Why didn’t cavemen need glasses?
I am frequently asked for advice about avoiding or ameliorating myopia, or short-sightedness, so I was interested to read a piece in The Courier Mail recently (“Caving in to the accidental diet”, Sue Dunlevy and Sarah Orr) discussing the Paleo Diet. Dunlevy and Orr quote Loren Cordain, an exercise physiologist at Colorado State University, advocate for the Paleo diet, and a researcher that has previously published an hypothesis that relates the modern observed increased prevalence of myopia to modern changes in diet. Online experts/advocates for the Paleo diet have claimed numerous health benefits on its behalf, so what’s this all about?
The Paleolithic Diet (“Paleo Diet” is trademarked) is an attempt to adopt assumptions about what early humans ate (during the “Paleolithic Era”, which ended around 10 000 years ago) and adapt them to a modern post-industrial society. The reasoning is that since we evolved to eat the diet of early humans, we should be more healthy doing so. The enthusiasm for it is based on observations made since the late 19th and early 20th century that many of the “diseases of civilization” (eg diabetes, heart disease, appendicitis, cancer, and myopia) were unknown in pre-agricultural societies when those societies were first encountered by modern doctors, but became common once these same societies transitioned to a Western diet and Western lifestyles were adopted. Some knowledge of the diet of early humans can be derived from the activity of modern hunter-gatherers, but much is derived from archeological enquiries that are properly in the field of paleo-anthropology.
From modern hunter gatherers we know that the prevalence of myopia is extremely low, around 1% or so, in pre-agricultural/pre-Industrial humans. In Western countries it is significantly greater (more than 15% in Australian adults), and higher still in Asian countries. There is significant evidence that this disparity is multifactorial, and correlates with parental education and numbers of books read amongst other factors, and inversely correlates with time spent outdoors. Cordain’s hypothesis is that high GI diets (diets high in refined carbohydrates that the body quickly breaks down to glucose, ie white flour, white rice, sugar) lead to an increase in insulin secretion, which has a number of biochemical consequences in the body. These include suppression of IGF-binding protein-1 (IGFBP-1), and increased IGF-1 (insulin-like growth factor-1). This can have effects on tissue growth, but also may lead indirectly to suppression of IGF-binding protein-3 (IGFBP-3), leading to effects on retinoid receptor signaling in the eye.
It’s an intriguing hypothesis, but there are a few problems with adopting a stone-age diet to treat your myopia. Firstly, working out what stone-age man ate on a day-to-day basis is more difficult that you might think, and adopting such a diet even harder. Hunter-gatherers forage and hunt, and eat everything when they hunt, often prizing organs such as liver and brain. Secondly, when modern hunter-gatherers hunt and forage, they typically cover a lot more ground than most city dwellers (15km daily for men, 9km daily for women). In the Paleolithic early man probably hunted by running-down prey, in what is termed “persistence hunting”, covering even more ground. So if you want to live like a caveman, get out and run!
That’s not to say that evolution can’t teach medicine anything. Quite the opposite. Learning about how we evolved to become human teaches us quite a lot about what it means to be human, at least as far as our bodies are concerned. Our modern diets and lifestyles would be unimaginable to our pre-agricultural cousins, and for all the technological improvements that have helped us treat infections and other diseases, we are spending more of our healthcare resources treating diseases that are, to a very large extent, caused by the way we live. Spending more time on our feet and eating less highly processed foods intuitively make sense, and find support in the available research. But the relationship of diet to myopia progression is more problematic.
Part of the problem is the interpretation of available data, which is often colored by the prism of the established low-fat diet dogma. A relatively recent study of Singaporean children in which diet was assessed by a survey completed by the children, and then stratified according to carbohydrate, total fat, cholesterol, saturated fat and polyunsaturated fat intake, concluded that myopia progression is associated with cholesterol intake. The data, included with the paper, is difficult to reconcile with the author’s conclusion, and appear to me to support only a conclusion that dietary fat, cholesterol and carbohydrate have no apparent impact on myopia progression.
We need to be very careful when assessing research that advocates changes in the way we live our daily lives. Having a better understanding of where we came from May help this. But recognizing that is understanding is a work in progress is paramount.