By Tracy Brighten
Carbohydrates have been left off the menu by dietary fads favouring protein, but a study on human brain evolution suggests a key role for starchy carbs
Paleolithic diets are increasing in popularity in the belief that a return to our ancestor’s diet can curb the upward trend in obesity and diet related health issues.
Scientists agree there are health gains when our diet aligns with our evolutionary past because our physiology is optimised for that diet. Yet there is little agreement about what makes up a healthy diet and what comprises the Paleolithic diet of Old Stone Age man.
There are gaps in evolutionary knowledge, including how and why we have evolved larger brains.
Previous studies on the development of the human brain have highlighted the significance of stone tools that led to a shift from a largely plant based diet to a meat based diet. Scientists also point to cooking as an explanation. Brain growth started around 2 million years ago, but accelerated 800,000 years ago when cooking became widespread. However, the role of starchy plants has mostly been overlooked.
Researchers in this new study, published in The Quarterly Review of Biology, propose that digestible carbohydrates in cooked starchy plant foods were essential to the increase in brain size and aerobic capacity during the Pleistocene, around 2.6 million to 12,000 years ago. The increase in carbohydrate consumption co-evolved with both the use of cooking by controlled fire and an increase in production of salivary amylase that breaks down starch.
Key features in human evolution that are linked to a change in diet include tooth morphology, a reduction in gut size, and an increase in brain size and aerobic capacity.
A change from fibrous plant to meat based diet has been credited with these physiological changes, but to shed light on the diet of modern hunter-gatherers, evidence is needed from a range of disciplines that analyse brain and body changes, food sources, and archaeological remains.
Lead author is archaeologist Dr Karen Hardy, ICREA Research Professor at the Universidad Autonoma de Barcelona and an Honorary Research Associate at the University of York. An expert in human adaptation in pre and early agricultural periods, Hardy is interested in the extraction of starch granules from archaeological sediment and dental calculus to reconstruct starchy foods in ancient diet.
Hardy and her research team, from the University of Sydney and University College London, analysed archaeological, anthropological, genetic, physiological and anatomical data. They report five reasons why starchy carbs played a key role in brain growth.
Starch provides efficient energy source for the growing brain
The human brain uses up to 60% of blood glucose, and although glucose can be synthesised from other sources, it is not efficient, so a diet low in starchy carbs would not have met these high energy demands. There is also a limit to the amount of energy that humans can derive from protein without toxicity occurring, suggested to be 35%-40% of the energy requirement. Preformed glucose available from plant sources provided an advantage in meeting the energy demands of a larger brain.
Glucose supports reproduction
Glucose is the main energy source for foetal growth and low glucose availability can compromise foetal survival. Glucose is also required for lactation. At peak lactation, mammary glands require an additional 70 g glucose/day for synthesis of lactose, the sugar in milk.
Availability of starches
Changes in climate led to drier periods and a more mobile lifestyle, which may have led to increased consumption of meat. However, an increase in open grassland also made high-starch plants in the form of tubers, seeds, and nuts more available.
Cooking to facilitate starch digestion
Humans are the only animal to cook food and this is considered to have transformed the human diet. Salivary amylase was not very effective at breaking down raw starch, but cooking facilitated digestion and led to an increase in plant consumption.
Adaptation of salivary amylase
Salivary amylase and pancreatic amylase production increased in the last 1 million years suggesting an adaption to take advantage of the cooked starchy carbohydrates. Salivary amylase genes are usually present in many copies, 6 on average in humans, but in only 2 copies in other primates. This increases the amount of salivary amylase produced, which increases the ability to digest starch. More preformed glucose is then available to the brain, red blood cells and developing foetus, which could account for the acceleration in brain growth.
The study provides further knowledge to support the important role of starchy carbs in human evolution and physiology.
On this basis of this research, it seems that diets advocating a blanket omission of carbohydrates are ignoring the diet that best suits our physiology.
It looks like potatoes are back on the menu!
Also published on Science Nutshell Aug 9, 2015