A Hydrolyzed Casein Standardized to Lactotripeptides for Support of Vascular Health

Abstract

Lactotripeptides (LTPs) found in fermented milk have been shown to have potential antihypertensive properties by acting as angiotensin-converting enzyme (ACE) inhibitors. A standardized LTP product, with levels of LTP higher than what is available from dairy products, has also been shown in both in vitro and animal models to induce nitric oxide (NO) production and attenuate atherosclerosis. Clinical research has confirmed that LTP can promote vasodilation, improve endothelial function, and mitigate arterial stiffness in prehypertensive and hypertensive subjects. In this review of published research on the standardized LTP product, we discuss its established mechanisms of action and the randomized placebo-controlled clinical trials that document its potential beneficial effects on cardiovascular health.

Introduction

Cardiovascular disease (CVD) encompasses a range of diseases that affect the heart and blood vessels. According to 2015 American Heart Association statistics, CVD is the leading cause of death globally and claims more lives than all forms of cancer combined.1 Yet CVD is often preventable; conditions such as hypertension, elevated cholesterol levels, diabetes, and obesity are all lifestyle-related. Lifestyle-related diseases are caused by risk factors that are modifiable because they are interconnected with diet, exercise habits, and tobacco use. Combined, these lifestyle diseases account for half the deaths from CVD in the United States. Hypertension alone is believed to be the risk factor associated with the largest percentage of preventable deaths due to CVD.2
 
Lifestyle-related diseases, as well as nonmodifiable risk factors such as advancing age, gender, and family history, often precede CVD, as they commonly degrade vascular endothelial function, one of the key factors for the progression of atherosclerosis and CVD.3-7 Endothelial dysfunction is an imbalance of vasodilators (particularly NO and prostacyclin), and vasoconstrictive substances (such as angiotensin II) stimulated by the endothelium. When endothelial dysfunction persists, it promotes atherosclerotic plaque formation through a number of mechanisms such as upregulation of adhesion molecules, increased chemokine secretion, and enhanced low-density lipoprotein oxidation. Endothelial dysfunction also encompasses diminished production/availability of NO. Fortunately, endothelial dysfunction can be reversed prior to atherosclerotic damage, reducing the risk of CVD.8
 
Pharmacological treatment of endothelial dysfunction and hypertension depends on the risk factor that contributes to it, as different risk factors have different underlying mechanisms. Options include angiotensin-converting-enzyme (ACE) inhibitors, beta-blockers, calcium-channel blockers, and statins,9 all of which have side effects such as dizziness, ankle swelling, headache, muscle cramping, and fatigue.10,11 Therefore, a nonpharmaceutical intervention is of particular interest, especially one that can be used prophylactically. 
 
One such treatment is milk-derived peptides (also called lactotripeptides or LTPs) found in sour milk. Sour milk is a common product in Japan and has been marketed as providing a number of health benefits. Sour milk is cultured milk produced by fermenting skim milk with a natural starter culture containing several microorganisms.12 Fermented milk has been shown to lower blood pressure in hypertensive patients.13-16 There are 2 active peptides found in fermented milk: valine-proline-proline (VPP) and isoleucine-proline-proline (IPP). They are collectively referred to as LTP and, when isolated, have also been the subject of numerous clinical trials and 5 comprehensive meta-analyses that concluded that the blood pressure-lowering effect is statistically significant in hypertensive or prehypertensive individuals.17-21 The most recent meta-analysis by Fekete et al included 30 trials and 2,200 participants. The pooled treatment effect was −2.95 mm Hg (95% confidence interval [CI]: −4.17,−1.73; P<0.001) for systolic blood pressure (SBP), and −1.51 mm Hg (95% CI: −2.21, −0.80; P<0.001) for diastolic blood pressure (DBP). The authors noted that although the hypotensive effects are less than those of many pharmaceutical antihypertensive drugs, LTP “represents a potentially important prophylactic strategy for reducing the risk of hypertension through the life span, which could reduce the need for hypertensive medication later in life.”19 They also point out that no adverse effects have been reported with LTP use. 
 
Bioactive peptides and protein hydrolysate products for treatment of hypertension have been marketed in Japan under Food for Specified Health Use (FOSHU) designation for a number of years. These include sardines, bonito, sesame peptides, and a proprietary standardized LTP product (known as AmealPeptide) derived from enzymatic hydrolysis of casein,22,23 which contains levels of VPP and IPP higher than what is found in common dairy products. This review discusses the mechanism of action of this standardized LTP product based on laboratory and animal studies, as well as results of published clinical trials. 

Mechanisms of Action

It is well-established that bioactive peptides, such as those derived from sardines, bonito, and casein, lower blood pressure by inhibiting ACE, the mechanism of action of many antihypertensive drugs.12,24-26 Angiotensin-converting enzyme inhibitors have also been reported to improve vascular endothelial function.9,27,28 They prevent production of the potent vasoconstrictor angiotensin II, and conversely increase the potent vasodilator bradykinin, resulting in blood vessel dilation and blood pressure reduction. Angiotensin-converting enzyme inhibitors also attenuate production of the inflammatory cytokine tumor necrosis factor alpha (TNF-α), which acts on vascular endothelial cells, accelerating the production of various adhesion factors that can cause atherosclerotic plaque formation.29
 
Studies have shown that standardized LTP product induces NO production in cultured human endothelial cells, promotes vasorelaxation of isolated rat aortic rings through its inhibitory action of ACE and its induction of NO,30 attenuates atherosclerosis development in apolipoprotein E–deficient mice,31 and mitigates arterial dysfunction in rats treated with N(G)-nitro-L-arginine methyl ester hydrochloride (a competitive inhibitor of NO synthase).32 These mechanisms provide evidence that its hypotensive effect has potential to ultimately translate to CVD prevention.

Clinical Evidence

Standardized LTP product has been the subject of a number of randomized, placebo-controlled trials in subjects with hypertension and prehypertension. Since the LTP is standardized and put into tablet form, it is more useful for double-blind research design than food products. In one placebo-controlled, double-blind study, 42 subjects with mild or moderate untreated hypertension were given either placebo or 2 tablets of standardized LTP product (1.26 mg of VPP and 0.82 mg of IPP) once a day at breakfast for 8 weeks. In the test groups, SBP and DBP were significantly decreased after 2 weeks and 8 weeks of treatment. In the placebo group, no changes in blood pressure were detected.33
Continuous intake of standardized LTP product might have the potential to improve arterial stiffness as well as central SBP and peripheral brachial blood pressure.
Another randomized, double-blind, placebo-controlled clinical trial investigated the effects of standardized LTP tablets containing 3.4 mg each of VPP and IPP on central blood pressure and arterial stiffness in 70 subjects with untreated stage 1 hypertension. Central blood pressure and brachial-ankle pulse wave velocity (baPWV), a marker of arterial stiffness, were measured at the beginning and end of the 8-week study. Central SBP, baPWV, brachial SBP, and radial mean blood pressure were significantly reduced in the treatment group compared with placebo. Overall, these results suggest that VPP and IPP might have beneficial effects on arterial health.34
 
Hirota et al examined the effects of standardized LTP product on endothelial function in mildly hypertensive patients. In this placebo-controlled, double-blind crossover trial, 24 male subjects received either a placebo or 1.25 g of standardized LTP product (3.42 mg of VPP and 3.87 mg of IPP) per day for 1 week. After a 2-week washout period, the subjects switched to the other treatment for 1 week. Vascular endothelial function was measured by vasodilator response and the effects of reactive hyperemia (the transient increase in organ blood flow that occurs following a brief period of ischemia) on forearm blood flow (FBF) using plethysmography as an index of vascular endothelial function. In addition, a number of serum biochemical markers of endothelial function were measured. Although the basal FBF did not change, the maximum FBF during reactive hyperemia increased significantly compared to baseline and to the placebo group. Systemic blood pressure did not change, which indicates that the improvement of the vascular endothelial function attributable to LTP may be independent of blood pressure-lowering effects. In conclusion, LTP improves the vascular endothelial function in subjects with mild hypertension. The continuous intake of standardized LTP product may help to prevent cardiovascular diseases in hypertensive subjects.35
 
In a smaller study of LTP in hypertensive patients, hemodynamic parameters improved when the treatment duration was increased. Twelve adults with brachial SBP over 140 mm Hg received daily treatment of 4 standardized LTP tablets containing a total of 1.13 mg of VPP and 2.05 mg of IPP. Compared with basal levels, treatment with LTP at 6 and/or 9 weeks showed a significant reduction in peripheral SBP and DBP, augmentation index, and central SBP. Although small and not placebo-controlled, this study suggests that continuous intake of standardized LTP product might have the potential to improve arterial stiffness as well as central SBP and peripheral brachial blood pressure.36
 
Menopause is another nonmodifiable risk factor for endothelial dysfunction, which can manifest as hypertension in women in their postmenopausal years.37,38 In 2 trials, Yoshizawa et al investigated whether combining regular exercise with a standardized LTP product in postmenopausal women would improve endothelium-dependent dilation and arterial compliance more effectively than either treatment alone. Both studies were 8-week randomized, placebo-controlled trials with 4 intervention arms and included healthy but sedentary postmenopausal women. The participants were randomly assigned to 1 of the 4 groups: placebo; exercise plus placebo; LTP; or exercise plus LTP. Participants in the LTP (alone) and exercise plus LTP groups ingested 2.8 g of standardized LTP product containing 2.4 mg of VPP and 4.3 mg of IPP per day (this daily dose was divided into 8 capsules).
 
Participants in the exercise plus placebo and exercise plus LTP groups underwent aerobic exercise training 3 to 5 days per week, beginning with low-intensity exercise (~ 60% of their individually determined maximal heart rate) for 25 to 30 minutes per day, 3 to 4 days per week. As their exercise tolerance improved, the intensity and duration of aerobic exercise were increased to 40 to 45 minutes per day, 4 to 5 days per week, at an intensity of 70% to 75% of maximal heart rate. One study measured flow-mediated dilation (FMD), a noninvasive method for measuring endothelial dysfunction, while the other study measured arterial compliance. In both studies, the monotherapies (either standardized LTP product alone or exercise alone) and the combined therapy of standardized LTP product plus exercise resulted in significant increases in FMD and arterial compliance compared with placebo. However, the magnitude of the increases was significantly higher in the combined-therapy group than in either the standardized-LTP-alone or regular aerobic exercise-alone group. These results suggest that regular LTP intake has isolated effects, as well as additive effects when combined with exercise, on improving endothelium-dependent dilation and arterial compliance in postmenopausal women.39,40

Conclusion

Research has concluded that endothelial dysfunction is both an early contributor to and an early predictor of the development of atherosclerosis. Therefore, reversing vascular endothelial dysfunction is the first line of defense for preventing atherosclerosis. Given the evidence that lifestyle diseases (eg, hypertension, diabetes, hyperlipidemia) and lifestyle factors (eg, smoking, obesity) are associated with vascular endothelial dysfunction, it follows that lifestyle modifications should play a primary role in any treatment plan. Nonpharmaceutical interventions, such as the LTP product, can complement lifestyle modifications without the side effects associated with pharmaceutical treatment. 
 
The ACE inhibitory activity and other vasodilating-supporting mechanisms of LTP are believed to be at least partly responsible for its clinically documented effects. While short-term intake (1 week) of the LTP product was shown to improve vascular endothelium function, long-term intake (6-9 weeks) resulted in improvement in arterial stiffness, central SBP, and peripheral brachial blood pressure. Based on the clinical research, it appears that supplementation with 3.4 mg each of VPP and IPP, taken for at least 8 weeks, may result in significant beneficial effects. Current evidence indicates that LTP can be used safely and with no side effects. Since both endothelial dysfunction and hypertension are associated with CVD, standardized LTP product may also help reduce CVD risk.
 
Abbreviation Definitions
 
Term Abbreviation
lactotripeptide LTP
cardiovascular disease CVD
nitric oxide NO
valine-proline-proline VPP
isoleucine-proline-proline IPP
angiotensin converting enzyme ACE
systolic blood pressure SBP
diastolic blood pressure DBP
brachial-ankle pulse wave velocity baPWV
forearm blood flow FBP
flow-mediated dilation FMD

 

Description (Sub-Heading): 
A review of the literature