How does partially replacing animal- with plant-based proteins affect nutrition status? Original paper

In general, the production of animal-based foods, such as meat and dairy, hurts the environment more than the production of plant-based foods.^[1] Moreover, plant-based diets have been associated with a variety of health benefits — such as a lower risk of death and noncommunicable diseases.^[2][3][4]

Plant-based diets aren’t always perfect, however: they can be poor in some micronutrients, such as iodine and vitamin B12,^[5][6][7][8] and can even reduce the absorption of some micronutrients (e.g., zinc absorption is reduced by phytates, which are found notably in legumes). The benefits and drawbacks of a plant-based diet will vary based not only on the specific foods consumed but also on how these foods are processed and prepared.^[9]

Since people on a Western diet tend to eat more animal-based foods than is good for their health,^[1] they may want to switch to a plant-based diet, but how much of their protein intake should come from plant-based foods and how much from animal-based foods still needs to be determined.

This 12-week randomized controlled trial conducted in Finland included 136 healthy adults aged 20–69 (107 women, 29 men). On average, at baseline, 62% of the participants’ protein intake came from animal-based foods.

The participants were divided between three groups: ANIMAL (70% of protein from animal-based foods), PLANT (70% of protein from plant-based foods), and BALANCED (50/50).

The participants were told to eat according to their appetite and maintain a stable weight. They received enough food items to cover around 80% of their energy needs, estimated at 2,000 kcal/day. All three groups receive the same amount of fish and eggs, but different amounts of poultry, red meat, and dairy products. In the PLANT and BALANCED groups, animal-based proteins were replaced by proteins from cereals (e.g., oatmeal), pulses (e.g., lentils, chickpeas), nuts, seeds, and plant-based dairy substitutes, none of which were fortified with B₁₂, folic acid, iodine, iron, or zinc. Furthermore, the participants were not allowed to consume dietary supplements.

The dietary intakes of three vitamins (B₁₂, C, and folate) and three minerals (iodine, iron, and zinc) were assessed via 4-day food records. Fasting blood samples were taken at baseline and at the end of the intervention to measure folate, holotranscobalamin II (an indicator of B₁₂ status), and three indicators of iron status:^[10] hemoglobin, ferritin, and transferrin receptor. Similarly, 24-hour urine samples were taken at baseline and at the end of the intervention to measure iodine status.

The following results were reported for micronutrient intakes:

• B₁₂ was lower in PLANT (2.3 µg/day) and BALANCED (3.4 µg/day) than ANIMAL (4.9 µg/day). It was also lower in PLANT than BALANCED.
• Iodine was lower in PLANT (180.9 µg/day) and BALANCED (188.1 µg/day) than ANIMAL (263.9 µg/day).
• Zinc was lower in PLANT (12.3 mg/d) than ANIMAL (14.1 mg/day).
• Vitamin C didn’t differ between groups.
• Folate was higher in PLANT (364.1 µg/day) than ANIMAL (305.6 µg/day).
• Iron was higher in PLANT (17.9 mg/day) than ANIMAL (14.3 mg/day).

The following results were reported for biomarkers of nutritional status:

• B₁₂ was lower in PLANT (97.7 pmol/L) than BALANCED (105.8 pmol/L) and ANIMAL (122.1 pmol/L).
• Iodine was lower in PLANT (128.7 µg/day) and BALANCED (123.5 µg/day) than ANIMAL (197.4 µg/day).
• Folate didn’t differ between groups.
• None of the iron biomarkers differed between groups.

A limitation of this trial is its homogenous sample: 79% of the participants were women, and 69% of these women were highly educated (i.e., had higher than a bachelor’s degree). Furthermore, the relatively high intakes of fiber and folate at baseline suggest that the participants were more health-conscious than the average person.

All in all, the present findings align with the body of evidence concerning the nutritional adequacy of plant-based diets. Most notably, there was a dose-dependent reduction in the intake of B₁₂: the average intake was greater in ANIMAL than BALANCED and in BALANCED than PLANT. Consequently, PLANT also had the lowest average holotranscobalamin levels, which had decreased in this group by 23% over the 12-week intervention.

Similarly, a 2019 trial observed a 30% decrease in holotranscobalamin levels after a 4-week vegan diet.^[5] A lower intake of B₁₂ with plant-based diets is also supported by a plethora of observational studies,^[11][6] including the EPIC-Oxford cohort study, which reported that meat-eaters had the highest B₁₂ intake, followed by progressively lower intakes in fish-eaters, lacto-ovo vegetarians, and vegans.^[12]

With that said, the average B₁₂ intake in PLANT (2.3 µg/day) exceeded the Nordic Nutrition Recommendations (2.0 µg/day), and the average holotranscobalamin levels (97.7 pmol/L) were far from inadequate.

Unfortunately, the present study’s authors aren’t always clear or precise. For instance, they state that three of the participants in PLANT and BALANCED fell below the deficiency threshold (<35 pmol/L) over the course of the study, but it isn’t clear if that means three participants in each of these two groups (meaning six people total) or three participants between these two groups (meaning three people total). Also, the paper doesn’t mention the baseline levels of these participants, so we cannot tell if the decrease they experienced was big or small.

Still, let’s seize this opportunity to mention that certain individuals are at a greater risk of B₁₂ deficiency, maybe for genetic reasons.^[13] Therefore, a B₁₂ supplement may be needed when replacing the majority of animal-based protein with plant-based protein.

The intake of iodine was greater in ANIMAL than in BALANCED and PLANT, but each group exceeded the recommended intake (150 µg/day). However, iodine intake may have been overestimated, since cooking losses — which range from 6% to 52%^[14] — were not accounted for. This would explain why iodine deficiency was more prevalent in PLANT and BALANCED than in ANIMAL, despite the average levels being adequate (>100 µg/day^[15]) in all three groups. Other studies have also reported a higher probability of iodine deficiency among vegans than among omnivores.^[11] Potential solutions include eating seaweed and adding iodized salt to meals.

The intake of iron was highest in PLANT. Correspondingly, cross-sectional studies have reported higher intakes in vegans and vegetarians than in omnivores.^[16][17][6] Despite the higher intake in PLANT, however, none of the iron biomarkers differed between groups, maybe because plant iron (aka nonheme iron) is less bioavailable than animal iron (aka heme iron), and maybe because some plant-based foods contain “antinutrients” that can reduce iron absorption (e.g., phytates).^[18]

However, the bioavailability of nonheme iron can be enhanced by vitamin C,^[19] and vitamin C intake in this study was about twice higher than the Recommended Daily Allowance (RDA). Moreover, over time, people can become better at absorbing and retaining nonheme iron, as shown by a progressive reduction of fecal iron.^[20][21] This improvement might be partly due to the body adapting to the presence of antinutrients, since the malabsorptive effects of antinutrients appear to decrease over time.^[22][23] Thus, even though vegetarians tend to have lower iron stores than omnivores,^[24] they are not at a much greater risk of iron deficiency.