Here's my take.
Firstly the low-carb diet recommended in the study was actually a low carb diet, where the recommendation was for subjects to maintain an intake of digestible carbohydrates of less than 40g daily. That's a rarity among low-carb studies, as to date many have instead focused on what might only be described as "lowish" or "lower" carb intakes - but more on this in a bit. As far as the low-fat dieters, they were told to keep total daily energy intake to less than 30% of total daily calories from fat, and to ensure that 55% came from carbohydrates. Worth mentioning too is the fact that participants in both arms received 1 meal replacement a day for the study's duration, and also received 20 hours of RD counselling and support over its course. Dietary recall was used quarterly to assess compliance and consumption.
Results wise, after a year the low carb folks lost on average 11.7lbs while their low-fat counterparts only lost an average of 4lbs.
Before we go any further, just a quick reminder, it's been known for some time that low-carb diets lead those on them to automatically consume fewer total calories. Some folks, including me, think the reasoning therein lies more with protein than carbs whereby folks on low-carb diets are usually more regular with their inclusions of protein at all meals and snacks which in turn is more sating. Consequently the lower the carbs, the higher the protein and likely the lower the total calories consumed due to increased overall satiety.
Now back to this data. Looking at it a bit more closely it turns out that 88% of the extra weight loss enjoyed by the low-carb folks was accumulated during their first 3 months on their diets - their honeymoon period if you will where participants would likely be paying more care and attention to their diets' details. The honeymoon concept is definitely borne out by the data as well, as looking at the initial daily dietary composition data reporting it would seem that during those first 3 months the low-carb folks were consuming 80g of digestible carbs daily representing 28.9% of their calories but by the end of the study these numbers had increased to 112g and 34% respectively.
And what of protein? The low carb folks started at 25.6% of total daily calories which fell to 23.6% by study's end, while the low-fat folks stayed around 19% throughout.
Looking at calories (because yes, they still count), calories were lowest during the first 3 months for both study arms, but especially so for the low-carb folks where during those first 3 months they reported consuming 190 fewer daily calories than the low fat folks. That difference decreased as the study went on. In fact compared with their first 3 months' reporting, by year's end the low-carbers had upped their total daily calories by 15%, while the low fat folks had only upped theirs by 7.5% with the gap between them now being fewer than 100 calories. Consequently I'd also have loved to see longer term outcomes as I don't think it's a given that there'd be any real difference 2 years out given the more rapidly rising calorie (and carb) counts of this study's so called low-carb arm.
But is this really a low-carb diet study?
Nope. This simply isn't a low-carb study. It's not a low-fat study either by the way.
It's plainly not a low-carb diet study as the low-carb folks, though they were certainly prescribed a low-carb diet, never adhered to one, where even during their diets' honeymoon phase they were consuming over 25% of their total daily calories from carbs, a percentage that rose to 34% by year's end - both far higher than a true low-carb diet would require. Similarly for low-fat where participants weren't even prescribed a low-fat diet, as a diet with 30% of calories coming from fat by definition isn't low-fat. All that said, I'd be willing to wager that the protein distribution among the low-carb diet folks was in fact markedly different from the low-fat folks, as during those 20 hours of RD counselling I've little doubt that the utility of consuming a protein source with every meal and snack in helping to stick to a low-carb approach was emphasized.
It's also important when considering this study and participants' losses not to forget the meal replacement they were provided daily.
So for me this study's overarching take home messages are firstly that our overly saturated-fat phobic national dietary guidelines that still steer people to diets consisting of 55% carbohydrates probably aren't necessary. Secondly, it would seem that for individuals, if you're not planning on tracking calories, having a daily meal replacement while reducing carbs somewhat may well be a viable way to go for a modest amount of weight loss, and perhaps more importantly, for improvements in many metabolic parameters. And thirdly, if the aforementioned approach only leads you to lose a little bit of weight (remember, in this study the average loss for the so-called low-carb dieters after a full year of dieting was only 11.7lbs) I'd encourage you to start keeping a food diary (with more on that from me here), to ensure you include protein with every meal and snack, to markedly reduce liquid calories, to make a concentrated effort to include more produce than products and to re-relegate restaurant meals to special occasions only.
Lastly, it's important to note that if the question is whether you personally should go low-carb, low-fat, or in-between this study certainly doesn't answer that. Ultimately the best diet for you is the one you actually enjoy enough to keep living with, as merely tolerable diets won't last, and any and all can work so long as you enjoy them enough to sustain them as seen in this meta-analysis published yesterday in JAMA.
Putting this all another way it's important not to forget that one person's best diet is undoubtedly another person's worst, and that folks who are stuck dogmatically promoting only one "best" diet can be safely ignored.