What to expect this week:

Do you find that you eat different foods in different seasons? A lot of us in the modern world do. We tend to eat more during the colder months, especially more carbohydrates. This is likely an evolved adaptation for energy conservation for times of the year when food has historically been more scarce.

However, most pre-industrial societies never could make this sort of choice. Their annual dietary fluctuations were rarely conscious decisions. The diets of modern hunter-gatherers shift according to the availability of different foods in their environments. This was likely the case throughout most of evolution.

Should we follow suit and put a pause on our glutenous “eat whatever, whenever” habits? Let’s take a closer look at this new trend of seasonal eating and whether or not it’s worth adopting.

Housekeeping:

If you read yesterday’s newsletter, you’ll know that this is the last bit of Evolve.2 content for a little. When I get back from my social media cleanse, get ready for even better content. Longer, higher-quality videos with even more detail on the subject matter are on the way!

Links to my e-Courses

• The Bare Minimum - FREE

• The Adaptive Scholar - $5

Imagine savoring the crisp sweetness of a freshly harvested apple amid autumn's vibrant foliage, or indulging in the juicy burst of flavor from a sun-ripened tomato on a balmy summer afternoon. Seasonal eating grants us access to these sensory experiences to the fullest. In a world where convenience often trumps tradition, rediscovering the joys and benefits of consuming foods in harmony with the changing seasons offers a pathway to better health and culinary delight.

Eating seasonally means consuming foods, especially plants, during the time of year they are naturally grown and harvested outdoors. Just like us, plants evolved under certain climatic settings, and are adapted to those settings. For example, try growing strawberries in New Jersey during the winter. The seeds won’t even penetrate the soil’s surface, let alone flower into a sweet, nutritious fruit. They are simply not adapted to such cold weather. 

Against the backdrop of an ever-industrialized food market, aligning our eating patterns with the natural ebbs and flows of plant harvests is pushing its way to the forefront of dietary conversations. This is partly a plea for healthier habits that are more harmonious with the environment. It is also an invocation of our ancestral eating patterns, considering that pre-modern societies did not have access to the same foods all year round.

In this article, I’ll explore both of these angles to see if the benefits of eating seasonally outweigh the risks. 

Macro-seasons and Human Evolution

When we look at the earth’s climate, it’s obvious that it is a highly dynamic system. What we see around us today has not always been. The temperature of the atmosphere rises and falls. The shorelines encroach farther onto the land and then recede. Volcanos erupt and mountains are formed. We can think of these fluctuations as macro-seasons. 

They occur over vast time spans, millions of years, and can dramatically impact the organisms living on Earth. Such drastic changes will often lead to adaptations, sometimes causing speciation or extinction. Extremely cold macro-seasons, like ice ages, will favor creatures like the woolly mammoth, which was adapted to the cold with its thick layer of subcutaneous fat and dense fur [1]. 

Our human lineage is no different. We too have experienced these macro-seasons and have responded with adaptations of our own. A prime example of this has to do with diet. An inevitable consequence of climate change is a disruption of food accessibility. Certain foods become more or less available depending on temperature changes and other environmental factors. We can see evidence of early human relatives adapting to these food shifts through the archaeological record - especially in their teeth.

Teeth are the hardest part of the human body, remaining well-preserved in the ground to be studied by archaeologists. Necessary for food consumption, teeth are closely intertwined with survival, which means that we can infer the foods ancient hominins were eating based on their dental adaptations. 

The Australopithecines were a genus of hominin living in Africa between 4-1 million years ago (MYA). There were multiple species within the genus, such as Australopithecus anamensis and Australopithecus aferensis (Lucy). Some species were more robust than others, but they all had rather large jaws and especially thick tooth enamel. This was probably an adaptation to the macro-season they were in.

During this macro-season, Africa's climate was drying and slowly transitioning from a land of dense forests to one of open savannas [2]. The preferred fruit and foliage of the Australopithecines' ancestors were becoming more scarce. Instead, they had to rely on fallback foods. These would have included things like seeds with hard shells and starchy roots and tubers. Anthropologists suggest that it was these tougher foods that required the Australopithecines to adapt by evolving harder, thicker teeth [3;4].

This is just one example of how ancient humans have responded to the food fluctuations of a macro-season in an adaptive way over the course of generations. But we also respond to micro-seasons, the annual seasons we are all familiar with, within our own lifetimes. 

Micro-seasons and the Gut Microbiome

Very few groups of people living today still practice hunting and gathering as their only subsistence strategy. Most small-scale societies have adopted some form of agriculture or pastoralism, and of those only a few also hunt or forage for plants - typically as a backup plan for when crops fail. The Hadza of Tanzania are one of the few communities that have rejected more modern subsistence strategies and retain their hunter-gatherer lifestyles.

Because of this, they are one of the only groups in which we can study the effects of natural, seasonal fluctuations in diet - the types of fluctuations that likely would’ve occurred for most people in our Homo sapiens lineage.

First, it’s important to note that the Hadza are not representative of all hunter-gatherers, especially in terms of diet. To quote anthropologists Herman Pontzer and Brian Wood, “The only unifying features among hunter-gatherer diets are breadth and variability” [5]. The very diverse range of hunter-gatherer diets is largely due to their differences in geography and climate. Those living in the Arctic or tundra rely more heavily on meat because they have minimal access to fruits. Annual seasonality is slim for many of them, so this is their diet most of the year.

But, in the lower latitudes, like in Tanzania, there is more seasonal change, and people have access to more alternatives to meat. Just as the Australopithecines had to adapt to their macro-season by adopting new foods, the Hadza respond to their seasons with different food choices depending on accessibility. For example, there are five categories of food that the Hadza eat: tubers, berries, meat, baobab, and honey [6]. As their environment fluctuates between wet and dry seasons, the proportion of calories that each food category accounts for also changes. Foraging for berries and honey is more common during the wet season, and those calories are largely replaced by meat during the dry seasons [5].

After these findings were published, scientists began wondering how this intra-annual variety in diet affects the gut health of the Hadza. If you aren’t already aware, your gut consists of countless microorganisms that feed off of what you eat and help you digest it. With this in mind, it’s reasonable to suggest that seasonal changes in diet would be reflected in seasonal changes in the gut microbiome.

This is exactly what microbiologist Samuel Smits and his colleagues discovered. They found that the gut microbiome of Hadza hunter-gatherers cycles seasonally, with certain microbial species disappearing in one season and reappearing in the following one [7]. Interestingly, they found a greater diversity of microbiota during the hunting and meat-eating seasons. 

This builds upon previous research showing that the gut microbiomes of the Hadza are simply more diverse than those of people from industrialized societies [8], and microbiome diversity is a predictor of health and longevity [9]. 

Lastly, the most recent study looking into the gut microbiome of the Hadza, published in 2023, specifically found 124 microbial species that are vanishing from the guts of industrialized populations [10]. We are still unsure of how important these vanishing species are to our modern gut health. Still, the researchers had this to say about the matter:

If we connect the dots, we see that the Hadza have healthy and diverse gut microbiomes, which is likely in part due to the variety of foods they consume as the seasons change, and industrial people who typically don’t eat seasonally have less diverse gut microbiomes. Should we adopt the practice of eating seasonally?

Pros and Cons of Eating Seasonally

Most of us do eat seasonally, but not in the way we’ve been discussing. Until now, we’ve been considering the natural fluctuations in food availability that influence human eating, which was probably the case for most of human history. Now, we have access to essentially the same foods year-round. Most of these foods are highly processed and the seasonal changes in our consumption are not in response to natural ripening and harvesting times, but simply due to our own behavioral choices.

A study published in 2016 found that, in modern societies, seasons determine both calorie intake and the types of food we eat [11]. In general, more calories tend to be consumed in the winter and post-harvest seasons. Notably, much of the fluctuation in this calorie consumption is attributable to things like alcohol and cereals, both of which are relatively new to our species. 

If we want to encourage seasonal eating aligned with more natural foods and food rhythms, there are two ways we can do this - globally or locally [12]. Local seasonality is pretty self-explanatory and typically what we think of when we think of eating seasonally. It is the idea of producing and consuming foods within their naturally optimal geography and time of year. Global seasonality is when the food is produced according to its natural phases, but consumed elsewhere. 

According to professor of nutrition Jennie I. Macdiarmid, local seasonality is accompanied by costs and benefits to our health, environment, and economies [12]. The first concern deals with general nutrition. Eating seasonally means that you would eat a wide variety of foods over the course of the entire year, but this also means that there will be times of the year when you may not meet nutritional requirements.

She says this with respect to local seasonality, “during the winter in the UK there would be no fruit and a very limited range of vegetables. Limiting fruit and vegetables to fewer and possibly less appealing items could risk reducing intakes of fruit and vegetables even further.”

Second, local seasonality may be more environmentally friendly. It could reduce the impact of food production on water stress, land use, and biodiversity.

Third, local seasonality may have negative economic consequences. This could manifest itself in two ways. First, it could reduce job stability by promoting seasonal labor jobs. Second, it would limit international trade because food markets would be constrained to local populations. 

Global seasonality could potentially solve two of these problems but with a caveat. It would allow countries to continue trading with one another, and it would allow those people in the UK, for example, to consume seasonally produced fruit from the tropics. 

The caveat is that fresh produce loses its nutritional value when stored and transported through the supply chain [12;13]. To prevent this, food must be frozen:

So it may be the case that mimicking the seasonal eating of the Hadza could improve the gut microbiomes of people in industrialized societies, but this is just one variable among many.

Conclusion

Our bodies respond to micro-seasonal changes in diet and our gut microbiomes are an example of this. At a broader timescale, they also respond to macro-seasonal changes through evolutionary processes, as shown by our dental morphology. The question is whether or not we allow these natural responses to occur.

If we are eating seasonally, at the local level, we must consider the possible consequence of not meeting nutritional requirements during the “off-season” of certain fruits and vegetables. If we adhere too strictly to this, we may deprive ourselves of important nutrients throughout large portions of the year, and how our bodies respond to that may not be ideal.

We can avoid this by a.) freezing locally grown food or b.) importing fresh produce, which must also be frozen to avoid losing its nutritional value. In doing so, we can continue to produce foods during their natural growth and harvest cycles, while holding on to their nutritional value for a later date.

All things considered, it’s likely ideal to at least produce our food in agreement with their natural seasons, but producing and consuming them within these condensed time spans poses more of a challenge. 

References:

[1] Smith, S., et al. 2027. “Evolutionary adaptation revealed by comparative genome analysis of woolly mammoths and elephants.” DNA Research 24(4):359-369.

[2] Hernández, M., and Vrba, E. 2006. “Plio-Pleistocene climatic change in the Turkana Basin (East Africa): evidence from large mammal faunas.” Journal of Human Evolution 50(6):595-626.

[3] Strait, D., et al. 2009. “The feeding biomechanics and dietary ecology of. Australopithecus africanus.” Proc. Natl. Acad. Sci. USA 106:2124-2129.

[4] Dominy, N., 2008. “Mechanical properties of plant underground storage organs and implications for dietary models of early hominins.” Evolutionary Biology 35:159–175.

[5] Pontzer, H. and Wood, B. 2021. “Effects of Evolution, Ecology, and Economy on Human Diet: Insights from Hunter-Gatherers and Other Small-Scale Societies.” Annual Review of  Nutrition 41:363-385.

[6] Marlowe, F. and Berbesque, J. 2009. “Tubers as fallback foods and their impact on Hadza hunter-gatherers.” American Journal of Physical Anthropology 140:751–58.

[7] Smits, S., et al. 2017. “Seasonal cycling in the gut microbiome of the Hadza hunter-gatherers of Tanzania.” Science 357(6353):802-806.

[8] Schnorr, S., et al. 2014. “Gut microbiome of the Hadza hunter-gatherers.” Nature Communications 5:3654.

[9] Deng, F., et al. 2019. “The gut microbiome of healthy long-living people.” Aging (Albany NY) 11(2):289-290.

[10] Carter, M., et al. 2023. “Ultra-deep sequencing of Hadza hunter-gatherers recovers vanishing gut microbes.” Cell 186(14):3111-3124.e13.

[11] Stelmach-Mardas, M., et al. 2016. “Seasonality of food groups and total energy intake: a systematic review and meta-analysis.” European Journal of Clinical Nutrition 70(6):700-8.

[12] Manga, M., et al. 2018. “Impact of transportation, storage, and retail shelf conditions on lettuce quality and phytonutrients losses in the supply chain.” Food Science and Nutrition 6(2).

[13] MacEvilly, C. and Peltola, K. 2003. “The Effect of Agronomy Storage Processing and Cooking on Bioactive Substances in Food.” In: Goldberg, G., Ed., Plants: Diet and Health, Blackwell Publishing Ltd., Oxford, 226-239.

Fit Fuel Song Suggestion

The Fit Fuel song suggestions are hand-picked by yours truly to elicit the motivation (and possibly aggression) needed to initiate or persist through a grueling workout. They consist of heavy, brutal guitar riffs and gruesomely guttural vocals. Additionally, I timestamp what I believe to be the best riff of the song - one that will kick your nervous system into overdrive when approaching a personal record (PR).