Some Results from a Truly Long-Term Diet Study


It isn't very often we see long-term results in a diet study. In fact, it isn't at all unusual to see a one-year study referred to as "long-term", even though it's clear that the one year mark is "just getting started". Especially when it comes to weight loss studies, I don't really get too excited at anything less than 3 or 4 years, because the long-term weight loss results are usually so dismal, with regain back to base weight (or higher) common even up to five years later.

This is because, as we have discussed often in the past, our bodies resist long-term weight loss. Usually, a long-term weight loss of 5% of body weight is considered a "success". Even though this is usually not what people were hoping for, that amount of weight loss does produce positive health benefits. (And, to be sure, I know many people who have lost and kept off much more -- there is just a lot of variability.)

This month's New England Journal of Medicine provides a rare counter-example -- a followup report 4 years after the end of this diet study published in 2008. Since the study was 2 years long to begin with, these people had been following (or not) their respective diets for six years. Yes, six years -- practically unheard of in the world of diet research. Although the study had its flaws (which I discussed here), any study with this much followup is going to be of interest to me.

Unfortunately, there isn't any data presented as to what people were actually eating at the follow-up points, but the authors state that 67% of the participants had continued to follow some form of the original diet they were assigned to, which is surprising to me. This could be at least partly due to the fact that this was a workplace study.

There continued to be support for one meal per day (lunch at the workplace) of following the diet they were assigned to, and they probably continued to see people in their group fairly frequently. This underlines the importance of support in making dietary changes.

Bottom line: We only have the averages of the three groups, which leaves a lot of information out. On average, people did not maintain a very big weight loss, but some health benefits remained. Also, since nutritionists and others always seem to be worried about the long-term effects of low-carb dieting, this should help allay the fears of some.

There were three groups in the original study: a calorie-restricted low-fat diet group, a calorie-restricted Mediterranean diet group, and a low-carb group (calories not restricted). I noted at the two-year mark that people weren't adhering very well to the original diet instructions; nevertheless, they had made dietary changes in the direction of the diet they were supposed to be following.

Here are some of the results:

Weight: The groups were further apart at the 2-year mark, with the low-carb group sustaining an average of 12 pounds lost at that point (10 for the Mediterranean group, 7 for the low-fat group). By the six-year point, those group retained 3.7, 6.8, and 1.3 lost pounds respectively. These kinds of averages, as we've seen before, don't tell us much about the likely result for any one person following a given diet. We've seen other studies where there were very large differences within groups, where some people lost a lot and others gained in each diet group. This is valuable information, especially when the researchers analyze what factors may have contributed to the differing results. But usually within-group differences are not reported, and this study is no exception.

Blood Lipids: Interestingly, the weight regain by the low-carb group was mirrored by a similar rebound in their triglycerides. Triglycerides are often a marker for how well a person is sticking to a low-carb diet, so it would make sense for people who weren't continuing their low-carb eating to rebound in both of these parameters. Still, the low-carb continued to maintain a significantly more improved LDL:HDL ratio than the other groups, leading the researchers to conclude that "a 2-year workplace intervention trial involving healthy dietary changes had long-lasting, favorable postintervention effects, particularly among participants receiving the Mediterranean and low-carbohydrate diets, despite a partial regain of weight."

Comments: Once again, the low-fat diet did not prove to be more effective than either of the other two. Also, on average, even with only minimal weight loss retained, the low-carb diet not only appears to be safe on the the parameters tested, but may have an advantage in some respects. On the other hand, there is a lot we can't tell from this study, including how any one person will respond (but you can figure that out for yourself pretty easily) and how closely the participants were sticking to the original diet. It would be really great if we had more detailed information. Still, this is an addition to the data we already have, even if it isn't too surprising.

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