Answer (Mag. nutr. Ivana Šimić):

By means of the balanced diet, you can introduce all the necessary nutrients, while vitamin B12 is normally taken by vegans through supplements or enriched foods. Usually we have reserves for about 5 to 6 years, and the lack is noticed only after ten years, which of course should be prevented. The natural source such as Spiruline is not an adequate source of vitamin B12. 

Proteins
Studies show that vegan diet provides the ideal amounts of protein, and in accordance with the recommendations of the World Health Organization and the Ministry of Health of the United Kingdom this is 0.8 g/kg of body weight, while for athletes it is slightly higher.

Given that the problem is not in the amount of proteins but  in their quality, it is necessary to have a varied diet that includes a variety of foods to provide essential amino acids - legumes (peas, beans, chickpeas, lentils, soybeans), grains (wheat, oat, rice, barley, buckwheat, millet, pasta, bread), nuts (hazelnuts, almonds, walnuts, cashews) and sunflower seeds, pumpkin seeds and sesame seeds.

It is not necessary to have consume them all in the same meal, they can be combined - for example, tortillas with beans or oatmeal for breakfast and chickpea fritters for lunch.
 

Iron
The recommendations for iron intake were 1.8 times higher due to the non-heme iron in a vegetarian diet and due to the high intake of phytate. Good plant sources of iron include dried fruits, whole grains (including wholemeal bread), nuts, green leafy vegetables, seeds and legumes.
Soaking, germination, fermentation and cooking reduces the amount of phytic acid. Pulses must be soaked before cooking and rinse thoroughly. Iron is better absorbed with the presence of vitamin C and it is therefore recommended to drink a glass of orange juice or always have a fresh salad and vegetables included in the meal.
Juices made of dark fruits and vegetables (blackberry, blueberry, chokeberry, beetroot) with the addition of green leafy vegetables rich in chlorophyll and lemon or orange will help fighting anemia, as well as the nettle tea.

Other foods rich in iron which are usually eaten in smaller amounts include soya flour, parsley, watercress, black molasses and edible seaweeds.
 

Calcium
Good plant sources of calcium include tofu (if prepared using calcium sulphate contains more than four times the amount of calcium than cow's milk buttermilk), green leafy vegetables, seeds (sesame) and nuts. Choose drinks made of soy, oats and rice with added calcium. Other calcium-rich foods include black molasses, edible seaweeds, watercress, parsley and dried figs and raisins.
Vitamin D is essential for a good utilization of calcium, and we can get it by exposing the face and arms to the sun for twenty minutes a day or by using enriched products.

Examples of juices for iron (for a juicer or blender):

2 leaves of kale
4 carrots
1 handful parsley
 

4-5 carrots
1 handful of dandelion leaves
1 head of fennel
lemon juice
 

half a head of red beet
2 carrots
1 apple
1 stalk celery
 

1 small red beet
4 carrots
1 orange
 

2 cups blackberries
1 apple
1 cube of ginger
 

1 kg of beetroot
1 kg of lemons
1 kg of carrots
Drain the beets and carrots in the juice extractor and mix them with lemon juice.
 

1 apple
1 banana
3 cups of kale
1 cup of fresh parsley
1 cup of water
 

2-3 kiwis
1 banana
2 cups of green leafy vegetables
¼ cup of soaked raisins
1 cup of water
 

1 handful of dandelion leaves
1 handful of parsley leaves
1 handful of spinach leaves
1 apple
1 small cluster of grapes
 

For anemia: nettle tea

Answer (Mag. nutr. Ivana Šimić):

There is a great variety of plant sources of calcium, but we can single out some of them: savoy cabbage, broccoli, cabbage, almonds, oats, sesame seeds, figs, oranges, arugula, beans, parsley, lentils, soybeans, chard, etc. There are also foods fortified with calcium, such as soy milk, tofu (if made with calcium sulfate), orange juice, and so on. You can find more information here. 
Salt reduces calcium absorption so it is advisable to limit the consumption of salty foods, although there are low-sodium salts. You should also avoid sugar and refined foods such as white rice and regular pasta, and give priority to whole-grain foods. 
The intake of vitamin D is also important because it enhances calcium absorption. We recommend regular sun exposure, consumption of mushrooms (leave them in the sun briefly before using) and foods fortified with vitamin D. 
Regular physical activity is important for healthy bones, so make sure to get enough exercise. 

Answer (Mag. nutr. Ivana Šimić):

A variety of disorders in your body, related or unrelated to diet, can cause low hemoglobin levels. The symptoms of anemia include constant fatigue, getting tired quickly when physically active, lack of energy, etc. Iron deficiency affects the immune system, so infections are more common.
Many people nowadays suffer from low hemoglobin levels, whether they are vegetarian or not. Switching to a vegan diet has helped many to deal with their low hemoglobin levels, by eating higher quality foods rich in vitamins and minerals. 
Vegans’ situation regarding iron is excellent since they get plenty of iron from their diet. Although iron from plant sources is more difficult to absorb than from meat, the high level of dietary vitamin C enhances iron absorption. Vegans are also likely to have high intake of green leafy vegetables and other vegetables. 
In order to be generally healthy it is very important to eat whole grains (whole-wheat pasta, whole-wheat bread) instead of white flour, which should be avoided. Your diet should include cereals such as oat flakes, nuts and seeds, legumes such as peas, lentils, chickpeas, beans and soybeans, and green leafy vegetables combined with lemon and other sources of vitamin C.
Choosing healthy foods will not only improve the level of hemoglobin but also your overall health.
We suggest taking vitamin B12 supplements because vegans and vegetarians should be especially mindful of this vitamin.

Answer (Mag. nutr. Ivana Šimić):

The situation with iron is excellent for vegans since their diet is rich in iron. Studies show that the iron status of vegans is usually normal, and iron deficiency is less common than among the general population.
Good sources of iron include tofu, quinoa, rolled grains, whole grains, whole-wheat bread, beans and legumes, spinach, almonds, pumpkin seeds, dried apricots and figs and green leafy vegetables. Even though it is harder to absorb iron from plant-based sources than from meat, the high level of vitamin C in the diet improves the absorption of iron, so you can toss your lettuce with lemon juice. 
Foods rich in selenium include Brazil nuts, whole grains, whole-wheat pasta, rolled grains, wheat germ, barley and sunflower seeds.

Answer (Mag. nutr. Ivana Šimić):

I would suggest you try Stevia - it exists in several forms, and the most valuable form is the green powder (dry milled leaves of the Stevia rebaudiana Bertoni). Since it has a bitter note, it is possible to find the purified white powder, tablet or liquid adapted to the market needs. Among other natural alternatives (unless you are allergic to them), I would recommend the rice and barley malt and agave syrup. The recommendation of a healthy diet is to maximally reduce the addition of refined sugar and enjoy the naturally occurring sugars in foods.