Why should you take a multivitamin multimineral formula? Well, I have always believed that the best investment a person can make is in themselves and their health. Without our health, we are nothing, and we often do not realise how important good health is until we lose it. People are often surprised to learn that mineral and vitamin deficiencies are a common cause of the metabolic abnormalities that can lead to chronic weight gain and serious ill health. Evidence shows that many people in developed nations are deficient in one or more vitamin or mineral. A good multivitamin and mineral formula can provide all the vitamins and many of the minerals you need to maintain optimal health and correct body weight. Of course, a high quality diet containing nutrient rich foods is also essential, but on the days when diet quality suffers, the supplement will act as an insurance policy for good health.
As with most things, you get what you pay for with a multivitamin multimineral, and so it is important to pick a quality supplement. There are many useless multivitamins on the market and avoiding the low quality supplements is a necessary and prudent step. A good quality multivitamin multimineral formula should contain high enough levels of all of the vitamins. These will generally need to be above the government recommended intakes, as these levels are really set to address the prevention of serious deficiency diseases, not optimal health. For example, some researchers now acknowledge that the government recommended levels for vitamin D are too low, and that they need to be revised upwards. Other vitamins, such as vitamin E and the B vitamins, have also shown positive effects against major chronic diseases in studies, but at much higher levels than recommended by government. 
Table 1. Typical values of various vitamins that would be expected in a good quality multivitamin multimineral formula.
In addition to the vitamins, you need to assure a good range of minerals at the right doses. The macrominerals such as magnesium, calcium, sulphur, chloride, potassium and sodium are generally needed in gram amounts per day (>100mg per day) and are best obtained from a healthy diet or additional supplement. However of great importance are the trace minerals, zinc, copper, selenium chromium, molybdenum, iodine, and manganese. Extensive agriculture and over farming has already lowered the quality of our soil and depleted the trace minerals that find their way into the plants we eat. With low levels of minerals in the plants, supplementation becomes essential for those wishing to maintain good health. Trace minerals act as important co-factors in the enzymes that drive our biochemical pathways. Deficiencies of these trace minerals are common for the reason mentioned above, and lead to metabolic defects some of which may cause weight gain. 
Table 2. Typical values of various minerals that would be expected in a good quality multivitamin multimineral formula. 1The macrominerals calcium, magnesium and potassium are required in too high amounts to fit into a multivitamin and mineral tablet. These are best obtained from the diet or an additional supplement. However, levels of calcium and magnesium over 100mg can provide additional intakes that may be beneficial. An additional calcium and magnesium supplement with 1000mg of each would be beneficial. Potassium is easily obtainable in gram amounts in a healthy diet. 2Additional sodium is not required as it is provided in adequate amounts in most diets. 3Additional iron is generally not required by men who consume red meat, but may be required by women and vegetarians or vegans. Iron tends to prevent the absorption of other trace minerals and so men who eat meat should use a supplement with the lowest amount of iron possible.
Research shows that large numbers of people are becoming increasingly deficient in certain vitamins and minerals. A number of these common nutrients deficiencies are now known to be associated with poor health and chronic metabolic dysfunction. For example, it is well established that chromium is required for the correct function of the insulin receptor, and that deficiency leads to the development of insulin resistance with associated abdominal obesity and weight gain. Vitamin C is known to show inverse associations with total mortality and selenium intake is inversely associated with some cancers. Calcium shows an inverse association with weight gain, and administering calcium to both animals and humans causes weight loss. To attain optimal health, it therefore makes perfect sense to supplement a high quality diet with a high quality multivitamin multimineral formula.