The Vegetarian Diet
The historical evidence reveals that our ancestors were mostly vegetarian. It is thought to be because of their ethical principles and spiritual beliefs. A survey report found that approximately 3% Americans, 3% Britannia, 1–2% of New Zealand and 3% of Australian follows a vegetarian diet. Whereas, countries like India (40%), Germany (9%), Israel (8.5%), Canadians (8%) and Ireland (6%) have significantly higher rates of vegetarians in the present century. 
There is no single definition for a complete vegetarian diet because some consider complete plant-based diet as a vegetarian diet; whereas some include dairy products, poultry products, and seafood in their vegetarian diet plan. Overall, the aim to limit animal products.
- Vegan (or total vegetarian) - Eliminates all animal foodstuffs, particularly meat, poultry, eggs, seafood and dairy products.
- Lacto-vegetarian - Eliminates meat, poultry eggs, and seafood but includes dairy products.
- Ovo-vegetarian - Eliminates meat, poultry, seafood, milk products but includes eggs.
- Lacto-Ovo vegetarian - Eliminates meat, poultry and seafood and includes dairy products and eggs.
However, it is necessary to know why the current trend promotes someone to become a vegetarian. The following may be some reasons which influence the plant-based diet:
• Animal welfare
• Ethical reasons
• Maintaining ecological balance
• Food security
• Health purposes [1,3]
Health Benefits of a Vegetarian Diet
Following are some evidence-based health benefits of a vegetarian diet:
Obesity occurs when increased calorie intake does not get utilized due to a sedentary lifestyle. People often try to control their satiety to get that ‘Bikini Body’. For vegetarians, a diet of fresh, fibrous whole fruits and vegetable containing diet is a good option to control their weight. A research study concluded that a vegetarian meal can decrease 15% energy consumption by 15% and increasing satiety.
Cardiovascular disease (CVD) protection
There are multiple studies which report that a vegetarian diet can reduce CVD risk.  A study revealed that green leafy vegetables can significantly reduce the relative risk of cardiovascular diseases.  Carotenoids present in colorful vegetables like carrots, broccoli, tomatoes, yellow squash, lettuce, and spinach. Also, in flavonoid rich fruits like berries, apple, etc. have coronary artery disease preventive effect. 
A vegetarian diet has been shown to reduce the risk of diabetes and diabetes-induced complications. A clinical trial recently conducted in Taiwan provided the evidence that a vegetarian diet has prospective health benefits against diabetes by improving insulin resistance. Whole-grain products like whole-wheat bread, brown rice, oats, and barley have lower glycemic index (GI) values, which can control high blood glucose condition.  In addition, increased fibre and phytonutrient intake can help to maintain healthy body weight and improve microbiome interaction in the intestine. 
What are the concerns of eating a vegetarian/unbalanced vegetarian diet?
Restrictive or unbalanced vegetarian diet may not provide complete nutritional requirements. The nutrition experts often address common deficiencies like protein, calcium, iron, zinc, omega 3 fatty acids, vitamin D and vitamin B-12 among vegetarians. A dietary supplement is one of the convenient ways to compensate for these deficiencies.
Zinc is an essential mineral for our normal biological function, such as enzyme action, cellular signalling, cell membrane stabilization and gene expression, particularly at a younger age. Zinc is also essential during the gestation period to fulfil the maternal and foetal requirements. Zinc deficiency can cause growth impairment, endocrinal dysfunction, immune system impairment etc. A systemic meta-analysis reported that vegetarian individuals have insufficient zinc intake, resultant of which leads to zinc deficiency in the blood.  Deficiency of zinc in vegetarian people is mainly due to the consumption of plant-based foods which have a lower zinc content along with lower bioavailability. Meat, poultry, fish, shellfish are animal food sources that naturally contain zinc.  Complete vegetarian diet consumers need to take zinc-fortified foods or supplementation to compensate for the physiological requirement.
Iron deficiency is one of the major problems associated with a vegetarian diet. Animal-based foods including meat, fish, and poultry items are the main sources of dietary iron for omnivore humans.  Iron is one of the key element of haemoglobin, this red component in the blood cells is responsible for oxygen supply. Therefore, iron deficiency anaemia is a major health issue in unbalanced vegetarian diets. A cross-sectional study reported that a significantly lower serum ferritin levels are found in the blood of adult vegetarians, in comparison to the non-vegetarian control group due to the lack of bioavailable iron content in the plant-based diet.  However, vitamin C rich vegetables and fruits enhance iron absorption.
Vitamin B12, other B vitamins
Vitamin B12 deficiency is one of the common deficiency found in the vegetarian diet. Animal sourced food items are only natural source of vitamin B12 and other B vitamins. Vitamins B12 is essential to maintain normal haematological condition along with neuronal function. Deficiency leads to megaloblastic anaemia due to impaired synthesis of red blood cells. Alteration of myelin sheaths and glial cell synthesis also develops due to vitamin B12 deficiency and cause incorrect nerve transmission. This can lead to weak cognitive functioning, involuntary movements, irritable bowel syndrome, hyper-pigmentation of the skin and other developmental issues. Multiple research studies reported that a vegetarian diet increases the risk of vitamin B12 deficiency, which may reverse the cardiovascular disease preventive benefits of vegetarian diets. Therefore, experts recommend supplementation as an option to combat vitamin B12 deficiency for vegetarians. 
Dairy products are a good source of calcium. Calcium is essential for development and maintenance of bone and teeth. Deficiency of calcium in older adult increases the risk of bone fracture due to the progression of osteoporosis. Even, children also may have physical growth retardation. There are plant-based foods like kale, broccoli, cereals, collards, Chinese cabbage, and some fruits which contains calcium but the daily recommended dose of calcium may not fulfill the requirements through these vegan sources. Therefore, a dietary supplement is required to prevent the calcium deficiency related conditions.
Vitamin D is naturally synthesized in the skin in the exposure of sun rays. But the rate of synthesis is unequal due to the variation of the skin pigmentation and climatic conditions. Moreover, maintaining a regularity towards sun exposure may not be possible due to busy work schedule. All these factors increase the scope of vitamin D deficiency in both vegetarians and non-vegetarians.  Vitamin D is an essential nutrient for bone health, but very limited dietary sources can compensate vitamin D deficiency. Vitamin D supplementation can help to maintain the balance of this essential micronutrient demand.
Omega 3 Fatty Oils
There is a variety of omega 3 fatty acids, including α-linolenic acid (ALA), docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), which are the essential dietary ingredients to maintain our normal physiological functioning. Vegetarian diet eaters have similar ALA level, but a significantly reduced level of DHA and EPA are found as compared with omnivores individuals. DHA and EPA are specifically important for maintaining brain functioning, eye health, the integrity of cell membrane and foetal development in the gestation period. DHA and EPA also have a preventive effect against various chronic conditions including cardiovascular diseases. However, research studies did not find any significant omega 3 fatty acids related deficiency diseases in vegetarian individuals, but experts suggest requirement of 200-300 mg supplementation per day to meet the need of people who have high demand like pregnant and lactating mother, elderly individuals, and diabetic patients or other chronic disease affected patients with reduced conversion ability. However, some vegan sources like flax, walnuts, camelina, canola, chia, and hemp are the natural sources of Omega 3 fatty acids. 
It is true that a vegetarian diet is good for health due to the nutritional and phytochemical constituents. But there are always possibilities to develop nutritional deficiencies of iron, calcium, vitamin B, and D, id the diet is not managed well. The inclusion of a specific dietary supplement helps to balance the deficit conditions.