Most athletes acknowledge that increasing training intensity is a key factor in improving performance. But the risk with any increase in training intensity is that one overdoes it and gets themselves sick. As such, there’s always a fine balancing act of increase training intensity and keeping the immune system in check.
But an exciting new study has provided encouraging support that a high protein diet (i.e. 3g/kg bodyweight/day) may be able to prevent the decrease in immune function normally associated with a significant increase in training intensity/duration.
Scientists from the UK, Netherlands, USA and Germany collaborated to test whether an increase in protein intake (i.e. from 1.5g/kg/day to 3.0g/kg/day) could prevent the decrease in immune function in response to a large (i.e 70%) increase in the relative volume of high-intensity cycling exercise. The study involved eight well-trained healthy male cyclists with an average age of 27 and VO2max of 64 mL/kg/min.
As shown below, the study was designed in a counter-balanced fashion such that each subject performed a week of their normal training (consuming a normal diet with 1.5g/kg/day of protein) which was followed by a week of high intensity training, consuming either a high protein diet (3.0g/kg/day) or a calorie-matched diet with a normal level of protein (i.e. 1.5g/kg/day).
The table below outlines the respective energy and macronutrient content of each diet. In the case of the control diet during the high-intensity training week, the calorie deficit was made up by increasing the fat content of the diet, while keeping protein and carbohydrate intake the same. This allowed researchers to see the specific effect of increased protein on the immune response to a week of high intensity training.
As far as the training was concerned during normal-intensity and high-intensity conditions, the study is a little short on specific details. It only mentions that overall training load (in terms of volume and intensity) was increased by ~70%. This meant that under normal training conditions, subjects typically exercised once per day, 4-5 days per week. In contrast, during the high-intensity training week, subjects regularly trained 2 sessions a day, 7 days a week. Prescriptions for exercise intensity were based on percentage of maximum heart rate as determined during initial VO2max testing.
As shown in the figure below, when following the ‘normal’ diet during high-intensity training, subjects showed a significantly impaired immune response. More specifically, when consuming a normal diet (i.e. control-diet), high-intensity training was associated with a smaller exercise-induced mobilisation of leukocytes (white blood cells) and granulocytes compared to normal-intensity training. During exercise it is normal for the number of white blood cells circulating in peripheral blood to increase two- or three-fold, and 1-2 h later, fall to half of normal levels. Granulocyctes are a type of white blood cell and they mediate much of the immune protection/repair in muscle following exercise. This process; called exercise-induced granulocytosis is responsible in large part for detecting and eliminating bacterial infections following exercise. However, the drop in both leukocytes and granulocytes numbers was largely prevented when consuming the high protein diet.
Another measure of immune function taken as part of the study involved specialised cells of the adaptive immune system called CD8+ T-cells (also called killer T cells or cytotoxic T cell) following normal-intensity training and high-intensity training. CD8+ T-cells primarily target viral antigens and as the graph below shows, following high-intensity exercise on the control diet, the ability to detect virus-infected cells is compromised considerably. Practically, this can translate into a higher risk of upper respiratory tract infections (URTIs) following demanding training sessions. However, when the cyclists consumed the high protein diet, this impaired adaptive immune response was largely prevented.
Additionally, the graph below shows the extent of egrees or redistribution of CD8+ T-cells following exercise. Redistribution refers to the process of immune cells moving to peripheral tissues such as the skin, lungs and gut where they are more likely to detect and eliminate infected cells. This process was impaired when the cyclists’ upped their training significantly while consuming the control diet. However, the high protein again nullified this effect.
Finally on a more practical level, the graph below shows the dramatic reduction in URTI symptoms when cyclists’ consumed the high protein diet as compared with the normal (control) diet.
Type & Amount of Protein For Immune Support
With such significant positive effects on immune function, it’s important to highlight the exact type of protein used in the study and the dose at which it was administered. The additional 1.5g/kg/day of protein in the high protein diet was achieved by giving subjects 3 daily serves of a specialised casein hydrolysate called PeptoPro.
PeptoPro is characterised by its unusually high content of di- and tri-peptides (i.e. minimum of 60%). Not many other hydrolysates (whey or casein) on the market can boast this high level of di- and tri-peptides. PeptoPro was given in three separate equal doses of 20g each day in conjunction with meal time. While the authors of the study make no mention of the possible role that the high di- and tri-peptide content of PeptoPro may have played in mediating the immune –enhancing effects of the high protein diet, it is conceivable that it could have played a role.
Witard OC, et al. High dietary protein restores overreaching induced impairments in leukocyte trafficking and reduces the incidence of upper respiratory tract infection in elite cyclists. Brain, Behavior, and Immunity. 2014;39:211-219.