**1. Introduction**

Nutrition and physical activity (PA) are the two main modifiable factors that determine health and well-being in modern civilization. These two factors are often studied and considered as separate entities, although they are, in reality, inter-related in various ways. To give some examples of this interplay, PA can shift the nutrient spectrum that human metabolism utilizes; lack of PA leads to accumulation of ectopic fat in liver, muscle, and potentially other organs [1,2]; malnutrition hampers physical and mental performance;

**Citation:** Strasser, B.; Pesta, D.; Rittweger, J.; Burtscher, J.; Burtscher, M. Nutrition for Older Athletes: Focus on Sex-Differences. *Nutrients* **2021**, *13*, 1409. https://doi.org/ 10.3390/nu13051409

Academic Editor: David C. Nieman

Received: 21 January 2021 Accepted: 12 April 2021 Published: 22 April 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

overfeeding leads to expansion of fat deposits, thus increasing body inertia and deteriorating physical fitness [3]. One motivation for this review, therefore, is to raise awareness of the interplay between nutrition and PA/performance for readers with either of the two backgrounds.

The topic of diet and PA becomes particularly relevant in combination with aging, as aging is generally associated with generalized inflammation and exaggerated disease burden [4]. Ample evidence suggests that regular exercise and optimized nutrition can help to reduce disease burden [5–7]. Therefore, the World Health Organization (WHO) has taken action to promote PA across the entire age spectrum [8], and many governments have developed national strategies also with the interest to mitigate age-related morbidity. However, older people become typically more and more sedentary with increasing age, and it is often challenging to engage them in regular physical exercise.

In this sense, masters athletes can offer interesting insights. These people train for and compete in athletic events beyond the typical age of retirement from sports. Many of them follow rigorous training regimens, often over decades or even their entire adult life. Beyond their intrinsic motivation to be physically active, they are driven by the desire to excel in sports, but also to maintain their health. The scientific literature has long neglected this topic, and only the past decade has seen an increasing number of publications on all aspects of masters athletics (Figure 1).

**Figure 1.** Publications on masters athletes, plotted against year of publication. Date are the result of a literature research on public Medline on 1 October 2021, using the following search terms: ((master athlete (Title/Abstract)) OR (masters athlete (Title/Abstract)) OR (veteran athlete (Title/Abstract)).

Naturally, a plethora of questions arise with regard to the nutritional support for these aging athletes. Thus, as with any athlete, also the masters athletes wish to reduce their body fat, in order to optimize athletic performance. But what are the energetic requirements, given that resting metabolic rate (RMR) typically dwindles with age? How can the requirements for the intake of protein, vitamins and other micronutrients be met when the total intake is reduced for the sake of body composition? Which nutrients are physiologically opportune for optimized performance, and which ones should be recommended to facilitate recovery and prevention of injury? The field of nutritional support for athletes has traditionally focused on young elite athletes, and there is no clear answer to these and other questions pertinent to the topic. Hence, the present paper attempts to collate the relevant information in order to (a) provide recommendations where evidence is available, and to (b) flag the most important open question for future research.

#### **2. Physiological Changes in Older Athletes**

### *2.1. Oxygen Delivery and Utilization Systems*

The exercising skeletal muscles rely on continuous oxygen supply that is precisely matched to the metabolic requirements of the exercise intensity. If, at any intensity, the oxygen supply becomes limited, anaerobic metabolism compensates for the lacking oxygen, indicated by increasing lactate concentration [9,10]. Oxygen has to be transported from the ambient air to the oxygen-respiring mitochondria of the working muscles. This is involving a series of steps, namely oxygen diffusion (driven by the existing pressure gradient) from the alveoli into the pulmonary capillary blood, the oxygenated blood (oxygen is primarily bound to hemoglobin) is pumped by the heart to the skeletal muscles, where oxygen is converted to adenosine triphosphate (ATP), providing energy to power the working muscles (Figure 2). Oxygen delivery (DO2) to the muscles is determined by the cardiac output (heart rate x stroke volume, Q), the hemoglobin concentration (Hb) and the level of its saturation (SaO2), according to the equation:

$$\text{DO}\_2 = \text{Q} \times \text{Hb} \times \text{SaO}\_2 \times \text{K}$$

K indicates the Hüfner coefficient for hemoglobin-oxygen binding capacity which is 1.33 mL/g.

**Figure 2.** The lung–heart–muscle axis involved in oxygen delivery and utilization. Main cardiorespiratory parameters specifying organ function at rest and during exercise: minute ventilation (VE), cardiac output (Q), and oxygen extraction in the skeletal muscle. Parameters listed below the organs represent those which are primarily affected by the aging process. Oxygen, O2; carbon dioxide, CO2; tidal volume, VT; breathing frequency, Bf; hemoglobin concentration, Hb; arterial oxygen saturation, SaO2; heart rate, HR; adenosine triphosphate, ATP.

The extraction of oxygen utilized by the mitochondria is represented by the arteriovenous oxygen difference (a-v O2 difference). As the need for oxygen is increasing with increasing exercise intensity, minute ventilation, cardiac output, muscle perfusion and oxygen extraction all have to increase. According to the Fick principle for individual aerobic capacity, the highest achievable rate of oxygen uptake (VO2max) equals Q × a-v O2 extraction. All organs involved in oxygen transport and utilization may contribute to VO2max decline in the aging athlete.
