```json
{
  "sym_variables": [
    ("x0", "milligrams of vitamin B12"),
    ("x1", "milligrams of vitamin B5"),
    ("x2", "milligrams of iron")
  ],
  "objective_function": "5*x0 + 9*x1 + 2*x2",
  "constraints": [
    "6*x0 + 20*x1 + 11*x2 <= 170",  // muscle growth index upper bound
    "12*x0 + 9*x1 + 8*x2 <= 76",  // energy stability index upper bound
    "7*x0 + 13*x1 + 8*x2 <= 132",  // cognitive performance index upper bound
    "9*x1 + 8*x2 >= 8",
    "12*x0 + 9*x1 + 8*x2 >= 23",
    "6*x0 + 11*x2 <= 98",
    "6*x0 + 20*x1 + 11*x2 <= 98",
    "9*x1 + 8*x2 <= 66",
    "12*x0 + 9*x1 + 8*x2 <= 66",
    "13*x1 + 8*x2 <= 106",
    "7*x0 + 13*x1 <= 67",
    "7*x0 + 13*x1 + 8*x2 <= 67",
    "x0 >= 0",
    "x1 >= 0",
    "x2 >= 0"
  ]
}
```

```python
import gurobipy as gp

try:
    # Create a new model
    m = gp.Model("vitamin_optimization")

    # Create variables
    x0 = m.addVar(vtype=gp.GRB.CONTINUOUS, name="vitamin_B12")
    x1 = m.addVar(vtype=gp.GRB.CONTINUOUS, name="vitamin_B5")
    x2 = m.addVar(vtype=gp.GRB.CONTINUOUS, name="iron")


    # Set objective function
    m.setObjective(5*x0 + 9*x1 + 2*x2, gp.GRB.MAXIMIZE)

    # Add constraints
    m.addConstr(6*x0 + 20*x1 + 11*x2 <= 170, "c0")
    m.addConstr(12*x0 + 9*x1 + 8*x2 <= 76, "c1")
    m.addConstr(7*x0 + 13*x1 + 8*x2 <= 132, "c2")
    m.addConstr(9*x1 + 8*x2 >= 8, "c3")
    m.addConstr(12*x0 + 9*x1 + 8*x2 >= 23, "c4")
    m.addConstr(6*x0 + 11*x2 <= 98, "c5")
    m.addConstr(6*x0 + 20*x1 + 11*x2 <= 98, "c6")
    m.addConstr(9*x1 + 8*x2 <= 66, "c7")
    m.addConstr(12*x0 + 9*x1 + 8*x2 <= 66, "c8")
    m.addConstr(13*x1 + 8*x2 <= 106, "c9")
    m.addConstr(7*x0 + 13*x1 <= 67, "c10")
    m.addConstr(7*x0 + 13*x1 + 8*x2 <= 67, "c11")

    # Optimize model
    m.optimize()

    if m.status == gp.GRB.OPTIMAL:
        print('Obj: %g' % m.objVal)
        print('Vitamin B12: %g' % x0.x)
        print('Vitamin B5: %g' % x1.x)
        print('Iron: %g' % x2.x)
    elif m.status == gp.GRB.INFEASIBLE:
        print('The model is infeasible.')
    else:
        print('Optimization ended with status %d' % m.status)


except gp.GurobiError as e:
    print('Error code ' + str(e.errno) + ': ' + str(e))

except AttributeError:
    print('Encountered an attribute error')
```