According to this study a vegetarian diet is associated with poorer health including higher incidence of cancer, allergies and mental health disorders.
https://journals.plos.org/plosone/artic ... =printable
Did you read the study? I would guess not, since it doesn't support what you seem to imply, that the study finds a causal inference. It does not.
This is a good time to say that both the media and individuals often misrepresents scientific research and articles, by latching on to the title and/or results, without reading the discussion of the article, or being unable to understand the research in context of the whole field of study (not an easy thing).
Let us look at the discussion of the article you linked (my emphasis in the quoted text below):
Overall, our findings reveal that vegetarians report poorer health, follow medical treatment more frequently, have worse preventive health care practices, and have a lower quality of life.Concerning the variable ‘‘eating behavior’’, we tried to generate avariable that would reflect the animal fat intake (1 = vegetarian,2 = carnivorous diet rich in fruits and vegetables, 3 = carnivorous diet less rich in meat, 4 = carnivorous diet rich in meat). The mean BMI of subjects is coupled in nearly linear progression with the amount of animal fat intake. This is in line with previous studies showing vegetarians to have a lower body mass index [1,4,5,7,9–12]. Our results have shown that vegetarians report chronic conditions and poorer subjective health more frequently. This might indicate that the vegetarians in our study consume this form of diet as a consequence of their disorders, since a vegetarian diet is often recommended as a method to manage weight  and health .
This should have been the first thought of any reader, as it should be well-known that becoming vegetarian is often a result of an already existing health issue (can be chronic, genetic), as a way of managing the health issue. If the person's health situation becomes better as a result, that person will often stay vegetarian. That speaks well for the vegetarian diet, but if you look at the results (as many do), you might think that being vegetarian causes
all these health disorders, but it's simply not supported in that data, as the authors acknowledges.
You need a much more complex study to take into account that a significant subset of the group of people that becomes vegetarians, suffer from health conditions, and thus you have a 'steady flow' of poor health people becoming part of the vegetarian group, and
they are removed from the carnivorous group (even though they most likely have been part of the group for most of their life).
Unfortunately, food intake was not measured in more detail, e.g. caloric intake was not covered. Hence, further studies will be necessary to analyze health and its relationship with different forms of dietary habits in more detail. When analyzing the frequency of chronic diseases, we found significantly higher cancer incidence rates in vegetarians than in subjects with other dietary habits. This is in line with previous findings, reporting that evidence about cancer rates, abdominal complaints, and all-cause mortality in vegetarians is rather inconsistent [5–7,19–22]. The higher cancer incidence in vegetarians in our study might be a coincidence, and is possibly related to factors other than the general amount of animal fat intake, such as health-conscious behavior, since no differences were found regarding smoking behavior and physical activity in Austrian adults as reported in other studies for other countries [9,13,14].
Here they discuss the higher cancer incidence rates in vegetarians. It should be no surprise that many people faced with cancer will try their best to become more healthy, and one relatively common response is to become vegetarian, or eat more vegetables and less meat, which fits with the high cancer incidence rate of the "Carnivorous diet rich in fruits and vegetables" group as well.
these people with cancer became vegetarian or began to eat more vegetables and less meat, in relation to their diagnosis, should reveal a clear trend. The same can be said for the rest of the study.
Therefore, further studies will be required in Austria in order to analyze the incidence of different types of cancer and their association with nutritional factors in more depth. Several studies have shown the mental health effects of a vegetarian diet to be divergent [9,15,16]. Vegetarians in our study suffer significantly more often from anxiety disorder and/or depression. Additionally, they have a poorer quality of life in terms of physical health, social relationships, and environmental factors. Moreover, the use of health care differs significantly between the dietary habit groups in our study. Vegetarians need more medical treatment than subjects following another form of diet. However, this might be due to the number of chronic conditions, which is higher in subjects with a vegetarian diet.
Again, that a large subset of the vegetarians in their study have poorer mental health, does not imply that a vegetarian diet causes
poorer mental health. It is much more likely that they are instead highly correlated due to other factors.
As for an explanation other than trying to improve one's physical health (or possibly also mental health) through diet, I would make the hypothesis that a significant part of the numbers can be explained by the idea that individuals suffering from anxiety and/or depression are more likely to be part of a subculture or counterculture than the average population, and that these are on average more associated with veganism or vegetarianism than not. This is just an idea however.
Potential limitations of our results are due to the fact that the survey was based on cross-sectional data. Therefore, no statements can be made whether the poorer health in vegetarians in our study is caused by their dietary habit or if they consume this form of diet due to their poorer health status. We cannot state whether a causal relationship exists, but describe ascertained associations. More-over, we cannot give any information regarding the long-term consequences of consuming a special diet nor concerning mortality rates.
And to put into a larger context, and the scope of the study:
To our knowledge this is the first study ever in Austria to analyze differences in terms of dietary habits and their impact on health. We admit that the large number of participants made it necessary to keep the questions simple, in order to cover the large sample. Overall, we feel that our results are of specific interest and contribute to extant scientific knowledge, not with-standing some limitations regarding causes and effects.
I can recommend reading this take on the study, made by the NHS: https://www.nhs.uk/news/food-and-diet/v ... dy-claims/
They have a "Behind the Headlines" section that looks into situations like this, where the media report something with little nuance.
In general I encourage you all to instead of posting articles as ammunition to settle a debate, to actually read them in-depth and practice critical thinking. Try to find flaws and limitations in the studies, and try to understand how this 'piece of the puzzle' fits into the larger picture of all the research of the field. This requires much more dedication that just linking articles that seems to support one's viewpoint, but that dedication is rewarded with hopefully more scientific literacy, which is valuable.