Health properties of kefir

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Health

properties of kefir

Kefir enjoys a

rich tradition of health claims. In the former Soviet Union, it is used

in

hospitals and sanatoria for a variety of conditions, including

metabolic

disorders, atherosclerosis, and allergic disease (1). It has even been

used for

the treatment of tuberculosis, cancer, and gastrointestinal disorders

when no

modern medical treatment was available. Its consumption has also been

associated with longevity in Caucasus (20) . Various scientists have

observed

digestive benefits of kefir (21, 22) , but controlled studies have yet

to

confirm their empirical findings.

Various

research teams around the world have reported encouraging results, but

several

methodological difficulties still need to be resolved. Most studies to

date

have been performed in vitro or using animal models, and human studies

are not

available. Further, the effects of kefir grains or their isolates are

often studied, rather than the product kefir, and there is no evidence

that the

observed effects would occur using the drink itself. Also, kefir

products vary

significantly according to the composition of the grains used and even

according to the region in which it is made, and therefore specific

effects may

not be demonstrated in all kefirs. Given these caveat, a variety of

health

benefits are being investigated. Table 3 presents recent studies using

kefir

products.

Several

studies have investigated the antitumor activity of kefir (20, 23, 24)

and of

kefir grains (25, 26) . Specific cultures isolated from kefir were also

shown

to bind to mutagenic substances such as indole and imidazole (27, 28).

Immune

system stimulation with kefir (24) and with sphingomyelin isolated from

the

lipids of kefir (29) have been demonstrated in both in vitro and in

vivo

studies.

Kefir (30)

possesses antimicrobial activity in vitro against a wide variety of

gram-positive and gram-negative bacteria (20, 31), and against some

fungi (20)

.. In Zacconi et al.ís recent study (30), the antagonistic effects of

kefir

against Salmonella kedougou were attributed to the complexity and

vitality of

the kefir microflora. De Vrese et al . (32) demonstrated

that fresh, but not heat treated,

disintegrated kefir grains suspended in kefir directly enhanced

intestinal

lactose digestion in minipigs. This effect was attributed to microbial

b-galactosidase activity of kefir. The above studies provide

encouraging

results, but much more research is necessary in order to demonstrate

similar

effects using kefir in humans. Further, a standardized, well-defined

product

must be used in order to provide useful information.

Abstract

Research

on fermented milks (FM) has grown

dramatically in the past 20 years. FM have probiotic effects since

their

consumption leads to the ingestion of large numbers of live bacteria

which

exert health benefits beyond basic nutrition. Major results of research

are as

follows. Yogurt consumption reduces symptoms of lactose maldigestion

compared

to milk. FM, may have antibacterial and immunological properties.

Ingestion of the lactic acid bacteria bifidobacteria improves the

colonic

microflora by increasing bifidobacteria levels. Lactobacillus casei

reduces the

duration of some types of diarrhea. Future research conducted using

human

subjects, with rigorous methodology and modern statistical analysis,

will

provide further information on the health benefits of FM.

 

 

 

 

Keywords: fermented milk,

probiotic, yogurt, kefir, Lactobacillus streptococcus,

Streptococcus thermophilus, Lactobacillus casei, bifidobacterium

Glossary

Lactic acid

bacteria (LAB): a large group of bacteria with the common

characteristic of

producing lactic acid as the principal end product of metabolism; found

in milk

and other natural environments LAB can be: a. homofermentative: produce

70-90%

lactic acid; e.g., L. bulgaricus, S. thermo-philus, L. acidophilus b.

heterofermentative: produce at least 50% lactic acid plus other

compounds such

as acetic acid, CO2, and ethanol; e.g., L. casei, bifidobacteria a.

mesophilic:

grow best at a temperature range of 25-30¡C; e.g., L. casei b.

thermophilic:

prefer a range of 40-44¡C; e.g., L. bulgaricus, S. thermophilus a.

Facultatively prefer anaerobic anaerobic: conditions for metabolism,

but are

aero-tolerant (most LAB fit in this b. Strictly anaerobic: survive only

in

anaerobic conditions; e.g., bifidobacteria

Functional

foods:

Foods that, by

virtue of physiologically active food components, provide health

benefits

beyond basic nutrition (Working definition of ILSI Functional Food Task

Force,

Brussels, February 17,1997). Interleukin, interferon, tumor necrosis

factor:

examples of cytokines, which serve as signals between cells involved in

immune

response. sIgA: secretory immunoglobulin A; principal antibody produced

by the

gut immune system.

Azoreductase,§-glucuronidase, glycocholic acid hydrolase,

nitroreductase:

colonic enzymes implicated in the conversion of procarcinogens to

carcinogens. LDL/HDL: ratio between blood levels of low density

lipoprotein and high density lipoprotein; level above 3 indicates

increased

risk of cardiovascular disease. Breath hydrogen test: measurement of

hydrogen

expired after oral lactose load of 12-50 g compared to base level; >

10-20

ppm indicates malabsorption.

From

legend to science: Historical perspective

For centuries,

fermented milks have been purported to provide a large gamut of health

benefits, from improving well-being to increasing longevity. One story

recounts

that in the sixteenth century, King Fran*ois the First of France

suffered from

persistent diarrhea, and after several unsuccessful treatments, a

Turkish

doctor was sent in. He brought with him sheep and a secret recipe for

yogurt.

The king was soon cured of his intestinal infection.

Scientific

interest began much later, in the early

twentieth century, when Elie Metchnikoff, a Nobel-prize winning

biologist at

the Pasteur Institute in Paris, first suggested that lactobacilli might

counteract the putrefactive effects of gastrointestinal metabolism (1).

In the

past twenty years, scientific research has blossomed, with an interest

in

topics ranging from antimicrobial effects to reduction of risk of

cancer. Much

valuable preliminary work has been done using animal or in vitro

models, which

allow for much greater control over variables than when studying

humans, and

which offer reproducible results. These models are also useful for

studying the

mechanisms involved.

 

 

Studying

the effects of FM on humans presents several

challenges. Fermented milksare unctional foods, and as such, their

impact on

human physiology is of a small amplitude and not easily detected. Also,

early

humans studies, though numerous, were generally case reports rather

than modern

experimental studies (randomized). Currently, researchers are beginning

to

address these methodological problems.

Yogurt,

the ever-popular fermented milk

According to the

Codex Alimentarius (5), yogurt is milk (usually cowís milk) that has

been

fermented by Streptococcus thermophilus and Lactobacillus bulgaricus

under

defined conditions of time and temperature. Each species of bacteria

stimulates

the growth of the other, and the products of their combined metabolism

produce

the characteristic creamy texture and mild acid flavor. Fermentation is

stopped

by cooling, and the final product, which contains100-1000 million live

bacteria

per ml, is refrigerated until use. As a fresh dairy product, it has a

limited

shelf-life.

1.

Milk digestibility

Given all the

research to date on FM, the fact that lactose is better digested from

yogurt

than from milk by lactase-deficient individuals is the most

well-established

health benefit (6). Yogurt ingestion leads both to less hydrogen

production in

the breath hydrogen test (lactose maldigestion) (Figure 1) and to

reduced

symptoms (lactose intolerance) (Marteau, 1990; Lerebours,1989; Kolars,

1984).

This effect is related to the living bacteria, the enzymatic content (

e.g

,§-galactosidase), and the texture of yogurt.

2.

Recovery from diarrhea

Yogurt reduces

the duration of certain types of diarrhea, especially in children (Niv,

1963;

Boudraa, 1990). The World Health Organization (WHO, 1995) recommends

that

during treatment of diarrhea, yogurt should replace milk when available

since

it is better tolerated than milk and can help prevent malnutrition or

reestablish nutritional adequacy.

3.

Immunomodulating effects

Yogurt has been

shown to enhance various parameters of the immune system in invitro

models (13)

and in mice (14-16). In humans, one study found an improvement in

clinical

symptoms of nasal allergy, but no changes in any parameters tested (17)

.. A

recent report with atopic subjects found no significant modification of

immune

system parameters, showing that there was no aggravation of the immune

system

caused by yogurt (18) . Very high concentrations of yogurt bacteria

have led to

increases in IFNy, B lymphocytes, and natural killer cells (19) , and

yogurt

consumption increased 2í,5í-a synthetase activity (a reflection of

production

of IFNy) (20).

4.

Reduction of risk of cancer

A recent

epidemiological study from France showed that people consuming yogurt

had less

risk of developing large colorectal adenomas (21). In addition, the

consumption

of yogurt in elderly subjects with atrophic gastritis led to a decrease

in the

procarcinogenic fecal enzymes nitroreductase and azoreductase (22).

Research in

this field is intriguing, but preliminary.

 

 

5.

Blood cholesterol levels

Mann and Spoerry

(23) reported over 20 years ago that Maasai warriors consumed several

liters of

FM per day and yet had low serum cholesterol levels. This observation

sparked a

series of conflicting studies on the possible hypocholesterolemic

properties of

yogurt and other FM. Results have been inconsistent (24). What is clear

is that

regular consumption of yogurt does not increase plasma cholesterol

concentration (24, 25); yogurt can be part of the daily intake of

individuals

who are concerned about heart disease.

Kefir,

another traditional fermented milk

Kefir is a

stirred beverage made from milk fermented with a complex mixture of

bacteria

(including various species of lactobacilli, lactococci, leuconostocs,

and

aceterobacteria) and yeasts (both lactose-fermenting and

non-lactose-fermenting). The small amount of CO2, alcohol, and aromatic

compounds produced by the cultures give it its characteristic fizzy,

acid taste

(26). Kefir fabrication differs from that of yogurt in that kefir

grains (small

clusters of microorganisms held together in a polysaccharide matrix) or

mother

cultures from grains (27) are added to milk and cause its fermentation.

Kefir

is actually a family of products, in that the grains and technology

used can

vary significantly and thus result in products with different

compositions.

Many health

benefits have been traditionally reported. Kefir has been used for the

treatment of atherosclerosis, allergic disease, and gastrointestinal

disorders,

among other diseases (28). Until recently, most research has been

limited to

studies lacking modern statistical practices or to reports written up

in Slavic

languages, rendering them inaccessible to most western scientists.

Recent

studies have investigated antibacterial (29), immunological(30),

antitumoral

(31), and hypocholesterolemic(32) effects of kefir consumption on

animals.

Results suggest potential benefits. Fresh, but not heat-treated grains

in kefir

enhanced intestinal lactose digestion in minipigs (33). While awaiting

more

research, it is important to remember that kefir, like yogurt, has been

and

continues to be a part of the regular diet in central and eastern

Europe for

centuries. Bifidobacterium: a natural inhabitant of the intestines

Bifidobacteria were first described in 1900 by Tissier (34) . Since

that time,

their classification has evolved continually, and currently includes

around

thirty species (35, 36) . In general, they are strictly anaerobic,

Gram-positive rods which often have special nutritional needs and grow

slowly

in milk. Very few strains are adapted well enough to milk that they

both grow

in sufficient numbers and survive well throughout the shelf-life of the

FM.

Although

bifidobacteria produce both lactic acid and acetic acid as major

end-products

of metabolism (heterofermentative), many microbiologists consider them

to be

lactic acid bacteria, albeit a special case. Tissierís hypothesis

almost

100 years ago that bifidobacteria might have health benefits(37) was

based on

the following observations. Bifidobacteria are normal inhabitants of

the human

intestinal tract throughout the life cycle, beginning just days after

birth.

Further, they are often the predominant microorganism in the gut of

breast-fed

infants. It has since been shown that breast-fed babies are less at

risk for

diarrheal disease than formula-fed infants (38). In addition to the

above

inherent characteristics of bifidobacteria, some strains of the

micro-organism

survive intestinal transit in sufficient numbers to exert a metabolic

effect in

the gut (39,40).

1.

Effects on the intestinal microflora

Ingestion of milk

fermented with bifidobacteria leads to an increase in fecal

bifidobacteria

levels, both in infants (43) and in adults (44) . Elevated levels

return to

normal after cessation of consumption (39). Ingestion of FM with

bifidobacteria

has also led to a decrease in §-glucuronidase activity, but not in

other

enzymes associated with colon (44).

2.

Effect on mild constipation

Slow intestinal

transit can be partially corrected in women by the regular consumption

of a

milk fermented with yogurt cultures and bifidobacteria (41). This

effect was

not observed with yogurt as a control, thus demonstrating the

specificity of

bifidobacteria for the increased colonic motility (42).

3.

Prevention of diarrhea

Few studies have

been performed. One double-blind study of infants demonstrated that a

formula

with added B. bifidum and S. thermophilus reduced the incidence of

hospital-acquired diarrhea compared to a standard formula. It also

lowered the

rate of rotavirus shedding into the environment (45).

4.

Immunomodulating effects

Ingestion of milk

fermented with B. bifidum led to an increase in phagocytic activity in

peripheral blood compared to milk consumption (46). A mixture of B.

bifidum and

L. acidophilus decreased chronic inflammation of the sigmoid colon and

increased humoral immunity in a group of elderly subjects (47).

Lactobacillus

casei: new interest in an old bacteria

The group L.

casei consists of several species of facultatively anaerobic and

hetero-fermentative, mesophilic lactic acid bacteria(48). Their

metabolism

provides organoleptic qualities to several traditional FM and cheeses,

and more

recently, to new fermented milks. L. casei have been detected in the

feces of

both infants (49) and adults (50). Their ability to survive transit

through the

intestinal tract in adequate numbers to have a physiological effect

(50) ,

coupled with their potential health benefits make L. casei an ideal

candidate

for a probiotic.

1.

Treatment of diarrhea

Several double

blind, placebo-controlled clinical trials have demonstrated that oral

consumption of L. casei reduces the duration of diarrhea (51), and in

particular, rotavirus gastroenteritis (52) in children. In addition, L.

casei

may help reduce the duration of diarrhea associated with children in

day care

centers (53), antibiotic treatment (54) and travelerís diarrhea (55).

 

 

2.

Effects on the intestinal microflora

In addition to

increasing lactobacilli count in feces (50), milk fermented with L.

casei has

been shown to lower the activity of the colonic enzymes §-glucuronidase

(50,

56), glycocholic acid reductase, and nitroreductase (56) in healthy

adults. A

recent study demonstrated a decrease in §-glucuronidase and

§-glucosidase

activities in infants after ingestion of a milk fermented with yogurt

cultures

and L. casei. This effect was not found with yogurt alone or with

gelled milk

(control) (57) , thus suggesting that the modification was due to L.

casei or

to the association between L. casei and yogurt.

3.

Immunomodulating effects

Challenge tests (

e.g ., using Salmonella typhimurium ) with oral ingestion of L. casei

in mice

has led to increased protection in animals infected with pathogenic

bacteria

(58, 59). A few reports using human subjects have shown an enhancement

of

non-specific immune system activators, such as y interferon and

interleukins

(ex vivo) (60) and of specific immune responses to various challenges,

including rotavirus vaccine (61). In a recent study infants with

atopic

dermatitis were given formula with added L. casei. Not only did the

concentration of fecal tumor necrosis factor-a decrease significantly

(a

measure of the immune response), but clinical symptoms improved as well

(62) .

Viability of the bacteria is an important factor of its effectiveness

(61).

 

Other

probiotics

In addition to

the probiotics discussed above, other bacteria, some well known and

some more

recent, offer additional health benefits. In particular, much research

has been

conducted on L. acidophilus. Several studies suggest a

hypocholesterolemic

effect of L. acidophilus (63) , while others have investigated its

ability to

prevent various types of diarrhea (64) and to reduce the incidence of

candidal

vaginitis (65).

In addition,

consumption of L. acidophilus has led to

modifications of various parameters of the immune system (46), and to a

decrease in several fecal enzymes associated with colon cancer (66).

Less

well-known bacteria include Lb. helveticus (67), L. plantarum(68) , and

L.

reuteri (69). These lactic acid bacteria have different microbiolog