December 2000
Harsha Vittal, MD
School of Medicine, University of Nevada
|
Highlights:
The DASH diet is a combination of a Mediterranean diet with low fat dairy
products.
- It
is the most effective non-drug treatment of hypertension.
- Its effects are potentiated by salt restriction.
|
High blood pressure is one of the most common medical diagnoses in the United States. Approximately, 45 million Americans have hypertension, defined as systolic blood pressure 140 mm Hg and diastolic blood pressure 90 mm Hg. Of this group, only 25% are adequately
controlled by lifestyle modification and/or medication to a normotensive
range. Considering evidence shows a 42% increase in strokes and 56% increase in cardiovascular deaths in people who have mild hypertension (defined as systolic blood pressure between 140-159 mm Hg and diastolic blood pressure 90-99 mm Hg) researchers and physicians have tried to find both pharmacological and non-pharmacological methods to more effectively treat mild hypertension.
Over the years, various ideas concerning dietary interventions for the treatment of hypertension have been investigated. Salt restriction was a hot concept for many years but has recently lost favor among many practitioners.
Although in carefully controlled circumstances salt restriction can lower blood pressure significantly, in the real-world of clinical practice these gains are difficult to attain. An analysis of the many studies addressing this issue came to the conclusion that salt restriction in people under 45 or normotensive people at any age would be of very limited benefit. In patients over 45 with hypertension, the benefit would be minimal and not nearly as significant as exercise and a low-fat diet. In fact, The British Hypertension Society recommendations place salt reduction at the bottom of the list of priorities with smoking cessation, weight loss, and reduced alcohol intake presented as more effective lifestyle modifications.
In addition, many researchers have focused on other dietary modifications such as increasing potassium, magnesium, or calcium intake. There is a good deal of evidence that people whose diets are high in potassium (usually diets high in fruits and vegetables) tend to have lower blood pressures. However, the results with potassium supplementation in hypertensives have been mixed. There is also some evidence that increasing potassium in the diet lowers the incidence of stroke regardless of the effects on blood pressure.
Although there are a number of proponents of calcium and magnesium, there is limited evidence to support their routine use in blood pressure reduction. However, patients do benefit from calcium supplementation in other disorders such as osteoporosis. Unfortunately, no study to date has evaluated the efficacy of using each of these supplements simultaneously.
The DASH Study
Table 1. Food Groups of the DASH Diet
|
Food Group
|
Daily Servings
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Serving Sizes
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Examples
|
|
Grains
and Grain Products
|
7-8
|
1 slice of bread
½ cup cereal
½ cup rice or pasta
|
Whole Wheat Bread
Cereals
Oatmeal
|
|
Vegetables
|
4-5
|
1 cup raw vegetables
½ cup cooked vegetables
6 oz vegetable juice
|
Tomatoes, Potatoes,
Carrots,
Beans
|
|
Fruits
|
4-5
|
6 oz fruit juice
1 medium fruit
¼ cup dried fruit
½ cup fresh, frozen,
canned fruit
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Bananas, Grapes,
Oranges, Apples
|
|
Low fat
or nonfat dairy foods
|
2-3
|
8 oz milk
1 cup yogurt
1.5 oz cheese
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Skim Milk
Nonfat Yogurt
Nonfat Cheese
|
|
Meat,
poultry, or fish
|
2 or less
|
3 oz cooked meats,
poultry, and fish
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Broiled, Roasted or Boil
Meat
Skinless Poultry
|
|
Nuts,
seeds, and legumes
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4-5 per week
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1/3 cup nuts
½ cup cooked legumes
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Almonds, Peanuts,
Lentils, Kidney Beans
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The initial DASH study compared the effects
on blood pressure of three diets: an average american diet (control diet); a diet rich in fruits and vegetables; and the DASH Diet (Table
2).
Table 2. Three Different
Diets Implemented in the DASH Study
|
DASH diet
|
Fruits-and-Vegetables Diet
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Control Diet
|
See Above
|
Rich in fruits and vegetables, but
similar fat content to the average American diet
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Low in fruits and vegetables with
similar fat content to the average American diet
|
People
included in the study had systolic blood pressures less than 160 mm Hg and
diastolic blood pressures between 80 and 95 mm Hg. The change in diastolic blood pressure at rest was the primary
endpoint. Change in systolic blood pressure and in ambulatory blood pressures
were secondary endpoints. The participants were started on a three-week
'run-in' diet low in fruits and vegetables with a fat content similar to the
average American diet. They were then
randomized to receive one of the three different diets for the next eight
weeks. Each meal was prepared in a
central kitchen, and participants were instructed not to eat anything that they
did not obtain from the DASH kitchen.
Furthermore, each group's diet contained equal amounts of sodium, and
the caloric intake was adjusted to maintain each person's body weight. These interventions were designed to
eliminate two important variables that could have affected the
outcome.
Results
Four hundred and fifty-nine
subjects were randomized. The average
age was 44.6 yrs, and baseline BP was 132/85 mmHg. Fifty-nine percent of subjects were African American, and 49%
were women. These characteristics were comparable in each intervention diet
group
The
main results of the study showed significant reductions in blood pressure among
the participants who were completed the DASH Diet compared to the other two
diets. The reductions of BP were
greater in African Americans than whites.
Furthermore, the
participants who had entered the trial with mild hypertension showed the
greatest reduction in blood pressure with DASH (Figure 1).
[dash figure1]
Participants with mild
hypertension on the DASH Diet had an approximately 11.4 mmHg reduction in
systolic BP as compared to the control diet, and a 7.3 mmHg reduction in
systolic BP compared to the fruits and vegetables diet. The difference in diastolic blood pressure
reduction was not as considerable, but there was a statistically significant
reduction. Furthermore, the reduction
in blood pressure was noted within the first two weeks and continued throughout
the duration of the trial. There have been many theories as to why the DASH Diet appears more
beneficial than the fruits and vegetables diet, but this study was not designed
to identify these differences.
Since
the original publication of this data, a number of sub-analyses have also been
presented. A subset of participants in
each group underwent 24-hour ambulatory blood pressure monitoring. This analysis showed persistent benefits to
the DASH Diet at all times of day.
Another report analyzed the results of people who already had mild
hypertension (Stage I Hypertension) prior to starting the trial. As mentioned above, this subgroup showed the
greatest relative benefit from the DASH Diet, and many of these participants
achieved normotensive blood pressure readings by the end of the trial. The treatment effect seen with the DASH Diet
is comparable to the effect seen in subjects of other trials who receive single
agent antihypertensive medication for mild hypertension.
DASH-Sodium Study
The
DASH investigators felt that a logical progression would be to try and combine
the benefits seen with the DASH Diet with sodium restriction. The DASH-2 or DASH-Sodium trials have been
completed, and the results are being prepared for publication. The study design compares people on a
regular American diet versus the DASH diet, but also randomizes participants to
three different levels of sodium intake.
The preliminary data was presented at the American Society of
Hypertension meeting on May 18, 2000.
This preliminary data indicated additive benefits of sodium restriction
and the DASH Diet, such that patients randomized to the lowest sodium intake on
the DASH Diet had the greatest reductions in blood pressure. However, this
finding must be reviewed with caution until the complete results of the
DASH-Sodium trial are published.
Comments
The
results of the DASH Diet trials give patients and physicians a proven, viable
alternative to the drug treatment of hypertension. Critics have pointed out that these trials were performed under
strict supervision with all meals prepared at a central kitchen (and at great
cost). Additionally, there is no data available to determine how strictly a
patient must follow the diet in order to receive the maximum benefit. However, many practitioners have found that
patients are generally able to tolerate the DASH Diet at home much easier than
low sodium diets or other dietary manipulations which have been tried in the
past. Additionally, there is increasing
evidence that the Mediterranean diet, which closely resembles the DASH diet, may have other health benefits
including a reduction in cancer rates.
And, perhaps most importantly, these blood pressure reductions are
obtained without the risk of unwanted side-effects which are often seen with
anti-hypertensive medications.
The JNC-VI recommendations
for mild hypertension include diet and exercise modification. The DASH Diet gives physicians an
intervention whose efficacy has been proven in quality clinical trials in
accordance with the principles of evidence-based medicine. Although it has not been compared
head-to-head with antihypertensive medication, the DASH Diet appears to lower
blood pressure as well as other proven monotherapies. Also, the effects noted in the first two weeks encourages both
patients and physicians that these lifestyle changes can have immediate
effects, comparable with pharmacological therapy. Of course, these modifications may require profound changes in
diet for many people, and may need to be implemented over a longer period of
time to ensure better compliance.
One of the remaining
unanswered questions is the effect of the DASH Diet on long-term cardiovascular
endpoints such as stroke, myocardial infarction, congestive heart failure, and
other cardiac events. Such a follow-up
study would take many years to perform, and is unlikely ever to be performed.
However, with careful physician follow-up, patients with high normal BP or mild
hypertension may be able to delay the initiation of medications, or even avoid
them entirely. As the DASH
investigators point-out, blood pressure reductions of this magnitude have
important public health implications. A population-wide blood pressure reduction of the degree seen in the DASH
study is estimated to reduce the incidence of coronary artery disease by 15% and
stroke by 27%. With an increasing number of studies showing that aggressive
reduction of blood pressure decreases cardiovascular events, the DASH Diet
should prove to be a powerful addition to the antihypertensive
armamentarium.
Selected References:
1.
Appel and colleagues.
A clinical trial of the effects of dietary patterns on blood pressure. New England Journal of Medicine:. 336, 1117,
1997.
2.
Midgeley JP and
others. Effect of reduced dietary sodium on blood pressure. A meta-analysis of
randomized controlled trials. Journal of
the American Medical Association 1996;275:1590.
3.
FL He and GA
McGregor. Potassium intake and blood pressure. American Journal of Hypertension 1999;12:849
4.
Mizushima S and
colleagues. Dietary magnesium intake and blood pressure; a qualitative overview
of the observational studies. Journal of
Human Hypertension1998;12:447-53
Related Links
The
Official Website of the DASH diet
http://dash.bwh.harvard.edu/ |
