Dec 14 2009
I have never been a fan of the low-carb diet craze – Atkins, South Beach, or whatever version you prefer. To me this was always a triumph of marketing over science. It is also an excellent example of how public opinion can be largely swayed by a few proponents and a compliant media, while the science goes off unnoticed in a different direction.
Dieting is also one of those issues that is more emotional than one might at first think. I have had the experience on more than one occasion of giving someone, in a very dry and calm fashion, my assessment of the evidence about low-carb diets, only to be met with righteous anger as if I had just trampled on a core tenet of their faith. People are often touchy about what they eat and how much they weigh, and for those who have lost weight and credit a low-carb diet, the published evidence seems irrelevant. “Well, it worked for me” is the almost ubiquitous response.
I also think that people have far too much confidence in their ability to estimate their own caloric intake. The suggestion that perhaps they consumed fewer calories on the low-carb diet and that was responsible for their weight loss is met with outright denial.
There have been some recent studies that cast further doubt on the efficacy and even safety of low carb diets. But first a little background.
The literature on dieting is vast and complex, and we must separate out many distinct questions. Different diets can be assessed for their efficacy in weight loss and control, their heart health, their risk for diabetes, satisfaction, and their overall healthfulness. Evidence also needs to be considered for short term and long term effects. Long term weight control efficacy seems to be largely a factor of ease of compliance.
Like most medical/biological questions, the clinical literature is complex and there are no perfect studies – so how the strengths and weaknesses of various studies are considered can have a dramatic effect on one’s overall opinion. Some studies are too short, they vary in how they monitor compliance with the diet and other variables such as exercise, and on which outcomes they measure. Weight, of course, is the obvious parameter to follow, but there is also mood, cholesterol, insulin levels, and cardiac and diabetes risk.
My quick summary of this complex literature is as follows:
– The evidence strongly supports the conclusion that weight loss and control is dominantly (if not completely) determined by caloric intake. All diets that result in weight loss do so by reducing calories, and the macronutrient make up of those calories is irrelevant (to weight loss). Apparent advantages of low-carb diets are likely related to decreased hunger, which results in decreased caloric intake – but this effect is short term (3-6 months at most) and there is no long term advantage.
– Weight loss diets, in general, do not work. Most dieters will experience short term weight loss, but about 95% will fail to maintain their weight loss long term. Regular exercise seems to be critical for long term weight management (and is good for overall health), as is making lifestyle changes in eating habits, rather than “going on a diet.” Even still – most dieters fail. This likely means that we need to focus our attention on cultural, lifestyle, portion-size, and food manufacturing variables rather than individual dieting as a public health strategy to reduce obesity.
– Low-carb, high-fat diets are likely not heart healthy, although this data is mixed. It seems that there are negative effects of a high-fat diet, but these are offset in the short term by weight loss. So if one loses weight on a low-carb, high-fat diet the net effect is positive for heart risk factors. But since weight loss on such diets is almost always short term, the long term health effects may be negative. But evidence is mixed.
A recent study adds to this literature. Researchers did a randomized controlled trial comparing low-carb (20% carbs and 60% fat) and low fat (60% carbs and 20% fat) diets for 8 weeks. All food was weighed and logged and designed to provide 500 calories less than expenditure. All subject lost weight on this diet to the same degree, supporting prior evidence that calories, not macronutrient content, determines weight loss.
They also found that insulin parameters were the same between the two groups. So there does not appear to be any difference in terms of risk of diabetes. However, they did find an increase in arterial stiffness in the low-carb group. This could translate to increased cardiovascular risk. Other studies have also linked low-carb diets to increased atherosclerosis, which would explain the increased stiffness.
A 20 year epidemiological study, however, showed no increased cardiovascular risk from low-carb diets. So as I said, there is reason for concern about the heart effects of low carb diets, but the data is inconclusive.
This latter study also found that getting fat and protein from vegetable rather than animal sources was associated with lower cardiovascular risk. And this highlights another bottom-line conclusion from the literature – the focus on low-carb vs low-fat may be entirely misplaced. For weight loss and maintenance, portion control and regular exercise seem to be the key factors and macronutrient ratios seem to be irrelevant.
In order to avoid type II diabetes, weight control is key. But also it may be helpful to eat low glycemic index carbohydrates – those that turn to sugar more slowly and therefore have less of a demand on insulin function.
For heart health, the amount of total fat may be a factor, but this is unclear. What is clear is that they type of fat is a significant factor. Vegetable sources of fat have a protective effect, while animal fat increases cardiovascular risk.
In other words – how much carbs vs fat one eats should not be a major concern, and may be largely irrelevant. While the types of carbs and especially fats is important for cardiovascular health.
To summarize, in my opinion here are the best diet recommendations that can be made from existing evidence:
– Eat a varied diet, mostly plant-based
– Limit carbohydrates with a high glycemic index (simple sugars and starches)
– Do not diet for weight loss. Rather, employ reasonable portion control and exercise regularly.
– Whatever you do for weight control, make sure it is sustainable long term. You should be happy with your diet and exercise should be fun and convenient. Anything that seems burdensome will likely not last and be of no long term utility.
– And most importantly – completely ignore diet fads, diet books, or any product that promises easy weight loss. They are scams.
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