If you lack energy and feel less motivated than you used to but tell yourself it's because you're growing older – you may be wrong. A lack of energy can have a knock-on effect on your mood, your muscles and how much you get out of your day. But more people are discovering that a magnesium deficiency in their diets could be the cause, as against simply getting older. If you lack energy and feel less motivated than you used to but tell yourself it's because you're growing older – you may be wrong A lack of energy is often the result of a shortage of key nutrients, normally obtained from a healthy, balanced diet. Food supplements can also be a useful additional source of these key nutrients.