"Veganuary": The effects of a short-term vegan diet on nutrient intake Original paper

In this 4-week observational study, switching from an omnivorous diet to a vegan diet led to reduced intakes of saturated fat, cholesterol, vitamin B12, and iodine.

The effects of a short-term vegan diet on nutrient intake.

81 adults (average age of 33; 80% women, 20% men) living in the United Kingdom.

In this 4-week observational study, the participants were categorized into one of the following groups based on their dietary history and intention to participate in "Veganuary" (i.e., the consumption of a vegan diet throughout the month of January):

• VV: vegans who remained vegan
• VegV: vegetarians who participated in Veganuary
• OO: omnivores who remained omnivorous
• OV: omnivores who participated in Veganuary

At baseline, dietary intake (including supplements) over the past 6 months was assessed using a food frequency questionnaire. A food frequency questionnaire was also completed at the end of the study to assess dietary intake over the past 4 weeks. Additionally, participants in VegV and OV filled out a “whoops” log to track the contribution of animal product consumption (accidental or deliberate) to nutrient intake.

Differences between groups were detected for changes in nutrient intake over time. Most notably, VegV and OV decreased their intakes of saturated fat (from 19 g/day and 26 g/day to 13 g/day and 12 g/day, respectively) and cholesterol (from 124 mg/day and 273 mg/day to 20 mg/day and 25 mg/day, respectively).

Without considering supplements, compared to baseline, vitamin B12 (from 5.1 to 1.8 µg/day) and iodine (from 125 to 38 µg/day) intake decreased, while vitamin B1 intake increased in OV (from 1.9 to 3.3 mg/day). When supplements were considered, there was no longer a decrease in vitamin B12 intake, but the decrease in iodine intake remained.

Professional organizations have stated that appropriately planned vegan diets are nutritionally adequate and can provide an array of health benefits.^[2][3][4] However, the phrase “appropriately planned” warrants emphasis. Whenever an entire group of foods is excluded from the diet, nutrient inadequacies can arise.

The summarized study reported that a 4-week vegan diet led to a decrease in vitamin B12 and iodine intake. There also appeared to be a decrease in protein intake (from 73 to 47 grams per day) and selenium (from 52 to 30 µg/day), but these findings weren’t statistically significant.

According to the data obtained from a questionnaire completed at baseline, micronutrient knowledge was relatively low among the participants: 30% or less of participants from each group reported being generally aware of micronutrients. The most commonly recognized micronutrients were iron (recognized by 54% of participants in VV; 53% in VegV; 43% in OV; 35% in OO) and vitamin D (recognized by 46% of participants in VV; 33% in VegV; 48% in OV; 50% in OO), and among symptoms of micronutrient deficiency, those related to iron (e.g., anemia) and vitamin D (e.g., rickets) were the most commonly reported. In comparison, 0%–8% of the participants from each group were familiar with niacin, vitamin B6, folate, or selenium.

Considering this lack of awareness, in combination with the lack of nutritional guidance provided during the study, it’s interesting that there weren’t more significant changes in nutrient intake in OV. Intakes of vitamin A, vitamin C, vitamin D, calcium, iron, zinc, magnesium, potassium, riboflavin, niacin, folate, and vitamin B6 did not appear to meaningfully differ from baseline.

Other studies that examined changes in nutrient intake following a short-term vegan diet have reported similar results. In other words, the consumption of a vegan diet led to reduced intakes of vitamin B12, iodine, and protein, while there were no meaningful changes in the intakes of other nutrients.^[5][6] Notably, in one study,^[6] these changes occurred despite the research staff providing continuous dietary guidance, as well as educational handouts, shopping lists, and recipes. This suggests that specific recommendations, as opposed to general guidance, are probably necessary to optimize the intake of certain nutrients when switching to a vegan diet.

Because changes in nutrient intakes over a month are not necessarily generalizable to the intakes of long-term vegans, it’s necessary to compare the findings of short-term dietary intervention studies to those derived from longer-term observational studies before drawing conclusions about which nutrients the average vegan diet may contain inadequate amounts of.

Evidence from systematic reviews generally indicate that vegan diets are usually deficient in vitamin B12 and contain inadequate amounts of iodine.^[7][8] Additionally, vegan diets may increase the risk of inadequate intakes of calcium, zinc, and selenium.^[7][8]

Vegan diets might also increase the risk of an inadequate intake of vitamin D, although it’s worth noting that vitamin D intake among omnivores is also commonly inadequate.^[8] Vegan diets tend to contain less protein than omnivorous diets, but on average their intakes meet or exceed the amount recommended by government agencies to support general health.^[3] In contrast, vegan diets tend to contain higher amounts of fiber, folic acid, thiamine, vitamin C, vitamin E, and magnesium than omnivorous diets.^[8]

Besides nutrient intake, nutrient status — generally indicated by blood levels of nutrients — can be assessed to obtain greater insight into which nutrients might be deficient in the average vegan diet. Deficiencies in iodine, vitamin D, vitamin B12, iron, and zinc appear to be more prevalent in vegans than omnivores.^[8] In contrast, blood levels of vitamin C are typically higher and the prevalence of folate deficiency is lower in vegans than omnivores.^[8] There don’t appear to be notable differences between vegans and omnivores for blood levels of vitamin A, vitamin E, thiamine, vitamin B6, magnesium, or phosphorus.^[8]

Collectively, the available evidence indicates that, when not appropriately planned, a vegan diet can increase the risk of an inadequate intake of several nutrients, most notably vitamin B12 and iodine. An inadequate intake of certain nutrients is not unique to vegan diets (the average omnivorous diet also tends to contain inadequate amounts of certain nutrients.^[8]) Vitamin B12 is virtually absent in plant foods, so it’s necessary to supplement with vitamin B12 or seek out foods fortified with vitamin B12, such as plant-based milks and yogurts, meat analogues, breakfast cereals, and yeast products. Iodine is also virtually absent in plant foods, with the exception of seaweed, as well as fortified foods. Iodine is primarily obtained through dairy products, seafood, and iodized salt. In the summarized study, the significant reduction in iodine intake in OV was mostly due to removing cow’s milk, dairy yogurt, and white fish from the diet.

Nutrients of concern, and select sources of these nutrients, on a vegan diet

The nutrients are ranked in order of the probability of an inadequate intake arising after switching from an omnivorous diet to a vegan diet, if appropriate planning does not occur. Supplementation is a viable option for each nutrient.

Although it’s true that it may be difficult to get enough of certain nutrients while following a vegan diet, it’s also true that vegan diets are suitable for all stages of life,^[2] and the consumption of a vegan diet can improve cardiometabolic risk factors^[9][10][11] and reduce the risk of various chronic diseases.^[12][9] However, not all vegan diets are created equal, as the quality of each diet ultimately depends on the quality of the foods it contains.^{[13][14][15][16]} Whether an individual is transitioning to a vegan diet primarily for ethical or health-related reasons, they should consult with a registered dietitian in order to prevent the possibility of nutrient inadequacies and optimize health outcomes.

A limitation of this study is that 40% of participants failed to provide data at the end of the study, so there was only complete data from 46 participants.

This study was conducted during the winter, and dietary intake can vary by season. This is particularly true in January, when more people tend to pursue weight loss goals^[1] (as a New Year’s resolution, for example), so these findings may have limited generalizability and should be interpreted with caution.