The paleo diet has jumped to the top of the diet fad list because of its supposed similarity to the diets that our ancestors ate: plenty of meat; and carbohydrates only from vegetables and fruits, instead of grains, which entered our diets only about 10,000 years ago. Our ancestors didn’t get the chronic diseases we do today, like obesity and diabetes, so eating what they ate must be the key, right? Or so the thinking goes. While the paleo diet has received as much criticism (it’s a diet high in saturated fats from meat, associated with higher risk of heart disease and certain metabolic problems) as acclaim, the reasoning behind it — returning to our ancestral eating habits — isn’t necessarily flawed. But, there might be a better dietary choice more soundly rooted in science. Say hello to microbiota accessible carbohydrates (MACs) and the MAC diet.

Do ancient microbes hold the key to better health?

Until about 10,000 years ago, humans obtained food through hunting and gathering. Today, only a few civilizations still maintain eating habits that are close to those ancient diets: 

• Rural Africans in Burkina Faso
• Groups in Malawi
• Venezuelan Amerindians
• The Hadza tribe in Africa 

As early as the 1970s, reports have chronicled the lack of chronic conditions such as diabetes, heart disease, and colorectal cancer among these groups.

The key seems to be not only what these groups eat (and don’t eat), but also how the foods they eat impact their resident gut microbes (the microbiome). In contrast to the modern high-fat, low-fiber Western diet, the ancient diets these groups follow are packed with fiber (60 to over 100 grams per day, compared to the average American diet of less than 20 grams per day) and completely lack processed foods. As a result, they have significantly different microbial communities in their guts than do Western populations. 

Have our modern eating habits caused us to lose beneficial microbes that can help keep us healthy? Let’s explore what the science says.

The microbiome diversity of ancient diets: Relevant?

Several studies have explored microbiome differences between modern hunter-gatherer groups and Western populations. They’ve made some important discoveries:

• Microbiome diversity is increased in “ancient” lifestyle groups from Burkina Faso, Malawi, and Venezuela, meaning that these groups have more different types of bacteria in their guts than people living a modern Western lifestyle.
• The Hadza group in Africa have more Prevotella and less Bifidobacterium in their guts than Italians living a Western lifestyle. 

However, these changes aren’t necessarily good or bad. They may simply reflect differences in diet; for example, Bifidobacterium-rich yogurt is a common food among Western populations but is not consumed by hunter-gatherers. 

But, one difference between our ancestors’ lifestyles and our own could hold the key to the differences we see: they ate a lot more fiber than we do.

The role of short-chain fatty acids (SCFAs)

Fiber is a key nutrient for the microbes living in our GI tracts. Such microbes break fibers down via a process called fermentation, producing several molecules during that process, including short chain fatty acids (SCFAs). The low levels of SCFAs in modern, Western peoples may be at the root of the increase in chronic conditions we see in modern peoples compared to our hunter-gatherer ancestors.

SFCAs include the molecules butyrate, propionate, and acetate and are the main source of energy for your intestinal cells. Researchers have observed several other important functions and characteristics of SCFAs, all connected to your health. SCFAs:

• Increase regulatory T cells, which maintain immune system homeostasis and prevent your immune system from reacting to things it shouldn’t.
• Regulate signalling by molecules called G-protein coupled receptors, which help regulate metabolism, inflammation, and disease.
• Protect against diet-induced obesity.
• Influence gluconeogenesis (a natural process that helps regulate blood sugar) in the intestine, improve metabolic profiles, and reduce weight gain (see here and here).
• Mediate the gut-brain axis.

Microbiota accessible carbohydrates (MACs) are more than just fiber

Because microbial fermentation of fiber results in increased SCFAs in the gut, many people feel that simply eating a diet higher in fiber is a simple solution. But fiber isn’t the only thing beneficial gut microbes eat, so we might want to expand our definition of “healthy” foods to include non-fibrous microbial food as well. 

Leading microbiome researcher Justin Sonnenburg calls the entire collection of molecules that beneficial microbes can break down (including but not limited to fiber) “microbiota accessible carbohydrates (MACs),” and he has been studying them for several years to figure what role they might play in health maintenance. Here’s what he’s learned about these important molecules:

• A MAC is any carbohydrate that gut microbes can metabolize/ferment.
• There are a variety of sources of MACs: host-derived animal glycans (such as the mucus that lines your intestines) and MACs synthesized by other microbes (consumed with food or already present in the gut), in addition to the most well-recognized MAC, fiber from plant material — the starting material for gut-healthy SCFAs.
• A MAC for you may not be a MAC for me: Japanese individuals have a type of bacteria in their guts that can digest carbohydrates from algae, but North American and European individuals do not have these bacteria. For the Japanese, algae is a MAC, but it is not a MAC for people without that bacteria that can break it down.

A lack of MACs in the diet is bad news: microbes eat the protective mucus lining your intestines if they don’t receive other MACs from what you eat, contributing to inflammation and related gut conditions.

“Rewilding” our microbiomes

All of this begs the question: could replacing “lost” microbial species, such as those observed in modern hunter-gatherers, help us to fight and evade chronic disease? There are some important things to consider:

• The hunter-gatherer groups studied are modern people that are simply approximations of our human ancestors, rather than exact replicas.
• We don’t know whether adding back lost microbes would be beneficial or harmful; for example, Prevotella is higher in groups lacking chronic conditions, but among Westerners has been associated with chronic inflammatory conditions such as arthritis.
• Similarly, we don’t know whether the Western microbiome will react positively to a diet higher in MACs (microbiota accessible carbohydrates).
• We don’t know whether the loss of bacteria or the loss of bacterial metabolites is at the root of chronic diseases among Westerners.
• We don’t know whether increasing MACs (microbiota accessible carbohydrates) could recover microbiome diversity, or whether lost species have been lost forever.
• The best approach could be a combination: collecting and isolating lost microbes from modern hunter-gatherers and reintroducing them to Westerners along with more MACs (microbiota accessible carbohydrates).

Key takeaways

Consuming more MACs (microbiota accessible carbohydrates), including fiber from plant foods, could improve your health and make you more resistant to chronic diseases such as obesity, diabetes, and cancer. Nevertheless, no two people respond the same to a given MAC or set of MACs due to differences in geography, medication history, diet, and even genetics. Because MACs encompass more than just fiber, approaches to improving health using MACs are likely to be even more personalized and complex than those focused on prebiotics and/or probiotics. And until we know more about the different kinds of MACs, the best approach for supporting a healthy lifestyle is to be kind to your microbes by avoiding unnecessary medications and eating a diet rich in fiber.