Nobody Painted Cave Walls to Celebrate a Salad
You have seen the cave paintings. Lascaux in France. Altamira in Spain. Chauvet. Across thousands of years and thousands of miles, early humans left us the same picture over and over again: animals being hunted, killed, and eaten.
Not a single field of grain. Not a vegetable garden. Not one human crouching over a pile of leaves.
This is not an accident. It is not a coincidence. It is evidence.
Before the food industry had a marketing budget and before nutrition guidelines were drafted by people with financial ties to agricultural corporations, humans ate what the land gave them. And what the land gave them, overwhelmingly, was animal protein and fat.
This article is not about going back to living in a cave. It is about asking a question that most of us were never encouraged to ask: if plants are so essential to human health, why does our body struggle so hard to actually use their nutrients?
The Body Does Not Lie
Let us start with anatomy, because your body has been trying to tell you something for years.
Human stomach acid sits at a pH of 1.5 to 2.0. That is extremely acidic. It is the kind of acid you need to break down dense animal protein and dissolve bone. Herbivores, animals that thrive on plant matter, have significantly less acidic stomachs because plants do not require that level of chemical force to process.
Our small intestine is shorter relative to body size than that of true herbivores. Herbivores need a long digestive tract to ferment and extract whatever nutrition they can squeeze out of cellulose. We do not have the enzymes to break down cellulose at all. The fiber you were told to eat more of passes through you largely undigested. That is not the digestive system of an animal designed to live on plants.
Our brains grew larger as we began eating more meat. Anthropologists call it the expensive tissue hypothesis: as the gut shrank (because dense animal food required less gut to process), the brain expanded. The nutrients that made this possible, long-chain fatty acids like DHA and EPA, and highly bioavailable protein, came from animal foods.
This is not an opinion. This is evolutionary biology.
What the Research Actually Shows
A 2005 paper published in the Proceedings of the Nutrition Society, reviewing what we know about ancestral diets, found that early modern humans obtained approximately 35% of their dietary energy from protein and 35% from fat. Animal foods dominated this intake, particularly in regions further from the equator.
Research from Aleph 2020, reviewing paleolithic nutrition across multiple archaeological studies, confirms that red meat and fat dominated the diets of Neanderthals and early modern humans in large parts of Eurasia. In some documented cases, like the Clovis culture in North America, populations obtained up to 95% of their calories from large animal prey. The shift toward more diverse plant consumption came later, primarily after megafauna extinctions forced humans to adapt.
The idea that we evolved on salads is simply not supported by the physical evidence.
The Nutrient Bioavailability Problem Nobody Talks About
Here is where the mainstream conversation falls apart completely.
When someone tells you that spinach is high in iron, or that lentils are a great protein source, they are technically reading a nutrition label correctly. The number is on the label. But a number on a label tells you nothing about how much of that nutrient your body can actually absorb.
This is the concept of bioavailability, and understanding this changed everything for me.
Plants contain compounds called antinutrients. These are not a fringe concept invented by carnivore advocates. They are documented in peer-reviewed literature, studied by food scientists, and acknowledged even in mainstream nutrition research. Antinutrients exist because plants evolved to protect themselves. They cannot run from predators, so they developed chemical defenses. When you eat those plants, you absorb those defenses.
The main antinutrients you need to know about:
Phytates are found in grains and legumes. They bind to minerals like iron, zinc, and calcium in your digestive tract and carry them out of your body before your body has a chance to utilize them. A 2022 review published in ACS Omega confirmed that phytates are among the most significant inhibitors of iron absorption.
Oxalates, found in spinach, almonds, and many other plant foods, bind to calcium and reduce how much you absorb. Spinach is often celebrated as a calcium source, but a study published in the American Journal of Clinical Nutrition found that calcium from spinach absorbs at around 5%, compared to roughly 27% from milk.
Lectins are proteins found in legumes and grains. They resist digestion, bind to the gut lining, and in some people contribute to what researchers call intestinal permeability, or leaky gut. Dr. Paul Mason, a sports medicine and exercise physician in Australia, has written and spoken extensively on how lectin sensitivity can silently stall fat loss and drive inflammation in people who appear to be doing everything right.
Tannins in coffee, tea, and many plant foods inhibit protein digestion.
Saponins in legumes can bind zinc and may disrupt gut barrier function.
A 2021 review in Nutrients, examining the clinical significance of antinutrients, confirmed that these compounds are not theoretical concerns. They alter mineral absorption, inhibit digestive enzymes, and affect the bioavailability of the nutrients you are counting on.
When you eat a plant food, you are not necessarily getting the nutrients that the label claims. You are getting whatever your body manages to extract after fighting through the plant's chemical defenses. For many women, that number is far smaller than they realize.
The Iron Story They Got Wrong
Iron deficiency is one of the most common nutritional problems in women. Particularly in women over 40, low iron shows up as fatigue, hair loss, brain fog, and poor recovery. Doctors often tell these women to eat more spinach.
This advice is almost useless, and spinach in particular is worth looking at more closely.
There are two forms of dietary iron. Heme iron, found only in animal foods, and non-heme iron, found in plants. These are not equivalent. They are not even close.
Heme iron absorbs at a rate of 25 to 30%. Your body recognizes it, your intestines have a dedicated pathway for it, and dietary factors do not significantly interfere with it.
Non-heme iron absorbs at 3 to 5%. According to a 2024 review in Nutrients examining dietary heme iron, this absorption rate is further suppressed by phytates, polyphenols, and calcium, all commonly present in the same plant-based meals where you find non-heme iron.
A paper published in NCBI reviewing iron absorption across food sources found that the absorption rate from organ meats sits at 25 to 30%, from green leafy vegetables at 7 to 9%, from grains at 4%, and from dried legumes at 2%.
You would need to eat an enormous amount of spinach to approach what a single serving of beef liver delivers, and most of what you eat passes through you.
But here is the part nobody tells you about spinach specifically. While you are getting very little iron, you are getting a significant dose of oxalates.
Oxalates are compounds that your body cannot break down or eliminate easily. When oxalate accumulates, it forms calcium oxalate crystals. Research published in PMC reviewing oxalate crystal disease confirmed that these crystals deposit in the kidneys, joints, tendons, cartilage, bones, and even the heart and eyes. Approximately 80% of all kidney stones are composed of calcium oxalate. Beyond kidney stones, research published in ScienceDirect found that oxalate crystals in the joints trigger inflammatory responses that can mimic gout and rheumatoid arthritis. The crystals damage renal tubular cells and drive chronic kidney disease progression in a cycle that is difficult to reverse once it begins.
The argument for eating spinach to get iron is this: consume a food that delivers very little absorbable iron while loading your body with a compound that can crystallize in your kidneys and joints over time.
That is simply not a good trade-off. And it is a trade you are making every time you put spinach at the center of your health strategy.
The research also confirms something called the meat factor: consuming animal protein with a meal actively increases your body's absorption of plant iron as well. Animal foods do not just feed you directly. They help you extract more from whatever you eat alongside them.
If you are a woman over 40 and you are tired all the time, please stop blaming stress and start asking about your ferritin level. Not just hemoglobin. Ferritin. A ferritin level below 30 ng/mL causes fatigue, hair loss, and cognitive fog even when hemoglobin looks normal. Eating more salad will not fix it. Eating liver will.
The Nutrients You Cannot Get From Plants
This is not an argument or an opinion. It is biochemistry.
Vitamin B12 exists naturally only in animal foods. Full stop. It is not found in any plant in a biologically active form that your body uses. Every vegan on the planet who is not supplementing is deficient. Every single one. Dr. Ken Berry, board-certified family physician and author of Lies My Doctor Told Me, has consistently pointed to B12 deficiency as one of the most common and most misunderstood consequences of plant-based eating. B12 drives red blood cell production, nervous system function, and DNA synthesis. You cannot fake it with kale.
Retinol is the form of vitamin A your body actually uses. It is found in animal foods: liver, eggs, and dairy. Plants contain beta-carotene, which your body must convert to retinol. The conversion rate in humans is notoriously poor. Research suggests that the body needs as many as 21 units of beta-carotene to produce 1 unit of retinol. Women, people with thyroid issues, and people with poor gut health convert even less efficiently. If you are eating carrots to get your vitamin A, you are likely not getting your vitamin A.
DHA and EPA are the long-chain omega-3 fatty acids that your brain, your heart, and your joints require. Plants contain ALA, a short-chain precursor. Your body must convert ALA to DHA and EPA. Multiple studies confirm this conversion is extremely inefficient, often under 5% for EPA and under 0.5% for DHA. Fatty fish and red meat deliver DHA and EPA directly. Plants do not.
Creatine, carnosine, and taurine are bioactive compounds found in animal muscle tissue. They play direct roles in muscle function, neurological health, and energy metabolism. They are not found in plants. Research published in PMC reviewing the role of meat in human nutrition confirmed that these compounds are unique to animal foods and that deficiencies have measurable consequences for brain and muscle health.
Heme iron, as discussed above, exists only in animal foods. There is no plant equivalent.
When vegans and vegetarians are deficient, and the research confirms they most often are, it is not because they are doing veganism wrong. It is because they removed the foods that contain these nutrients.
Plants Are Not Food. They Are Medicine.
This is the reframe that changes how you think about all of this.
Plants are not without value. Some plants contain compounds that reduce inflammation, support gut bacteria, or provide short-term therapeutic benefits. But medicine and food are not the same thing.
You would not take ibuprofen three times a day because it helped your headache once. You would not drink cough syrup with breakfast because it contains something useful. Yet that is essentially the logic behind building your diet around plants.
Plants developed antinutrients to deter consumption. They are not trying to nourish you. They are trying to survive. When you eat them occasionally, in small amounts, as a complement to an animal-based diet, your body handles the tradeoff. When plants become the foundation of every meal, the cumulative burden of antinutrients, poor bioavailability, and nutrient displacement adds up in ways that show on lab work and on how you feel.
Dr. Eric Westman, a physician at Duke University who has spent decades studying low-carbohydrate and ketogenic nutrition, has pointed out consistently that the nutrients humans need for optimal function are concentrated and most bioavailable in animal foods. The plant-based framework, he argues, requires constant supplementation and workarounds precisely because it is fighting the biology of the human body.
What This Means for You
If you are a woman over 40 and you have been doing everything right, eating your vegetables, limiting red meat, choosing whole grains, and you are still tired, still carrying weight you cannot lose, still watching your lab numbers move in the wrong direction, this article is for you.
The advice we have received was not based on human biology. It was based on population-level studies funded by agricultural interests, filtered through dietary guidelines that have not materially changed since the 1970s, and repeated so often that most doctors repeat it without questioning the source.
Your body was not designed to live on plants. It was designed to thrive on protein and fat from animal sources, with plants playing a supplementary role at best.
The cave painters knew something we have forgotten. They chased animals across the landscape and put their hunts on walls made to last thousands of years. Nobody painted a grain field. Nobody celebrated a harvest of legumes.
They were telling us something. It has taken us a long time to start listening.
This article is for informational purposes and reflects current research in ancestral nutrition and nutritional biochemistry. Always work with a qualified healthcare provider when making significant dietary changes. Referenced research includes work from peer-reviewed journals including Nutrients, ACS Omega, and NCBI, as well as the work of clinicians including Dr. Ken Berry, Dr. Eric Westman, and Dr. Paul Mason.
Disclaimer: The content shared here is for informational and educational purposes only and should never be taken as medical advice.
In writing this blog post, my goal is to distill research findings into a clear, approachable format that encourages critical thinking and empowers you to make informed decisions about your health.
