Starchy Dangers in Human Evolution
Mashed potatoes are Uncle Mike’s favorite food; at family dinners he would spoon mound after mound of them onto his plate long after everyone else was done eating. Many people seem to feel the same way about these creamy mountains of starch, but is it possible something sinister lurks within, threatening some people with weight gain? To answer that question we first need to understand starch and its role in the human diet, as well as what happens to starch as we digest it, and how this process might differ in different people.
Starch, a principle component of potatoes, corn, pasta, bread, and rice, as seen in Figure 1, is composed of long, branching chains of glucose, which is a sugar and the primary source of energy for living cells. In fact, starch is so rich in glucose, it serves as energy storage for plants to help them survive when the climate is cold or dry. These starchy stores are also exploited by animals, like humans, for energy to grow, stay warm, fight illness, and reproduce.
Figure 1 ~ (a) Starch molecules are long, branching chains of the sugar, glucose, linked together. Pink shading denotes individual glucose molecules. Image credit, modified from Wikipedia, Amylopektin Sessel. (b) Many plant foods, such as potatoes, pasta, bread, rice, and corn have high starch content. Purified starch, such as this corn starch, is a fine, white, tasteless powder. Photo credit, clockwise from top: Wikipedia, Potatoes, Starchy Foods, Corn Starch Mixed with Water.
But, some human populations have historically eaten more starch than others. For example, humans living in tropical or arctic environments eat less starch than those living in more dry or temperate climates. This is because the amount of starch plants produce, and thus the amount that is available for human consumption, depends on climate [1]. In tropical rainforests with plentiful sun and rain, plants have little need to store energy, focusing instead on capturing it by growing big leaves, and also growing rich fruits to attract animals to disseminate their seeds. Humans living in these environments, therefore, eat lower starch diets, consuming more meat, fruit, and honey. Additionally, arctic human populations in Siberia and northern Canada have little access to plant food at all, relying principally on animal foods. However, in somewhat dry or more temperate seasonal climates, plants grow large starch supplies, so humans living there have long dug for starchy tubers like potatoes as staples of their diets. Around 10,000 years ago, many of these populations cultivated plants like wheat, maize, and rice to maximize starch stores. This innovation actually changed how starch is digested in different people’s bodies—a divergence with serious ramifications for people’s health today.
When starch is consumed, it dissolves into glucose molecules with the help of molecular machines, known as enzymes. Specifically, enzymes called amylases aid in breaking starch into glucose with the help of water. The first amylases to act are those found in the saliva, encoded in the genome (the full set of heritable material of an organism) by one gene, called AMY1. However, mistakes that happen when the genome replicates sometimes cause AMY1 to duplicate, so some people end up with many copies of AMY1—up to twenty! It turns out that people who have more copies of AMY1 actually produce more amylase enzymes in their saliva, and more efficiently digest starch in their mouths [1]. This seems to have provided a nutritional benefit in populations who domesticated plants and increased starch consumption; over time these agriculturalist populations got more and more copies of AMY1, while the number of copies in non-agriculturalists has remained relatively low [1].
This difference in efficiency of starch digestion in saliva also has surprising downstream effects on the body. People with lower efficiency of starch digestion and a lower number of AMY1 copies actually have more dramatic spikes in their blood glucose levels after eating starch than people with a higher number—even though people with a higher number of copies break starch into sugar faster [1]. When food is digested into sugars, the sugars are absorbed into the bloodstream in order to feed tissues like muscle or fat around the body. In order for these tissues to take up glucose from the blood a hormone called insulin is required. Insulin is supposed to make sure tissues take up the energy-rich glucose from the blood, preventing blood glucose levels from getting too high and becoming toxic, as in diabetes. As it turns out, people with a lower number of AMY1 copies also have lower insulin levels after eating starch, so their glucose stays in the blood instead of entering tissues, which may explain why those people end up with bigger spikes in blood glucose levels [1, 2]. So maybe high starch consuming populations have become adapted to not only efficiently digest starch into sugars, but also use those sugars, keeping blood glucose levels moderate.
Unfortunately, lower insulin levels and higher glucose spikes also relate to a risk of obesity. According to a 2014 study of over 5,000 people from Europe and Asia, each fewer copy of AMY1 was associated with a 20% increase in risk of obesity, as depicted in Figure 2 [3]. Furthermore, variation in AMY1 copies may account for somewhere between 2.5% and 20% of all variation in risk of obesity among people. Prior to this study, all of the hundreds of genetic variants found to be associated with obesity accounted together for only 2-4% of the genetic risk of obesity, or less than 3% of the overall risk of obesity.
Figure 2 ~ Copies of AMY1 in the genome for different individuals. Fewer copies of AMY1 increases risk of obesity and also impacts blood glucose and insulin—traits closely linked to diabetes.
We still would like to know more about how starch affects health. Researchers want to figure out why exactly people with low amylase are at greater risk for obesity and insulin resistance, and how this relates to starch. For example, one study found that people can actually perceive the changes in viscosity of their saliva as the starch digests in their mouths [1]. If starch digests too slowly in some people’s mouths, would they think it tastes bad? Or would they feel less full from eating starch? Researchers are also trying to figure out if salivary amylase has different important functions in other tissues, like fat, where it is also found at high levels [4].
In spite of these mysteries, current research tells us two things. First, though huge variation exists within any given population, humans from different places are adapted to different diets, at least as far as starch is concerned. And, second, such adaptations can lead to vastly different health outcomes for people eating modern diets rich in processed, starchy foods. Uncle Mike certainly has never had a problem with eating potatoes, but I can’t really say if that’s because he’s an extremely efficient starch digester or because he’s been a champion runner since high school. Other factors like lifestyle and development also influence food digestion and obesity risk. However, studying what people’s ancestors ate and why people’s bodies differ in what they do with the food they eat will help tailor nutritional guidelines to suit individuals.
Elizabeth Brown is a graduate student in the Department of Human Evolutionary Biology at Harvard University.
References
[1] American Society for Nutrition:
[2] Muneyuki, et al. “Latent associations of low serum amylase with decreased plasma insulin levels and insulin resistance in asymptomatic middle-aged adults.” Cardiovascular diabetology. 11, 1: 80.
[3] Imperial College London/News: http://www3.imperial.ac.uk/newsandeventspggrp/imperialcollege/newssummary/news_31-3-2014-9-58-51
[4] NewScientist/Health:http://www.newscientist.com/article/dn25325-obesity-linked-to-our-ability-to-digest-carbohydrates.html
Further Reading
The Guardian:
http://www.theguardian.com/society/2014/mar/30/salivary-carb-breakdown-gene-obesity-study