Will going vegan make me tired or weak?

The objection

“Plant iron is poorly absorbed and plants have no B12 at all, so vegans run low on both, and a diet that only works with supplements and careful planning isn’t one most people can keep up for life.”

The answer

A planned vegan diet shouldn’t leave you tired or weak. The NHS says that with good planning a vegan diet can give you all the nutrients your body needs [1], the British Dietetic Association that carefully planned plant-based diets can support healthy living at every age and life stage [2], and the US Academy of Nutrition and Dietetics, in its 2025 position, that appropriately planned vegan diets can be nutritionally adequate for adults [3]. For children, the nutrition committee of ESPGHAN, the European Society for Paediatric Gastroenterology, Hepatology and Nutrition, said in 2025 that the evidence is inconclusive on whether a strictly vegan diet supports normal growth, although the studies it reviewed found no significant difference in height or body mass index between vegan and omnivorous children, and recommended regular checks on vegan children’s diet, growth and nutritional status, with supplements including B12 (Verduci et al., 2025) [19]. The objection is right that B12 and iron need deliberate attention: B12 because plants don’t provide it, and iron because the body absorbs it less well from plants than from meat [1][2].

Weakness, in the sense of muscle and strength, is a fair worry, and the trials are mostly reassuring. A 2025 meta-analysis of randomised trials (Reid-McCann et al.) found that plant protein built slightly less muscle than animal protein overall, a gap that showed up with rice, chia, oat and potato proteins and in trials of whole plant-based diets, while soy matched milk protein [4]. Strength differed less: across 14 trials upper-body strength was similar, and across 11 trials leg strength showed a small edge for animal protein that was statistically clear only in adults aged 60 and over [4]. In a 2021 Brazilian study of 38 untrained young men, 19 of them already vegan, whose protein was topped up to 1.6 g per kg of body weight a day with soy or whey, the vegans and the omnivores both gained 1.2 kg of leg lean mass over 12 weeks of weight training, and their strength gains did not differ (Hevia-Larraín et al., 2021) [5]. Those groups weren’t randomised, and whether it is harder to build muscle on a vegan diet has its own answer.

Over the long term, the evidence is reassuring on some diseases and not on others. EPIC-Oxford, a UK study that recruited adults between 1993 and 2001 and included a large share of people who ate no meat, followed 48,188 of them for about 18 years. Vegetarians, vegans included, had 22% less ischaemic heart disease than meat-eaters, 10 fewer cases per 1,000 people over ten years, but 20% more strokes, three more per 1,000 over ten years (Tong et al., 2019) [6]. A 2023 meta-analysis of 13 cohort studies found 21% less ischaemic heart disease in vegetarians than in non-vegetarians and no clear difference in stroke (Dybvik et al., 2023) [20]. In EPIC-Oxford, vegetarians were also 37% less likely than regular meat-eaters to be admitted to hospital with diabetes or to die with it, a gap that was no longer clear once their lower body weight was allowed for (Papier et al., 2019) [7]. Studies that observe people in this way can show links, but they can’t prove the diet caused them.

Bones are where the long-term evidence is least reassuring. In EPIC-Oxford the vegans had 2.31 times the meat-eaters’ rate of hip fracture, 14.9 more per 1,000 people over ten years (Tong et al., 2020) [8], and a 2026 meta-analysis of four cohort studies with 529,672 participants found a 75% higher risk of hip fracture in vegans than in meat-eaters and a 25% higher risk in vegetarians, while rating the certainty of that evidence very low for vegans and low for vegetarians (Ballarin et al., 2026) [21]. Allowing for calcium and protein intakes narrowed the EPIC-Oxford gap only slightly, and its authors concluded that vegans’ bone health needs more research [8]; the NHS, which says calcium is needed to keep bones healthy, lists it among the nutrients a vegan diet can miss without planning [1].

Relying on fortified foods and supplements isn’t something only vegans do. Because it is hard to get enough vitamin D from food alone, the NHS advises everyone to consider a daily vitamin D supplement in autumn and winter [22], and in the UK calcium is added by law to white and brown flour, so white and brown bread carry it whatever else you eat [1].

Why might I feel tired after going vegan?

Many cases of tiredness come down to stress, too little sleep, poor diet or other parts of daily life, the NHS says [9], and on a vegan diet the dietary causes to rule out are eating too little, missing B12 and running short of iron. The NHS’s list of common causes also includes depression, hormonal changes such as pregnancy or the menopause, and illnesses such as glandular fever and COVID-19 [10], so feeling flat in the months after a change of diet doesn’t by itself point to the food. Some reviews have linked vegetarian and vegan diets with more depression, but a 2026 umbrella review that assessed nine reviews of plant- and meat-based diets and depression found their results conflicting, drawn mostly from observational studies and rarely backed by any measure of how closely people kept to the diet (Lee et al., 2026) [23]. Its authors add that the reasons people take up these diets, such as health problems or ethical concerns, may be linked with mental health in either direction [23].

Eating less than you need is the first thing to rule out. In a 2021 trial at the US National Institutes of Health, 20 adults allowed to eat as much as they liked ate 689 kcal a day less on a low-fat plant-based diet than on an animal-based ketogenic one, over two weeks on each (Hall et al., 2021) [11]. Among people already eating this way the gap is smaller: averaging the 65 studies that measured it, a 2021 systematic review by two Unilever scientists put vegans at 1,947 kcal a day and meat-eaters at 2,101, a difference the reviewers called similar [12]. To keep energy up through the day, the NHS suggests regular meals and healthy snacks every three to four hours [9].

Can low B12 or iron make vegans tired?

Both can, because a shortage of either can cause anaemia, and shortages of both are more common in vegans than in meat-eaters. Plants can’t supply B12, the British Dietetic Association says, so vegans must get it from fortified foods or supplements, and too little of it can cause fatigue, anaemia and nerve damage [2]. The NHS lists extreme tiredness, a lack of energy and muscle weakness among the symptoms of deficiency, and urges prompt treatment because some of the problems it causes can be irreversible [13]. In EPIC-Oxford blood samples taken from 1994 to 1997, 52% of 232 vegan men had serum B12 below the cut-off for deficiency (Gilsing et al., 2010) [14]. A 2024 meta-analysis of 17 studies found lower serum B12 and higher homocysteine, a marker of functional B12 deficiency, in vegans than in omnivores, and better results on every marker in vegans who took a B12 supplement than in those who didn’t (Niklewicz et al., 2024) [24]. The B12 answer covers how much to take.

Iron stores run lower in vegetarians and vegans on average. A 2018 meta-analysis of 24 studies found vegetarians’ ferritin, the blood measure of stored iron, averaged 29.7 µg/L lower than non-vegetarians’ (Haider et al., 2018) [15], and in the four studies in the 2021 review that counted iron deficiency, it affected 15% of vegans against 7% of meat-eaters on average [12]. Lack of iron causes iron deficiency anaemia, whose symptoms include tiredness and lack of energy [16]. So iron is worth planning for, especially for women with periods. The NHS names heavy periods as a very common cause of iron deficiency anaemia [16], and a 2018 review of 13 studies found iron-deficiency anaemia more common among vegetarians, particularly women before the menopause (Pawlak et al., 2018) [17]. In the studies of vegetarian women in that review, between 12% and 79% had depleted iron stores, with ferritin below 12 µg/L [17]. Pulses, wholemeal bread, iron-fortified breakfast cereals, dark green leafy vegetables, nuts and dried fruit all supply it, and the NHS advises eating foods with vitamin C at the same meal and keeping tea and coffee away from main meals [1]. The iron answer has the detail.

2021 review averages	Meat-eaters	Vegetarians	Vegans
Energy, kcal a day	2,101	2,098	1,947
Ferritin, µg/L	55.5	33.8	31.3
Iron deficient	7%	11%	15%

The energy figures come from 65 studies, ferritin from 17 and iron deficiency from four, averaged across studies without being pooled in a meta-analysis [12]. Iodine belongs on the list as well: vegans had the lowest intakes of it in the review, and in all three studies that compared them directly with meat-eaters, lower levels of iodine in their urine, the usual measure of iodine status [12]. A 2023 meta-analysis of 11 studies covering 4,421 adults also found the poorest iodine intake and the lowest urinary iodine in vegans, and concluded that vegans living in countries with no programme to iodise all salt remain at increased risk of iodine deficiency (Eveleigh et al., 2023) [25]. Too little iodine is a rare cause of an underactive thyroid in the UK, but extreme tiredness is one of that condition’s common symptoms [18]. The British Dietetic Association advises vegans who drink soya or oat drinks to choose one fortified with iodine and, with very few rich sources in the diet, to consider a suitable supplement of no more than 150 mcg a day; it warns that seaweed’s iodine content varies and can be excessive, so brown seaweed or kelp shouldn’t be eaten more than once a week and seaweed or kelp supplements shouldn’t be used as a source [26].

When should I see a GP about tiredness?

If you’ve felt tired for a few weeks and can’t tell why, see a GP rather than guessing; a blood test can check for anaemia, low B12, thyroid problems and diabetes [10][13][18]. A test matters because the cause often isn’t the diet. The commonest cause of B12 deficiency in the UK is pernicious anaemia, in which the immune system stops the body absorbing B12 from food, and a diet short of B12 is a less common cause, though it can happen to vegans who take no supplement and eat no fortified foods [13]. Iron deficiency anaemia often comes from blood loss, such as heavy periods or bleeding in the stomach or gut, which a GP will want to find before recommending treatment, and the NHS recommends iron tablets when a blood test shows a low red blood cell count [16]. Keep iron tablets out of children’s reach, because the NHS warns that an overdose of iron can be fatal for a young child [16].

If cutting out foods has become a way of restricting how much you eat, a GP or an eating-disorder charity can help: Beat in the UK (beateatingdisorders.org.uk), Bodywhys in Ireland (bodywhys.ie), ANAD in the US (anad.org), NEDIC in Canada (nedic.ca) and the Butterfly Foundation in Australia (butterfly.org.au), while in New Zealand EDANZ (ed.org.nz) supports families and carers.