II. Chronic diseases
It has been demonstrated in Dyett et al. (2013) study that people following a vegan diet have a lower susceptibility to develop chronic diseases, these results are concordant with other research cited in this study. However, this low prevalence can also be associated with the lifestyle that vegans of this study were following: regular exercising, rarely smoking, rarely drinking alcohol. Consequently, these factors can distort the results of cardiovascular diseases and cancers because they take part of the health of the participants.

1. Obesity
Dyett et al. (2013) and Spencer et al. (2003) have shown in their study that globally vegans, both men and women, have a lower body mass index (BMI) than non-vegan people (Fig. 1). Spencer et al. (2003) study also showed that vegans have a lower prevalence of obesity than omnivores: 1.8% and 1.9% for vegans respectively women and men versus 5.7% and 5.0% for omnivores respectively men and women; pesco-vegetarians (don’t eat meat but fish) and vegetarians.
Figure 1: Mean BMI for different diet groups by age and sex (Spencer et al., 2003)
These differences of BMI and obesity prevalence are partially due to the lifestyle behaviors of participants within the four different diet groups of this study. But this is mostly explained by the difference of nutrient and energy intake between vegans and omnivores, especially fiber and protein. Vegan people globally have a higher fiber intake than other diet groups because of their diet. A lower BMI is a result for a higher fiber intake. Vegan people globally consume less protein than other diet groups and they just consume plant protein, which has no significant consequence on the BMI. A lower animal protein intake bring a decrease of the BMI.

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2. Cardiovascular diseases
Kim et al. (2012) and Szeto et al. (2004) studies have proven that long-term vegetarians (vegans included in this group) had a diet richer in vitamin A and C than healthy omnivores, specifically because of the higher consumption of fruits and vegetables. Vitamin A and C are antioxidant; therefore, vegetarians are less exposed to oxidative stress than omnivores. Higher antioxidant and in particular vitamin C dietary intake (and presence in plasma) is associated with a lower susceptibility to develop cardiovascular diseases, especially ischemic heart diseases and stroke (Szeto et al., 2014).
Obesity is equally an important risk factor to develop cardiovascular diseases. The lower level of obesity, and the lower BMI in vegans play a significant role in the prevalence of cardiovascular diseases.
Moreover, age-adjusted and sex-adjusted death rate ratio for ischemic heart disease for vegans and omnivores have been compared in Key et al. (1999) study: the vegan death rate ratio (0.74) is lower than the omnivore one (1.00 for regular meat eaters and 0.80 for non-regular). Otherwise, when pesco-vegetarians and vegetarians eating dairy products and eggs are compared with vegans in this same study, the death rate ratio for ischemic disease is higher for vegans (0.74 compared to a ratio of 0.66 for both pesco-vegetarians and vegetarians). These last data may be distorted because the number of vegans participants to this study was way smaller than the number of participants of the other diet groups. Similarly, Appleby et al. (2002) compared mortality from ischemic heart disease for vegetarians and non-vegetarians (vegans were included in the vegetarian group). This study showed a difference of death rate from this disease between the two diet groups: vegetarians had a 24% lower death rate than non-vegetarians.

3. Hypertension and cataract
Hypertension is the first risk factor for a stroke. Appleby et al. (2002) showed that vegans had a significantly lower systolic and diastolic blood pressure than omnivores, pesco-vegetarians and vegetarians. This difference is partially explained by the BMI difference between diet groups, the vegan group being the one with the lower BMI. A high BMI is an important risk factor for all cardiovascular diseases but especially for hypertension (Dua et al., 2014). With a lower blood pressure, vegans have a lower prevalence of hypertension than omnivores. In Appleby et al. (2002) study, the hypertension incidence for omnivores was 15.0% for men and 12.1% for women and only 5.8% for vegan men and 7.7% for vegan women and intermediate values for pesco-vegetarians and vegetarians. Like the blood pressure, this difference is partially attributed to a lower BMI for vegans. Age-adjusted and BMI-adjusted values still showed a significant difference between omnivores and vegans, the adjusted hypertension incidence for omnivores (globally, men and women) was 12.9% and 6.1% for vegans. Because of this difference, vegans have a lower risk to have a stroke or any other coronary heart disease.
Yu et al. (2014) have demonstrated in their study that a high blood pressure is associated with a higher cataract risk. Thus, hypertension is a risk factor of cataract if we don’t consider the different types of cataracts. However, the severity and the duration of hypertension both play a significant role in the cataract risk: a higher severity and a longer duration are associated with a higher cataract risk. Appleby et al. (2011) demonstrated in their study that there is a difference between diet groups for cataract risks: vegans have the lower cataract risk and omnivores have the higher risk. According to this study, people following a vegan diet have almost half less risk of developing cataract than omnivores who eat more than 100 grams of meat per day. This difference can partially be explained by the fact that hypertension is a risk factor for cataract. It also can be explained by the difference of nutrients and antioxidants intake between omnivores and vegans but in this study they didn’t found any particular nutrient that is significantly associated with a higher or lower cataract risk.

4. Cancer
Cancer is also a common argument to begin a vegan or a vegetarian diet as red and processed meat consumption is commonly known as a cancer risk factor. In Diallo et al. (2017) study with 61 476 participants, it has been demonstrated that red meat was significantly associated with a higher overall cancer risk and breast cancer risk, even when the values were BMI-adjusted. Moreover, this study showed that the consumption of red meat increased the breast density in women. A high breast density positively impact the breast tumor risk. Identically, Anderson et al. (2018) found that red meat consumption is a risk factor for breast cancer but, counter to Diallo et al. (2017), they also found a significant association between processed meat and breast cancer. Among 262 195 women in Anderson et al. (2018) study, those who ate processed meat had a 2.55 per 1000 incidence for breast cancer against a 2.46 per 1000 incidence for those who don’t eat any processed meat. In Sinha et al. (2009) study on men, red meat consumption was directly associated with a higher prostate cancer risk as well, and in Cross et al. (2007) study, it was also associated with a higher colorectal, lung, oesophageal and liver cancer risk. In this same study, processed meat consumption was associated with a higher colorectal and lung cancer risk.
On the other hand, in Cross et al. (2007) study, red meat consumption was associated with lower endometrial cancer risk and processed meat was associated with lower lymphotic leukemia and melanoma risk.
The vegetable consumption has also an incidence on cancers. A plant-based diet can be protective against prostate and breast cancer according to Ferdowsian & Barnard (2007). In Anderson et al. (2018) study, women who ate a lot of vegetables had a lower breast cancer incidence: on women who don’t consume processed meat: incidence of 2.46 per 1000 for those who don’t eat many vegetables, 2.01 per 1000 for those who eat a lot of vegetables. Beezhold et al. (2018) showed in their study that vegetarians and vegans are less likely to develop breast tumors because of their alimentation full of vegetables and their higher phytochemicals dietary intake compared to omnivores or pesco-vegetarians.
In Key et al. (2014) study, comparing cancer risk between vegans and other diet groups, they found that vegans had almost 20% less risk than omnivores for overall cancers and vegan diet was the diet with the lower global cancer incidence (Table 1). This is coherent with Diallo et al. (2017) study but unlike them, Key et al. (2014) didn’t find a significant difference of incidence for colorectal, breast and prostate cancer between vegans and omnivores.
Table 1: Ratio rates and number of cancers by diet group

III. Nutrients status
1. Vitamin B12 (cobalamin)
Multiple studies agree on the fact that people following a vegan diet have the lower serum concentration mean of vitamin B12 comparing to omnivores, pesco-vegetarians and vegetarians; the omnivore group being the one with the higher mean of serum vitamin B12 (Fig. 2). In both Gilsing et al. (2012) and Donaldson (2000) studies, it has been found that approximately half (respectively 57% and 50%) of vegans had a serum vitamin B12 concentration lower than the normal range. Gilsing et al. (2012) study also compared the difference of serum vitamin B12 between participants consuming cobalamin supplements and participants who didn’t use supplements. They didn’t find any significant difference between these two groups. This study also showed that 37% of vegans using vitamin B12 supplements and 95% of vegans not using supplements were below the United-Kingdom reference nutrient intake for cobalamin (1.5 mg/day).
Even if according to the last study, it doesn’t make any difference in the serum concentration of vitamin B12, it is important for vegans to consume dietary supplements in vitamin B12. The serum concentration of cobalamin is not that important because it doesn’t consider the level of cellular vitamin B12, which is more important (Mitsuyama ; Kogoh, 1988). The cellular vitamin B12 is the one being bioavailable; only 10 to 30% of the serum vitamin B12 is bioavailable. Thus, it is hard to really evaluate a cobalamin deficiency with a serum concentration test. However, globally, when the serum concentration of vitamin B12 is lower than the reference range (;200pg/mol), the person is deficient but a deficiency is also possible with a higher serum concentration of vitamin B12.
According to Haddad et al. (1999), cobalamin is important for the body, and a defect in this vitamin can alter both DNA synthesis and erythropoiesis by creating immature and large red blood cells which can lead to anemia. The alteration of DNA synthesis can also lead to neurotransmitter and hormone disturbance that can cause neurological and psychological problems. That is why it is recommended for vegans to supplement themselves with active vitamin B12 to have a daily intake of vitamin B12 at least close to the reference nutrient intake. Moreover, a study on young vegans (Larsson ; Johansson, 2005) have demonstrated that with a healthy vegan diet and supplement in vitamin B12, people can have all the necessary nutritional intakes.
Figure 2: Serum concentration of cobalamin in different diet groups in function of the age (?: omnivores, ?: vegetarians, ?: vegans) (Gilsing et al., 2012)
It can take many years to detect a cobalamin defect because first, as said earlier, the serum cobalamin concentration is not that significant and, second, it can decrease under the reference range after a long time. However, Gilsing et al. (2012) showed in their study that once cobalamin serum concentration goes down, it usually stabilizes even if there is a low intake (Fig. 3). This phenomenon appears because of body mechanisms permitting reabsorbing cobalamin from the bile to use it again and to reduce the excretion.
Figure 3: Serum concentration of cobalamin depending on the time people are on their diet (?: omnivores, ?: vegetarians, ?: vegans) (Gilsing et al., 2012)

2. Calcium and vitamin D
Multiples studies demonstrate that a vegan diet has a detrimental impact on the calcium present in the body. Appleby et al. (2002) study has shown that compared to omnivores or vegetarians, vegans consume twice less calcium. More than 70% of the calcium intake is dairy products (Omidvar et al., 2015) and vegans don’t eat any of these. Moreover, vitamin D is necessary to absorb calcium and the vitamin D dietary intake in vegans is lower than in other diet groups (Dyett et al., 2013). Vitamin D can especially be found in animal products such as fish, dairy products or eggs but the vitamin D status is mainly determined by the sunlight exposition. The difference in vitamin D intake is, thus, not significant for the calcium absorption (Appleby et al., 2007).
Because of this low calcium intake, and a global lower protein intake in the vegan diet, vegans tend to have a significantly lower bone mineral density (BMD) than omnivores and pesco-vegetarians but not different from lactovegetarians according to Lau et al. (1998) study and significantly different from lactovegetarians according to Appleby et al. (2007) study. However, it has been proven that BMD and calcium intake are correlated (Lau et al., 1998). BMD is also correlated with a lot of other factors, like the dietary protein intake but also some factors that are not linked with the diet.
One other consequence of the low calcium intake in vegans is the fracture risk. It is increased compared to the other diet groups, even lactovegetarians this time. The fracture risk is directly correlated with the calcium intake (Appleby et al., 2007) and with the BMD (Lau et al., 1998). In Appleby et al. (2007) study, it has been demonstrated that vegans had 30% more fracture than other diet groups.

IV. Conclusion
Nowadays, more and more people are interested in a vegan lifestyle and diet. The vegan diet influences multiples factors in the human health. First of all, vegan people have a lower obesity incidence and a lower BMI. According to some studies, a vegan diet can also have benefits for some cancers, especially breast, prostate and colorectal but the absence of processed meat consumption can increase the risk of leukemia and melanoma. Further researches on cancer incidence are necessary because of the difference of results between the studies.
A vegan diet can highly prevent chronic diseases like hypertension and ischemic heart diseases in an advanced age but is not necessarily the best diet compared to a pesco-vegetarian or a vegetarian diet. On the other hand, when people follow a vegan diet, they have to be careful with their nutrient intakes, especially for vitamin B12 and calcium because they can easily be deficient in these nutrients and that can cause severe diseases. For this, they have to take dietary supplements in vitamin B12 because they can’t have all the recommended vitamin B12 intake with non-animal products and they have to eat enough calcium via seeds, kale, or other calcium-fortified food.
Globally in all the studies, vegan participants have a better lifestyle than non-vegans: they smoke less, drink less alcohol and have a more regular physic activity. These factors can slightly affect the results of the studies.

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