Diet, exercise, and sodium: what the heart trials show

Advice about food, salt, and movement is everywhere, and much of it is stated with more certainty than the science supports. Some of the underlying trials are genuinely strong. Others measured blood pressure but never tracked whether people had fewer heart attacks. A few well-run studies found no benefit at all. This piece walks through the main trials on diet, sodium, physical activity, and alcohol, reports the numbers they produced, and flags where the evidence is thinner than the headlines suggest. It reports findings. It does not tell you what to do.

The DASH trials and blood pressure

The clearest diet evidence is about blood pressure, not heart attacks. In the original DASH trial, researchers fed 459 adults a controlled diet for eight weeks and measured the effect on blood pressure (Appel et al., NEJM, 1997). Everyone ate food prepared by the study, which removed the usual guesswork about who followed the plan. The diet rich in fruits, vegetables, and low-fat dairy, with less saturated fat, lowered systolic blood pressure by about 5.5 mm Hg and diastolic by about 3.0 mm Hg compared with a typical American control diet.

The effect was larger in people who started with high blood pressure. Among participants with hypertension, the DASH pattern lowered systolic pressure by about 11.4 mm Hg and diastolic by about 5.5 mm Hg versus control. Those are sizable shifts for a diet change, on par with what a single blood-pressure drug often delivers. The important caveat: DASH measured blood pressure over eight weeks. It did not run long enough to count strokes, heart attacks, or deaths. The link from a lower reading to fewer events is inferred from other bodies of evidence, not from DASH itself.

Sodium: a real dose-response, thinner data on outcomes

DASH-Sodium tested salt directly, and found a graded effect. Building on DASH, this trial assigned participants to high, intermediate, and low sodium levels while eating either the DASH diet or a control diet (Sacks et al., NEJM, 2001). As sodium fell, so did blood pressure, step by step. On the control diet, cutting sodium from the high level to the low level lowered systolic pressure by about 6.7 mm Hg. Combining the low-sodium level with the DASH diet, versus the high-sodium control diet, lowered systolic pressure by about 7.1 mm Hg in people without hypertension and about 11.5 mm Hg in those with it.

Broader reviews agree that salt moves blood pressure, modestly. A Cochrane analysis of longer-term trials found that moderate salt reduction lowers blood pressure, with a larger drop in people with hypertension than in those without (He et al., Cochrane, 2013).

But the harder question is unsettled. A separate Cochrane review that looked for effects on heart attacks and death, rather than on blood pressure alone, concluded the available trials were too small and too short to give a reliable answer (Adler et al., Cochrane, 2014). So the blood-pressure effect of salt is well documented. The proof that cutting salt prevents cardiovascular events, at the level of a randomized trial, is not.

PREDIMED, the Mediterranean diet, and a retraction worth understanding

PREDIMED is the largest diet trial to report hard heart outcomes, and it has a complicated history. The study enrolled adults at high cardiovascular risk in Spain and assigned them to a Mediterranean diet supplemented with extra-virgin olive oil, the same diet supplemented with nuts, or a control diet with advice to reduce fat (Estruch et al., NEJM, 2013). The first publication reported roughly a 30% lower relative risk of major cardiovascular events (a composite of heart attack, stroke, and cardiovascular death) in the Mediterranean-diet groups.

In 2018 the paper was retracted and republished. Reviewers found that a portion of participants had not been individually randomized. Some were enrolled as households or as whole clinic sites, which breaks the assumption that each person was assigned by chance. The authors reanalyzed the data with those irregularities accounted for and republished the trial (Estruch et al., NEJM, 2018). The main finding held up broadly: a similar reduction of about 30% in relative risk of the primary endpoint.

Read the size of the effect carefully. A 30% relative reduction sounds dramatic, but the absolute difference was modest, because the yearly event rate was low to begin with. Over roughly five years, the gap between groups amounted to a small number of events per hundred people. That is still meaningful across a population, and it is one of the few diet results anchored to actual cardiovascular events rather than to a surrogate like blood pressure.

Physical activity: a strong signal, mostly from observation

The evidence linking movement to lower heart risk is large but not from randomized trials. The US Physical Activity Guidelines summarize decades of research and conclude that more physical activity is associated with lower rates of cardiovascular disease and death (Piercy et al., JAMA, 2018). The 2019 primary-prevention guideline from cardiology’s main bodies reaches the same conclusion and recommends regular activity (Arnett et al., Circulation, 2019).

The candid part: most of this rests on cohort studies. People who exercise more tend to have fewer heart attacks. But those people also differ in other ways that are hard to fully separate out, such as smoking less or having fewer illnesses that limit activity in the first place. There is no large randomized trial that assigned sedentary adults to exercise or not and then counted heart attacks over many years. The strongest randomized exercise evidence comes from cardiac rehabilitation after a heart problem, where supervised exercise programs improved outcomes in people who already had coronary disease (Anderson et al., Cochrane, 2016). That is a different population from healthy adults deciding whether to start walking.

LOOK AHEAD: the null result that matters

Not every sensible lifestyle program lowers heart events, and one large trial showed it plainly. LOOK AHEAD assigned more than 5,000 overweight and obese adults with type 2 diabetes either to an intensive lifestyle intervention aimed at weight loss through diet and exercise, or to a comparison group that received diabetes support and education (Look AHEAD Research Group, NEJM, 2013).

The intensive program worked on weight but not on the primary endpoint. Participants in the lifestyle arm lost more weight and improved fitness and several risk markers. Even so, the trial was stopped early for futility because the rate of the primary cardiovascular outcome was no lower than in the comparison group. This is a useful check against assuming that anything which improves risk factors must reduce events. In this trial, in this population, it did not, at least over the years studied. It does not erase the benefits seen elsewhere, but it argues for humility about extrapolating from surrogate measures to hard outcomes.

Alcohol: the story has moved

For years, moderate drinking was described as good for the heart. That claim has weakened. Older observational studies often showed a J-shaped curve, with light-to-moderate drinkers appearing to have less heart disease than abstainers. Researchers increasingly think much of that pattern reflects confounding, including the tendency to group former drinkers who quit because of ill health in with lifelong abstainers.

A large global analysis found no clearly protective level. The Global Burden of Disease alcohol study concluded that, weighing all health effects together, the level of consumption that minimizes overall harm is zero, and that risk rises with the amount consumed (GBD 2016 Alcohol Collaborators, Lancet, 2018). For heart disease specifically the picture is mixed, with some analyses still showing a modest association between light drinking and lower ischemic heart disease, offset by higher risks of other conditions. The direction of travel in the research has been away from the idea that alcohol protects the heart.

This article reports what specific trials and guidelines found, along with their limits. It is not medical advice, it does not recommend a diet, a salt level, an exercise amount, or a drinking level, and it is not a substitute for care. Individual circumstances change how any of this applies, and the study populations here may not resemble you. Decisions about your diet, activity, blood pressure, and any medication belong with your own clinician, who can weigh your history against the evidence.

Common questions

Does the DASH diet lower blood pressure for everyone?

In the DASH trial, the average drop was about 5.5 mm Hg systolic across all participants, and larger, about 11.4 mm Hg, in those who already had high blood pressure (Appel et al., NEJM, 1997). Averages hide variation, and the trial measured blood pressure over eight weeks rather than tracking heart attacks or strokes.

Is there proof that eating less salt prevents heart attacks?

There is good evidence that cutting salt lowers blood pressure (Sacks et al., NEJM, 2001; He et al., Cochrane, 2013). Evidence that salt reduction itself prevents heart attacks or death is weaker: Cochrane reviewers found the outcome trials too small and too short to give a reliable answer (Adler et al., Cochrane, 2014).

Should I trust PREDIMED after it was retracted?

The 2013 paper was retracted because some participants were not individually randomized. The authors reanalyzed the data to account for that and republished the trial in 2018, and the main result, roughly a 30% lower relative risk of major cardiovascular events on a Mediterranean diet, broadly held up (Estruch et al., NEJM, 2018). It remains one of the few diet trials measuring actual heart events.

If exercise is so good for the heart, why call the evidence weak?

The association between activity and lower heart risk is strong and consistent, but it comes mostly from observational cohorts, not randomized trials in healthy adults (Piercy et al., JAMA, 2018). And LOOK AHEAD showed that an intensive diet-and-exercise weight-loss program did not lower cardiovascular events in people with type 2 diabetes (Look AHEAD Research Group, NEJM, 2013), a reminder that improving risk markers does not guarantee fewer events.