The ongoing debate surrounding the health implications of veganism has captivated discussions for ages, particularly gaining momentum with the rise of social media platforms like Facebook. This controversy is intensified by passionate arguments from both advocates and critics. While many long-term vegans attest to enjoying robust health, former vegans often share stories of a significant decline in their well-being, whether gradual or abrupt. Fortunately, scientific research is gradually uncovering the reasons behind these varying responses to diets that are low or entirely devoid of animal products, with a significant emphasis on factors like genetics and gut health.

Regardless of how nutritionally complete a vegan diet may appear on paper, individual metabolic differences can greatly influence whether a person flourishes or struggles when eliminating meat and other animal-derived foods from their meals.

1. The Conversion of Vitamin A

Vitamin A is undeniably a powerhouse among nutrients, playing a crucial role in maintaining vision, bolstering the immune system, promoting healthy skin, facilitating normal growth and development, and is essential for reproductive health, among many other functions. It's a common misconception that plant-based foods contain actual vitamin A (officially known as retinol). In reality, these foods are rich in vitamin A precursors, with beta carotene being the most notable.

Within the intestines and liver, beta carotene undergoes conversion into vitamin A through the action of an enzyme called beta-carotene-15,15′-monooxygenase (BCMO1). When this process functions efficiently, your body can derive retinol from plant sources like carrots (gajar) and sweet potatoes (shakarkandi). In stark contrast, animal-derived foods provide vitamin A in the form of retinoids, which bypass the need for BCMO1 conversion altogether.

However, there's a downside. Various gene mutations can significantly impair BCMO1 activity, obstructing the conversion of carotenoids and making plant foods less effective as sources of vitamin A. For instance, two common genetic polymorphisms in the BCMO1 gene (R267S and A379V) can together diminish beta carotene conversion by an alarming 69%. A rarer mutation (T170M) may cause a staggering 90% reduction in conversion for individuals with two copies of this variant.

Overall, approximately 45% of the population carries polymorphisms that classify them as “low responders” to beta carotene. Additionally, several non-genetic factors can further hamper the conversion and absorption of carotenoids. These include low thyroid function, poor gut health, chronic alcoholism, liver diseases, and zinc deficiencies. When any of these factors combine with genetic predispositions, the ability to convert plant-derived sources into retinol can diminish even further.

Yet, why isn't this widespread issue leading to mass vitamin A deficiency? The answer lies in dietary sources: in Western countries, carotenoids account for less than 30% of vitamin A intake, while over 70% comes from animal products. An omnivorous individual with a BCMO1 mutation can usually rely on vitamin A from animal sources, blissfully unaware of the ongoing challenges related to carotenoid conversion.

Conversely, for those who avoid animal products, the impact of a dysfunctional BCMO1 gene becomes evident—and can ultimately be harmful. For individuals with low conversion rates who adopt a vegan lifestyle, consuming carrots to the point of turning orange does not guarantee sufficient vitamin A intake for optimal health. This can lead to elevated carotenoid levels (hypercarotenemia), while vitamin A status plummets (hypovitaminosis A), resulting in deficiency despite seemingly adequate consumption.

Even for vegetarians with lower conversion capabilities, the vitamin A found in dairy products and eggs (which are far less nutrient-dense than meat sources like liver) may not suffice to prevent deficiency, especially if absorption challenges are also present. Unsurprisingly, the implications of insufficient vitamin A resemble the health issues reported among certain vegans and vegetarians. Complications such as thyroid dysfunction, night blindness and other vision-related issues, weakened immunity (leading to more frequent colds and infections), and dental enamel problems can all stem from inadequate vitamin A levels.

On the other hand, vegans with normal BCMO1 function who consume a variety of carotenoid-rich foods typically manage to produce enough vitamin A from plant-based sources to maintain good health. Individuals who are effective converters of carotenoids can generally obtain sufficient vitamin A on vegan diets. However, those with poor conversion abilities might face deficiencies even if their dietary intake aligns with recommended levels.

2. Understanding the Gut Microbiome and Vitamin K2

The gut microbiome, a diverse community of microorganisms residing in your colon, undertakes numerous essential functions, including the synthesis of nutrients, fermentation of fibers, and neutralization of toxins. This intricate ecosystem is not static; it exhibits remarkable flexibility, adapting to various factors such as diet, age, and environmental influences. However, a significant portion of your gut inhabitants is inherited or established during early life stages.

For instance, elevated levels of Bifidobacteria correlate with the gene associated with lactase persistence, highlighting the genetic aspects influencing the microbiome. Additionally, infants delivered vaginally acquire their initial microbial community from the birth canal, resulting in distinct bacterial compositions compared to those born via cesarean section. Furthermore, disruptions to the microbiome, whether from antibiotics, chemotherapy, or specific infections, can lead to lasting alterations in a previously healthy gut flora.

Some studies suggest that certain bacterial populations may never fully recover after antibiotic treatment, stabilizing instead at diminished levels. This indicates that while the gut microbiome is generally adaptable, certain characteristics may remain due to factors beyond your control.

So, why is this important for vegans? The gut microbiome plays a crucial role in how individuals respond to various foods and synthesize essential nutrients. Some microbial communities may be inherently more favorable for a plant-based diet than others. For example, specific gut bacteria are necessary for the synthesis of vitamin K2 (menaquinone), a nutrient known for its unique benefits related to skeletal health (including teeth), insulin sensitivity, cardiovascular health, and possibly in preventing prostate and liver cancers.

The primary producers of vitamin K2 include certain species of Bacteroides, Prevotella, Escherichia coli, and Klebsiella pneumoniae, along with various gram-positive, anaerobic, non-sporing bacteria. Unlike vitamin K1, which is plentiful in leafy greens, vitamin K2 is predominantly found in animal products, with the notable exception of natto, a fermented soybean dish that is often described as having an 'acquired' taste.

Research indicates that the use of broad-spectrum antibiotics significantly diminishes vitamin K2 levels in the body by eradicating the bacteria responsible for its synthesis. One intervention study revealed that in participants following a high plant, low meat diet (consuming less than 60 grams of meat daily), the primary factor affecting their fecal K2 levels was the presence of Prevotella, Bacteroides, and Escherichia/Shigella species within their gut.

If an individual’s microbiome lacks sufficient vitamin-K2-producing bacteria—whether due to genetic predispositions, environmental factors, or antibiotic usage—and if animal products are excluded from their diet, vitamin K2 levels can plummet to concerning lows. Although research on this matter remains limited, it is plausible that such deficiencies could deprive vegans (and some vegetarians) of the numerous benefits that vitamin K2 offers, potentially leading to dental issues, an increased risk of bone fractures, and diminished protection against diabetes, cardiovascular diseases, and certain cancers.

On the other hand, individuals possessing a robust microbiome capable of synthesizing K2 or those who enjoy natto may successfully obtain adequate amounts of this vitamin while adhering to a vegan diet. However, vegans who lack sufficient bacteria necessary for vitamin K2 synthesis may face complications arising from inadequate intake, including a heightened risk of dental problems and chronic diseases.

3. Amylase and Starch Tolerance

While there are certainly exceptions, it is generally observed that meat-free diets are richer in carbohydrates compared to their omnivorous counterparts. Indeed, several renowned plant-based diets are noted for comprising around 80% carbohydrates, primarily sourced from starchy grains, legumes (dal), and tubers (like potatoes or sweet potatoes). Notable examples include the Pritikin Program, Dean Ornish Program, McDougall Program, and Caldwell Esselstyn’s heart disease reversal diet. These diets have demonstrated impressive health benefits overall; for instance, Esselstyn’s program has been shown to significantly reduce cardiac events among committed participants. However, some individuals report less favorable outcomes after transitioning to high-starch vegan diets.

So, what accounts for these contrasting responses? The underlying explanation may reside in our genetic makeup and the composition of our saliva. Human saliva is enriched with alpha-amylase, an enzyme responsible for breaking down starch molecules into simpler sugars through a process called hydrolysis. The quantity of this enzyme can vary significantly, depending on the number of AMY1 gene copies you possess, as well as lifestyle factors such as stress and circadian rhythms. Consequently, amylase levels can fluctuate from being nearly undetectable to constituting as much as 50% of the total protein content in your saliva.

Typically, individuals from cultures that emphasize starch in their diets, such as the Japanese, are likely to have higher AMY1 gene copies and, thus, increased levels of salivary amylase compared to those from populations that historically relied more on fat and protein. This suggests a connection between selective pressure and dietary habits over generations. Essentially, AMY1 gene patterns appear to be closely related to the traditional diets of one's ancestors.

This relationship is crucial because the production of amylase significantly impacts how one metabolizes starchy foods, affecting whether these foods cause blood sugar levels to spike dramatically or rise more steadily. Individuals with lower amylase production tend to experience sharper and prolonged blood sugar spikes when consuming starch, particularly in its refined forms, compared to those with naturally higher amylase levels. Not surprisingly, people with low amylase production have an increased risk of developing metabolic syndrome and obesity when following typically high-starch diets.

What implications does this have for vegetarians and vegans? Although amylase levels are relevant to anyone, those adhering to plant-based diets rich in grains, legumes (dal), and tubers (like those in the previously mentioned programs) may inadvertently highlight any underlying carbohydrate intolerance. For individuals who produce little amylase, significantly increasing starch consumption could lead to detrimental outcomes, such as poor blood sugar control, decreased feelings of fullness, and potential weight gain. Conversely, individuals who possess the metabolic capability to produce ample amylase may find that adjusting to a high-carb, plant-based diet is relatively straightforward.

4. The Role of PEMT Activity in Choline Metabolism

Choline is a vital nutrient that often flies under the radar despite its crucial roles in metabolism, brain function, neurotransmitter production, lipid transport, and methylation processes. While it may not have received as much attention as trendy nutrients like omega-3 fatty acids and vitamin D, its significance cannot be understated. In fact, a lack of choline can lead to serious health issues, including fatty liver disease, which has become increasingly prevalent in Westernized societies.

Moreover, insufficient choline intake may elevate the risk of neurological disorders, cardiovascular diseases, and developmental complications in children. Generally, animal-based foods are the richest sources of choline, with egg yolks and liver leading the list, followed by other meats and seafood that also contribute substantial amounts. In contrast, plant-based foods tend to contain only modest levels of choline.

Interestingly, our bodies can synthesize choline through an enzyme known as phosphatidylethanolamine-N-methyltransferase (PEMT), which converts phosphatidylethanolamine (PE) into phosphatidylcholine (PC). In many scenarios, the modest choline provided by plant foods, alongside the choline produced via the PEMT pathway, might adequately fulfill your choline needs without requiring egg or meat consumption.

However, for those following a vegan diet, obtaining sufficient choline can be a challenge. Firstly, despite attempts to establish adequate intake levels for choline, individual needs can vary widely, meaning that what appears to be a sufficient intake on paper may still lead to deficiency. A study indicated that 23% of male participants exhibited signs of choline deficiency even while consuming the recommended daily intake of 550 mg.

Additionally, research indicates that choline needs can significantly increase during pregnancy and lactation as it is transferred from the mother to the fetus or through breast milk. Secondly, not every individual is equally efficient at synthesizing choline. Estrogen plays a crucial role in enhancing PEMT activity; thus, postmenopausal women, who experience reduced estrogen levels and diminished choline synthesis, require higher choline intake compared to those in their reproductive years.

Moreover, certain genetic mutations affecting folate metabolism or the PEMT gene can pose risks on low choline diets. For instance, a study revealed that women with the MTHFD1 G1958A polymorphism (related to folate metabolism) were 15 times more likely to develop organ dysfunction when consuming inadequate choline. Further findings suggest that the rs12325817 polymorphism in the PEMT gene—prevalent in about 75% of the population—significantly increases choline requirements, while the rs7946 polymorphism may necessitate a higher choline intake to avert fatty liver disease.

Although more research is warranted, preliminary evidence indicates that the rs12676 polymorphism in the choline dehydrogenase (CHDH) gene may increase susceptibility to choline deficiency, suggesting that individuals with this variant need to consume more choline to maintain optimal health. So, what implications does this have for those removing high-choline animal products from their diets? If an individual has standard choline needs and a favorable genetic profile, it may be feasible to maintain adequate choline levels on a vegan diet—certainly for vegetarians who consume eggs.

However, for new or expectant mothers, men, or postmenopausal women with lower estrogen levels, along with individuals harboring genetic mutations that heighten choline demands, relying solely on plant sources might not suffice to provide this essential nutrient. In such situations, adopting a vegan lifestyle could potentially lead to muscle deterioration, cognitive issues, heart disease, and an accumulation of fat in the liver.

The Bottom Line

When the optimal genetic (and microbial) factors align, vegan diets—supplemented with essential vitamin B12—stand a better chance of fulfilling an individual's nutritional requirements. This is particularly relevant in diverse dietary contexts such as in India, where traditional plant-based diets are prevalent, or in Australia, where veganism is gaining traction. However, complications arising from vitamin A conversion, the composition of the gut microbiome, levels of amylase, or specific choline needs can significantly diminish the likelihood of thriving on a vegan regimen.

Recent scientific research increasingly supports the notion that personal differences play a crucial role in how individuals respond to various diets. Some individuals possess a unique ability to efficiently extract the necessary nutrients from plant-based sources, or they may effectively synthesize what they need through the remarkable capabilities of the human body, as observed in traditional Japanese diets rich in fermented foods or among Canadian populations embracing local plant harvests.