye.

As R. H. Nagaraj et al. explain in “The pathogenic role of Maillard reaction in the aging eye”:

The proteins of the human eye are highly susceptible to the formation of advanced glycation end products (AGEs) from the reaction of sugars and carbonyl compounds. AGEs progressively accumulate in the aging lens and retina and accumulate at a higher rate in diseases that adversely affect vision such as, cataract, diabetic retinopathy and age-related macular degeneration. In the lens AGEs induce irreversible changes in structural proteins, which lead to lens protein aggregation and formation of high-molecular-weight aggregates that scatter light and impede vision. In the retina AGEs modify intra- and extracellular proteins that lead to an increase in oxidative stress and formation of pro-inflammatory cytokines, which promote vascular dysfunction. (3)

Nagaraj and colleagues widely recommend eliminating high glycemic index (GI) foods from the diet. These foods release sugar rapidly into the bloodstream, and the body compensates by releasing large amounts of insulin quickly. Fructose has been found to cause up to 10 times as much glycation as glucose. The researchers suggest consuming lots of vegetables, some fruit, and zero high fructose corn syrup is advisable. Research on the relationship between eye sight and low-carbohydrate or ketogenic diets is in the preliminary stages, with one study showing improvement in mice susceptible to glaucoma (4).


Notes

  1. See “Increased prevalence of myopia in the United States between 1971 and 1972 and 1999-2004,” which notes the National Eye Institute reports a 25% rate in 1970 versus a 40% rate in 2000.
  2. See “Opposite effects of glucagon and insulin on compensation for spectacle lenses in chicks.”
  3. See “The Maillard reaction in eye diseases,” which argues, “Advanced glycation end products (AGEs) form though a range of pathways within Maillard chemistry, and there is evidence to suggest that these adducts accumulate in the intracellular and/or extracellular environment of ocular structures.” This review evaluates the ever-growing literature on AGEs in biological systems and draws relevant links to diseases such as diabetic retinopathy, age-related macular degeneration, and cataract formation.
  4. See “Structural and functional of chronic metabolically stressed optic nerves through respiration.” 

Comments on Sugar and Sight

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Mary Dan Eades
January 16th, 2020 at 9:11 pm
Commented on: Sugar and Sight

In addition to a high sugar and refined carb intake, another issue that often contributes to myopia in indigenous populations who Westernize (and the rest of us as well) is reading, which isn't a paleo activity. Paleo eyes were designed to see to the horizon, as there was a survival advantage to seeing afar and less need for most people for near work and up-close vision. And, according to discussions we've had with Dr. Cordain (often discussed over wine in summers on the shores of Lake Tahoe) there is something especially damaging about the printed page -- especially black print on a white page.


Prolonged time spent focusing the eyes on reading material causes the ciliary muscles to work overtime to 'accommodate' (temporarily deform the lens) to let the eye see the up-close printed page clearly. The responsive effort deforms the lens over time and results in blurry distance vision. Which gets fixed with glasses. Which the get read through. Which incurs more deformity. Which blurs the vision further. Which gets corrected with stronger glasses. And on and on and on as the person becomes more and more myopic.


It is just this sad progression that the USAF academy noticed in their cadets, many of whom entered training as sharp-eyed freshman with 20/20 or better vision and under the heavy reading load of college became myopic, a problem for those hoping to become pilots. The solution they employed to combat this progression was to put the freshman into weak reading glasses to 'relax' the eye while reading -- in effect to make the near object appear distant to the eye and forestall the deformations.


Based on this research, we put all three of our school-aged sons into mild reading glasses (about a half diopter of plus) that they used any time they were reading for prolonged periods of time. And lo and behold, after college for all of them and in one case years of grad school for both an MBA and a JD, their distance vision remains sharp even now in at 42, 46, and 49 years old.


To be fair, of course, they also ate a fairly lowish carb whole food carb at home (though not stringently so--they had their share of donuts and pizza on occasion) for most of their adolescent and teen years. Mostly they still do eat that way.


All of which is to agree that diet is of critical, foremost importance. Avoiding sugar and refined flours is of especially critical importance. But you might also consider what a set of reading glasses might do to preserve distance vision into adulthood, especially among young scholars.

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Tyler Hass
January 17th, 2020 at 12:44 am

Mary, thanks for sharing that story of your kids. It’s always fascinating to hear what doctors do in their own lives and with their own kids. And I wish I could have been a fly on the wall during your conversations with Dr. Cordain!

I came across a paper recently called “Has Near Work’s Star Fallen?”. It argues that the evidence linking near work to myopia is relatively weak compared to genetic predisposition (parents with myopia). It mentions spending time outdoors could be preventative. That’s paleo-approved! The paper doesn’t mention sugar or nutrition at all, which is unfortunate. I would still put money on near work as one of many factors contributing to myopia. The USAF study you mention seems to support the near work hypothesis, so its star hasn't fallen completely.

journals.lww.com/optvissci/Fulltext/2009/02000/Has_Near_Work_s_Star_Fallen_.3.aspx

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Ryan Martin
January 16th, 2020 at 5:02 pm
Commented on: Sugar and Sight

I really appreciate this piece. Being a North American Indigenous health and wellness researcher, this article supports the Indigenous struggle in a number of unseen ways. Our current state of public health in Indian Country is based on old mid-century models from the Department of Health and Human Services and status quo of positional authority is choking our progressing to being well. This article also touches on the disparity of processed carbohydrates, and if this point is moved upstream in a causal context, the social determinates of access to quality nutrition can be critiqued. I would love to see more Indigenous articles that are vetted by CrossFit. It seems that we are drowning in a world of fake science and it's time consuming to weed out the one with a conflict of interest. Thank you.

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