Vitamins and Minerals for Energy, Fatigue and Cognition: A Narrative Review of the Biochemical and Clinical Evidence
by Anne-Laure Tardy Nutrients 2020, 12(1), 228;
Vitamins and minerals are essential to humans as they play essential roles in a variety of basic metabolic pathways that support fundamental cellular functions. In particular, their involvement in energy-yielding metabolism, DNA synthesis, oxygen transport, and neuronal functions makes them critical for brain and muscular function. These, in turn, translate into effects on cognitive and psychological processes, including mental and physical fatigue.
This review is focused on B vitamins (B1, B2, B3, B5, B6, B8, B9 and B12), vitamin C, iron, magnesium and zinc, which have recognized roles in these outcomes. It summarizes the biochemical bases and actions of these micronutrients at both the molecular and cellular levels and connects them with cognitive and psychological symptoms, as well as manifestations of fatigue that may occur when status or supplies of these micronutrients are not adequate.
There is a strong biological and physiological rationale that indicates that the long-known involvement of vitamins and minerals in cellular energy production translates into functional and physiological outcomes in humans, including perceived physical and mental fatigue as well as psychological and cognitive functions. Indeed, the organs supportive of these functions, skeletal muscle and brain, are the most energy-demanding ones in the human body. In addition, vitamins and minerals, especially the B vitamins, vitamin C, iron and magnesium addressed in this review, are mandatory to extract this energy from food and present it in a physiologically usable form. Furthermore, because there is a close interplay between these micronutrients across the successive steps of energy production, all of them should be available simultaneously as the whole system may be slowed down by a lack in a single one of them. While this role in energy production is pivotal, other functions fulfilled by micronutrients are also key in relation to fatigue and cognition. Besides energy, muscles and brain also need oxygen, which should be delivered by the hemoglobin present in red blood cells; this process requires an efficient erythropoiesis, for which iron and also several B vitamins are critical.
Inadequate intakes in these micronutrients result in anemia, with resultant fatigue and weakness symptoms. Oxygen is mandatory, but also generates oxidative reactive species, which are potentially harmful so need to be controlled by the anti-oxidative defences of the organism, in which vitamin C plays a key role. Furthermore, many vitamins and minerals have a role in the synthesis of neurotransmitters, development and maintenance of neuronal membranes, and brain receptor modification, with obvious consequences for brain activation and thus cognitive function.
Clinical confirmation of the essential nature of micronutrients and of their role in alleviating fatigue and cognitive dysfunction comes from observations in subjects with low or very low intakes or deficient biological status and from reversal of symptoms following supplementation. Severe deficiencies are no longer common, but there is some evidence that sub-clinical or marginal deficiencies can have negative consequences on physical and mental fatigue, as well as on cognitive and psychological functions. This is confirmed by supplementation studies, although many are enrolling subjects without a clear assessment of their baseline nutritional status, so the populations may include subjects both with and without a deficient or sub-deficient status. In such cases, it may happen that the supplementation is not effective, because a significant proportion of subjects already has a sufficient micronutrient intake and will not benefit from the supplementation. However, when restricted to subjects with inadequate intakes or status, conclusions are most often favorable.
Supplementing individuals with vitamin and minerals is thus highly likely to result in health benefits in the areas of mental and physical fatigue, as well as cognitive and psychological functions. However, efficient and safe supplementation should carefully consider practical dimensions, such as the chemical form of vitamins and minerals, which can affect their bioavailability, or the duration of treatment, which should be sufficient to ensure the restoration of adequate status in people where intakes are not sufficient, and who are the more likely to benefit from supplementation. In addition, caution should be exerted to avoid side effects and deleterious consequences of overdosing. It is for example known that neurotoxicity can be observed following excessive doses of vitamin B6, approximately 1 g daily, whereas recommended intakes are in the mg range [223]. Public Health Authorities have reviewed the existing evidence about health hazards associated with excessive intakes of all vitamins and minerals and have provided guidelines about tolerable upper limits of daily intake of vitamins and minerals that should be followed [223]. In most cases, remaining within the order of magnitude of the recommended daily allowance will provide the needed safety level.
In this review, detailed information has been provided for 12 micronutrients, which highlights that consuming adequate intakes of vitamins and minerals contributes to maintenance of normal cognitive function and to enhanced well-being by decreasing perceived mental and physical fatigue and favoring positive mood. Health professionals are often confronted by complaints about fatigue and perceived psychological or cognitive difficulties. Once pathological aetiologias have been discarded, it is relevant to question the supply and status of vitamins and minerals and to provide nutritional advice first aiming at promoting balanced food choices, then focusing on the use of vitamin and mineral supplementation.
