1. Introduction

The essential nature of vitamins and minerals for human health was demonstrated more than a hundred years ago. Recommendations for appropriate dietary intakes aim to ensure that most of the population receive amounts fulfilling their physiological needs [1]. The link between biochemical and physiological functions is established for some vitamins and minerals, as is their role in clinical outcomes. As an example, vitamin A is a component of the pigment rhodopsin located in the retina, that enables visual processes and prevents blindness. In many other instances, however, the nature of the involvement of micronutrients in molecular and cellular reactions that translate into physiological and functional effects is poorly understood.

Claims regarding the effects of vitamins and minerals on fatigue, cognition or psychological functions are authorized in many countries [2]. However, the available scientific rationale for these is often based on theoretical biochemical grounds and clinical features seen during frank clinical deficiencies, rather than on robust, empirical, physiological data. Such clinical deficiencies are relatively uncommon, particularly in developed countries. Conversely, subclinical or inadequate intakes (sometimes referred to as ‘insufficiencies’) are frequent worldwide, albeit with variations according to age groups and country [3,4]. A varied and balanced diet, rich in nutrient-dense foods such as fruits, vegetables and dairy products, is able to provide the amounts of vitamins and minerals needed. There is ample evidence, however, that food choice or availability often preclude such a diet. This can lead to a significant proportion of the population not meeting their optimum dietary needs, in emerging and developed countries. For example, 68% of Mexican women have folate (vitamin B9) intakes below the estimated average requirement (EAR) [5], and thiamine (vitamin B1) dietary intakes are below EAR in 55% of Turkish adults of both genders [6]. Although the majority of Americans consume sufficient amounts of most nutrients to offset clinical symptoms, in many individuals intake falls below the EAR or Adequate Intake levels [7]. In such cases, vitamin and mineral supplementation may become a means to meet adequate intake. Indeed, this is one of the most frequent reasons for consumption given by supplement users. An enhanced feeling of well-being, a reduction in mental and physical fatigue and improvements in psychological and cognitive functions are also among the commonly reported motivations for taking supplements [8,9].

This narrative review aims to examine the scientific evidence that supports the role of key selected vitamins and minerals in health outcomes related to fatigue, as well as psychological and cognitive functions. Nine vitamins (vitamins B1, B2, B3, B5, B6, B9, B8, B12 and C) and three minerals (iron, magnesium and zinc) have been selected based on the health claims dealing with those health outcomes that have been authorized in Europe for these nutrients. Firstly, attention will be given to how the concept of energy is understood and how it can relate to physical and mental fatigue and performance. This section will briefly address both the biochemical/physiological perspective and the perceptual and psychological manifestations of energy. This is followed by a detailed, up-to-date review of the evidence for these micronutrients playing a role in physical and mental fatigue as well as in cognitive functions, focusing on biochemical pathways, with clinical information where available. Priority will be given to human data, especially those obtained in the healthy general population and in subjects with inadequate nutrient intake or status. Cognitive functions, as well as physical and mental fatigue, will be reviewed based on observed symptoms of deficiencies or subdeficiencies in human populations. Results from recent supplementation trials will also be considered. Most of the reported clinical data concern adult (or adolescent) populations. Some selected data on younger and older population will be included where relevant.