2. Effect of Peptides on Bones, Muscles, and Joints
The relationship of the immune system with bone is reciprocal, meaning that bone cells also influence immune cells. In this regard, bone cells are responsible for creating the so-called “endosteal niche”, as these cells are implied in the mobilization of hematopoietic stem cells 
. Similarly, in terms of muscles, it has been described how the variations in the phases of myogenesis during muscle regeneration after having suffered from an injury concur with changes in the phenotype and activation state of leukocytes that invade the damaged, regenerating tissue. In fact, TNF, interferon-γ (IFNγ), (IL-10), and insulin-like growth factor 1 (IGF1) play an important role in managing the inflammation and the myogenic response to muscle damage that is required to achieve muscle regeneration 
Koning et al. 
reported how collagen peptides, under the circumstances assessed, improved bone mineral density and bone markers in postmenopausal women, through a randomized, placebo-controlled, double-blinded study. Over one year, postmenopausal women (n = 131, mean age of 64 years old) with a primary, age-related reduction in bone mineral density, ingested 5 g daily of the test item (collagen peptides) or a placebo, as the control. The primary endpoint was the change in bone mineral density of the femoral neck and the spine. On top of this, plasma levels of bone markers (i.e., amino-terminal propeptide of type I collagen and C-telopeptide of type I collagen) were also analyzed. According to the authors, the subjects who ingested the test item showed an increased bone mineral density of the spine and of the femoral neck in comparison to the control group. Concerning the markers, the propeptide increased significantly in the peptide-ingesting group, whereas the C-telopeptide was found to be higher significantly in the control group. This is similar to when chicken collage type II was ingested (2.5/day) over 8 weeks, and the target population, consisting of adults with joint discomfort, but no co-morbidities (n = 47, mean age of 66 years old), were found to have a significant reduction in joint-related discomforts 
Brown et al. 
aimed to assess whether a whey protein hydrolysate (WPH) supplementation could be useful for exercise-induced muscle damage and recovery in females (n = 20). The stimulus was a repeated-sprint exercise, and then the subjects ingested two doses of 70 mL of the test item (whey protein hydrolysate) or a control consisting of isoenergetic carbohydrate, over 4 days post-exercise. The endpoints measured were muscle soreness, limb girth, flexibility, muscle function, and creatine kinase, which were evaluated before, immediately after, and 24, 48, and 72 h post-exercise. According to the authors, time effects were observed for all variables except limb girth. It was observed that flexibility improved beyond baseline measures following WPH by 72 h and that the reactive strength index was higher throughout recovery in the test item group compared with the control. In addition to that, reductions in creatine kinase were higher following the intake of whey hydrolysate compared with the carbohydrates at 48 h post-exercise-induced muscle damage.
Concerning fish proteins, Nygard et al. 
aimed to demonstrate the benefits of cod peptides in older adults (n = 86, mean age of 73 years old) who ingested 3 g for a year. Parameters such as short physical performance battery, grip strength, gait speed, and dietary intake were considered, but at the end of the study, authors reported no significant differences observed after the test item ingestion.
Funglsang-Nielsen et al., 
evaluated the effects of 12 weeks of whey protein supplements on markers of bone turnover in adults with abdominal obesity. For this purpose, a population of adults with abdominal obesity (n = 64, mean age of 66 years old) ingested 60 g of peptides daily. The main endpoints evaluated were plasma markers of bone turnover, as well as u-calcium and u-carbamide excretion, bone mineral density, and insulin resistance. However, no major changes were observed in the parameters assessed, except for a slight increase of parathyroid hormone levels. In the same line, recently Kerr et al. 
evaluated whether a Vicia faba hydrolysate, which showed beneficial muscle recovery and anti-inflammatory properties by in vitro and animal studies, was effective in humans. For this purpose, a randomized, double-blind, placebo-controlled trial with healthy males (n = 30, mean age of 38 years old) was carried out, in which the subjects ingested 2.4 g of the test item/day for two weeks. According to the results, strength recovery was increased, and muscle fatigue decreased compared to the placebo over the 72-h period that post-resistance exercise was observed. In addition to that, other acute markers increased such as IL-6, IL-15, or Erythropoietin (EPO).
In studies related to inflammation and recovery of bones and/or muscles, a relevant parameter to be taken into account is the age of the individual, as ageing has a strong impact on the physical status of humans, deteriorating the organism and increasing the susceptibility of the individuals to suffer from inflammation. More parameters related to inflammation could be useful in order to unravel the mechanism of action by which the peptides exert the effects observed in the reported studies.
3. Effect on Gut Microbiota
The relationship between gut microbiota and the immune system is clear, although it is not fully described and characterized nowadays. Briefly, maintaining intestinal homeostasis and reducing inflammation depend heavily on the dynamic interactions between the gut microbiota and the host’s innate and adaptive immune systems. Proteins and complex carbohydrates are metabolized by gut microbiota, implying a communication between gut epithelium and immune cells. Gut epithelial cells create a mucosal barrier as a protective measure to separate the bacteria from host immune cells and lessen intestinal permeability. A compromised epithelial barrier and a greater susceptibility to infections can result from an abundance of potentially harmful Gram-negative bacteria and the metabolic changes they cause in the gut microbiota and mucosal immune system 
Recently, the alteration of gut microbiota by bioactive peptides was reviewed by Guo et al. 
and Yeo 
, among other authors, but most of the studies refer to in vitro assays or animal studies and are thus not in the scope of this research.
In terms of human studies, Moreno-Perez et al. 
carried out a randomized pilot study in cross-country runners (n = 12, mean age of 35 years old). The subjects followed, over ten weeks, a diet supplemented with a whey isolate and beef hydrolysate, whereas the control was maltodextrin. The microbiota was evaluated by collecting fecal samples and the results showed an increase of the Bacteroidetes phylum, whereas a decrease of Roseburia, Blautia, and Bifidobacterium longum, usually related to good health, was reported, thus implying a negative effect according to these parameters.
Furthermore, the body composition, gut microbiota, and serum metabolomics of old women (n = 60, mean age of 61 years old) with an energy-restricted diet were evaluated following an intervention consisting of 20 g of whey peptides over 8 weeks. The main outcomes were a reduction in body weight and the upregulation of the tricarboxylic acid cycle pathway, but no significant changes were reported in the microbiota 
Further studies aiming to evaluate how the ingestion of food-derived peptides from different sources affect the gut microbiota are needed in order to fully understand whether the impact can be considered biologically relevant, and the consequences derived from that. The differences found among studies are most likely due to the background diet, especially fiber, which has a relevant effect on microbiota, as well as the protein composition ingested and the high interindividual variation among subjects.
4. Effect on Glucose Homeostasis and Blood Lipids
Regarding diabetes, which is also one of the most prevalent diseases worldwide, it has similarly been demonstrated how inflammation plays a key role in the advancement of metabolic abnormalities, and bidirectionally, that metabolic factors modulate immune cell functions. An impairment of the metabolic status of subjects has an impact on the immune cell infiltration and inflammation while also implying an increase in reactive oxygen species. The consequence is the development of insulin resistance 
, while at the same time, it has an impact on gut microbiota dysbiosis, which is related to an alteration of the functions of innate and adaptive intestinal immunity 
. The effects of cholesterol-lowering dietary compounds on the immune system have been widely reviewed, as well as the impact of cholesterol on molecular and cellular events and subsequent biological responses of immune cells 
: for instance, initiating NLRP3 inflammasome-mediated cascades 
or inducing apoptosis by ROS production. Peptides appear as a relevant component in managing diabetes-related risk factors.
The safety and efficacy, for instance, of a beverage containing lupine (Lupinus angustifolius
L.) peptides on the immune, oxidative, and lipid status in healthy subjects was evaluated in an intervention study (open-label, n = 33, mean age of 30 years old). The daily dose was 1 g, ingested over 4 weeks, and implied changes in several biochemical parameters in fasting peripheral blood and urine samples. This increased the anti-/pro-inflammatory response of peripheral blood mononuclear cells and improved the cellular anti-oxidant capacity. The test item also reduced the low-density lipoprotein-cholesterol (LDL-C)/high-density lipoprotein-cholesterol (HDL-C) atherogenic index. The test item’s effect was beneficial on decreasing not only the LDL-C/HDL-C index but also serum total cholesterol levels in the male cohort 
In terms of potential antidiabetic properties of whey peptides, the concentration–time curves of glucose and insulin were monitored after a supplementation of 1.4 g of the test item over 6 weeks by prediabetic patients (n = 21, mean age of 62 years old). Authors reported a significant impact on a postprandial blood glucose profile with more glycemic than insulinotropic properties 
. Similarly, Saleh et al. 
aimed to demonstrate the antidiabetic effect of casein (17 g/d, over 8 days, mean age of 31 years old) in patients with mild gestational diabetes (n = 26). Among the parameters assessed (plasma glucose, insulin and C-peptide levels, and the insulin-to-glucose ratio), no insulinotropic effects occurred, but a moderated reduction of plasma glucose levels occurred.
However, Jensen et al. 
showed that, in a randomized, double-blind study with adults suffering from metabolic syndrome (n = 15, mean age of 53 years old), the supplementation with 4 g of cod peptides over eight weeks had an impact on the levels of high-sensitivity C-reactive protein, whereas the other parameters evaluated (e.g., fasting and postprandial levels of acylated ghrelin, and fasting levels of adiponectin, leptin) did not show any statistically significant differences. These authors also reported that two months supplementation with 4 g of cod protein hydrolysate had no effect on fasting or postprandial levels of insulin, glucose or GLP-1, lipid profile, or body composition in individuals (n = 15) suffering from metabolic syndrome 
Tackling the risk factors from the beginning is crucial in order to reduce the risk of inflammation, especially upon the ingestion of different food components. Protein has shown to have a beneficial effect, as the peptides can exert their action in different targets. However, the mechanisms of action of the interactions occurring are not fully understood, and studies with enhanced methodology and more clear results are required.
5. Effect on Blood Pressure
The relationship between hypertension and proinflammatory cytokines, as well as the cells of the innate and adaptive immune systems, has been shown. In this regard, the sympathetic nervous system, which is described as a key determinant of hypertension, stimulates the bone marrow, spleen, and peripheral lymphatic system, and is proinflammatory. Additionally, an increase in the cytokine levels leads to structural changes in the resistance vessels, causing elevated blood pressure 
Concerning the antihypertensive potential of peptides, Musa-Veloso et al. 
carried out a randomized, double-blind, placebo-controlled, multicenter trial to evaluate whether 1.2 g of shrimp peptides could have an effect on adults with mild to moderate hypertension (n = 144, mean age of 55 years old). Authors indicated a significant reduction of blood pressure, possibly due to a reduction in angiotensin II levels.
The intake of 3 g of peptides obtained from egg ovalbumin over 12 weeks (including an intermediate wash-out period) from middle-aged people (n = 70, mean age of 57 years old) with high to normal blood pressure was evaluated for the antihypertensive effect, considering the evaluation of blood pressure, arterial stiffness anthropometry, plasma lipids, and glucose. However, the results obtained did not lead to any decrease in blood pressure, nor any improvement in markers of cardiovascular diseases risk 
. On the other hand, the use of plant-derived peptides has been growing in recent years but only a study with rice bran peptides (43 µg of the peptide LRA ingested daily for 12 weeks) was reported, aiming to determine hypotensive effects in middle-aged people with high to normal blood pressure or grade 1 hypertension (n = 50, mean age of 54 years old). According to the authors, a decrease of systolic blood pressure was observed, while no serious adverse events occurred 
Hypertension is currently one of the most problematic diseases in modern societies, as it is directly correlated with the development of cardiovascular disease. The efficacy of peptides to regulate blood pressure in humans might be one promising non-medical strategy to reduce the worsening of risk factors. Inflammation markers could add value to the explanation of the changes in blood-pressure-related parameters, in order to understand its complete relationship with the immune system.
6. Effect on Neurological Parameters
Neurological disorders (e.g., epilepsy, dementia, motor neuron diseases, headache disorders, sleep disorders) related to inflammation are a growing problem in modern societies. The pathogenesis of these diseases is related to a dysregulation of the immune system, and can cause immune activation of the central nervous system (CNS), leading to an increase of cytokine levels and/or infiltration of immune cells into the CNS 
. An adequate management of the inflammation is subsequently crucial in order to have a healthy ageing and avoid the development of neurological problems. Two studies related to memory in humans were identified, although the parameters evaluated did not take into account inflammatory markers.
A chicken extract (670 mg) was orally administered to healthy middle-aged people under mild stress (n = 90, mean age of 42 years old) for 8 weeks in order to assess changes in neurocognitive tests and biochemistry markers. An improvement in verbal short- and long-term memory and spatial working memory was found 
. In another study performed in healthy adults (n = 76, mean age of 55 years old), the effect of the intake of silk fibroin peptides (0, 280, 400, or 600 mg) over 3 weeks was assessed. The endpoints considered were a learning and memory test, learning gradient, memory maintenance, retrieval efficacy, and drawing/recall scores. A dose-dependent amelioration of the memory quotient score, the learning gradient, the numbers of words remembered, the retrieval efficiency, and drawing/recall was recorded, while no adverse effects were reported 
Evaluating whether a diet has an impact on the development of neurological diseases is a challenge, and the identification and validation of markers appearing at early stages could be one approach to enhance the relevance of the studies in order for them to be applied and evaluated properly.