Vegan diets: the cardiometabolic upside and the nutrient gaps

A healthy vegan diet improves several cardiometabolic markers — in a controlled trial it lowered LDL cholesterol, fasting insulin and body weight within eight weeks — and the pattern is linked to lower rates of several chronic diseases [s1][s2]. The same diet comes with genuine nutrient gaps that have to be managed: vitamin B12 must come from fortified foods or supplements, and iron stores tend to run lower than in people who eat meat [s2][s3].

The cleanest test: identical twins

Diet studies are usually confounded by genetics and lifestyle, because the people who choose to eat one way differ from those who choose another. A trial published in JAMA Network Open in 2023 sidestepped much of that by randomising within pairs of identical twins: 22 pairs, 44 people in all, with one twin assigned a healthy vegan diet and the other a healthy omnivorous diet for eight weeks [s1]. For the first four weeks meals were delivered; for the last four, participants cooked for themselves [s1].

Compared with their omnivore twins, the vegan-diet twins had significantly larger reductions in LDL cholesterol (−13.9 mg/dL; 95% CI, −25.3 to −2.4), fasting insulin (−2.9 μIU/mL; 95% CI, −5.3 to −0.4) and body weight (−1.9 kg; 95% CI, −3.3 to −0.6) [s1]. Because the twins share a genome, those differences are hard to attribute to anything but the food.

Two limits deserve equal billing. The trial ran only eight weeks, so it measured markers, not heart attacks; and one of its authors reported funding from a plant-based meat company, a disclosure worth carrying whenever the result is cited [s1]. The finding is a real short-term signal on risk factors, not proof of long-term protection.

What the broader position says

The Academy of Nutrition and Dietetics, the largest body of registered dietitians, holds in its position paper that appropriately planned vegetarian and vegan diets are nutritionally adequate and appropriate for all life stages, including pregnancy, infancy and older age [s2]. It notes that vegetarians and vegans are at reduced risk of ischemic heart disease, type 2 diabetes, hypertension, certain cancers and obesity, and attributes much of that to low saturated-fat intake and high intakes of vegetables, fruit, whole grains, legumes, nuts and seeds, which lower total and LDL cholesterol and improve blood-glucose control [s2].

The load-bearing word is "planned." The same document states plainly that vegans need reliable sources of vitamin B12, such as fortified foods or supplements [s2]. B12 is made by bacteria and reaches the human diet almost entirely through animal foods; a vegan diet without a deliberate source of it risks a deficiency that can cause anaemia and irreversible neurological damage. This is the one gap that is not a matter of careful eating — it requires an external source.

The iron trade-off

Iron is the second recurring concern, and the picture is more nuanced than "vegans are iron-deficient." A systematic review and meta-analysis pooled 24 cross-sectional studies and found that adult vegetarians had significantly lower serum ferritin — the standard marker of iron stores — than non-vegetarians, by a mean of 29.71 µg/L (95% CI, 19.73 to 39.69) [s3]. The gap was widest in men, at 61.88 µg/L (95% CI, 38.17 to 85.59), and smaller in premenopausal women, at 17.70 µg/L (95% CI, 5.60 to 29.80) [s3].

Plant (non-haem) iron is less readily absorbed than the haem iron in meat, which is the mechanism behind the difference [s3]. But the review's authors added a caveat that cuts against a simple reading: high iron stores are themselves a risk factor for conditions including type 2 diabetes, so they recommended that everyone, not only vegetarians, monitor iron status rather than assume more is better [s3]. Lower ferritin is not automatically a deficiency, and the meta-analysis measured stores, not rates of anaemia.

The honest summary

The cardiometabolic case for a well-constructed plant-based diet is solid and points the same direction as the evidence for cutting dietary cholesterol from foods like eggs and for the Mediterranean pattern, which is itself largely plant-based. The gaps are specific and manageable: B12 is non-negotiable and must be supplemented, iron status is worth checking, and both are the reason the guidance keeps returning to the word "planned" [s2].

This is general information, not dietary advice. Nutrient status, supplementation and whether any eating pattern suits a particular person are matters for a clinician or registered dietitian.

Sources

• Cardiometabolic Effects of Omnivorous vs Vegan Diets in Identical Twins — JAMA Network Open, 2023-11-30
• Position of the Academy of Nutrition and Dietetics: Vegetarian Diets — Journal of the Academy of Nutrition and Dietetics, 2016-11-22
• The effect of vegetarian diets on iron status in adults: A systematic review and meta-analysis — Critical Reviews in Food Science and Nutrition, 2016-11-23