Tuesday June 14 2011
A low-fat diet has proven benefits in other areas of health
Eating a diet low in saturated fat “could help stave off Alzheimer's disease”, according to The Daily Telegraph.
The news is based on short-term research that tested two types of diet in 20 healthy adults and 29 people with memory problems. It tested one diet containing low saturated fat and foods with a low glycemic index and compared it to a diet high in saturated fats and with high glycemic index foods (glycemic index is a measure of how fast a food releases its sugars into the blood). It found that the low saturated fat/low glycemic index diet had an effect on the levels of a protein linked to Alzheimer’s disease in the fluid surrounding the brain and spinal cord. The low saturated fat/low glycemic index diet also improved one aspect of mental performance.
The study was small and very short (four weeks), although conducting longer-term controlled trials on these types of diets is unlikely to be feasible. Because Alzheimer’s was not an outcome assessed by the trial, it is not possible to say conclusively whether the diets would affect a person’s risk of developing the disease. More research is needed to study the effects of diet on Alzheimer’s risk.
In the meantime, a diet high in saturated fat is known to be bad for health, and eating less of these fats is likely to have some benefit on heart health, even if it’s not clear whether it will reduce the risk of Alzheimer’s.
Where did the story come from?
The study was carried out by researchers from the Veterans Affairs Puget Sound Health Care System and other research centres in the US. It was funded by the US National Institute on Aging, the Nancy and Buster Alvord Endowment, and the Veterans Affairs Puget Sound Health Care System. The study was published in the peer-reviewed medical journal Archives of Neurology.
The Daily Telegraph reported this study uncritically, although its coverage did feature quotes highlighting its limitations.
What kind of research was this?
This double-blind, randomised controlled trial looked at the effects of specific diets on cognition and various markers for Alzheimer’s disease, both in healthy people and in people with mild cognitive impairment. The researchers say that, to their knowledge, no study has looked at the effects of a dietary intervention on Alzheimer’s-related proteins in the fluid surrounding the brain and spinal cord (called cerebrospinal fluid or CSF).
The researchers say that recent reviews of observational studies have suggested that increased saturated fat intake is associated with an increased risk of Alzheimer’s disease or cognitive impairment, and that increasing monounsaturated or polyunsaturated fat intake is associated with reduced risk. However, they say that trials testing the effects of specific fatty acids in people with Alzheimer’s disease have been disappointing. Therefore, they wanted to test the effects of a “whole diet” intervention that did not just change one dietary factor, but instead controlled the levels of multiple components of the diet.
This type of study is the best way to investigate the researchers’ question. However, it is important to note that the study did not look at the risk of developing Alzheimer’s disease, and it is not possible to say what effects the diets would have on this risk. Carrying out a long-term randomised controlled trial into the effect of these diets on Alzheimer’s risk may not be feasible, as people may not be happy to remain on their prescribed diet for that long. Also, it may not be ethical to ask them to do so.
What did the research involve?
The researchers enrolled 20 healthy older adults (average age 69.3 years) and 29 older adults (average age 67.6 years) with a type of mild cognitive impairment involving memory problems (called amnestic MCI). They were randomly assigned to eat either a diet high in saturated fat and with a high glycemic index (HIGH diet), or a diet low in saturated fat and with a low glycemic index (LOW diet) for four weeks.
The HIGH diet provided:
- 45% of its calories from fat (25% saturated fat)
- 35–40% from carbohydrates (glycemic index greater than 70)
- 15–20% from protein
This diet was designed to be similar to a dietary pattern associated with an increased risk of insulin resistance and type 2 diabetes, which are themselves risk factors for Alzheimer’s disease.
The LOW diet provided:
- 25% of its calories from fat (less than 7% saturated fat)
- 55–60% from carbohydrates (glycemic index less than 55)
- 15–20% from protein
Both diets provided the same amount of calories that the participants would normally consume. Food that complied with these limits was delivered to the participants' homes twice a week. The participants recorded what food they ate to check how closely they stuck to the diets.
At the start of the study and again four weeks later, the researchers tested for levels of Alzheimer’s-related proteins in the cerebrospinal fluid (CSF). To do this, a sample of CSF was taken through a needle inserted into the base of the spine. The participants also completed various biological tests and assessments of cognitive (mental) functioning, including a series of cognitive tests, and measurements of insulin, glucose, and blood lipid (fat) levels.
The various Alzheimer’s-related proteins measured in the CSF included two forms of amyloid beta (Aβ42 and Aβ40) and tau protein. These proteins build up and form abnormal deposits in the brains of people with Alzheimer’s disease.
The researchers looked at whether the allocated diets had any effect on these outcomes in either the healthy participants or those with amnestic MCI.
What were the basic results?
The researchers found that the LOW diet had different effects on the concentrations of the Aβ42 protein in the CSF in people with amnestic mild cognitive impairment (aMCI) compared to healthy individuals.
In people with amnestic MCI, the LOW diet increased concentrations of the Aβ42 protein in the CSF compared to the HIGH diet. The researchers say that this is the opposite of what is usually seen in people with Alzheimer’s disease, who typically have lower levels of Aβ42 in their CSF.
In contrast, in healthy individuals on the LOW diet, there was a decrease in the concentrations of Aβ42 in their CSF, compared to those on the HIGH diet. The researchers suggest that higher levels of Aβ42 in the CSF may be found in a pre-symptomatic stage of Alzheimer’s disease, before the amyloid beta protein builds up in deposits in the brain.
The researchers did not see any effect of diet on levels of another form of amyloid beta (Aβ40) or tau protein.
Finally, they found that the LOW diet improved delayed visual memory (recalling a visually presented information after a delay) in both healthy and amnestic MCI individuals, compared to the HIGH diet. Other measures of delayed memory were not affected by diet, nor were other cognitive tests of immediate memory, executive cognitive functioning, or motor speed.
How did the researchers interpret the results?
The researchers concluded: “Diet may be a powerful environmental factor that modulates Alzheimer’s disease risk”.
This research suggests that diet can have an effect on certain proteins related to Alzheimer’s disease in the fluid surrounding the brain and spinal cord, and also on one aspect of mental performance. As the study was very short (four weeks), it is not possible to say conclusively whether the diets would affect a person’s risk of developing Alzheimer’s disease.
Other points to note include:
- The study was very small, and involved only 20 healthy adults and 29 people with a form of mental impairment (amnestic MCI). These groups were further spilt into smaller subgroups when they were given the two diets. This may make it difficult to detect important effects, and may also make the study more susceptible to chance findings.
- The researchers themselves note that the existing research in the area of diet and Alzheimer’s is “complex”. This study will need to be considered in the context of the research that has been done before.
More research is needed to study the effects of diet on Alzheimer’s risk. In the meantime, it’s known that a diet high in saturated fats is not good for health. Eating less of these fats is likely to have some benefit on heart health, even if it’s unclear whether it will reduce the risk of Alzheimer’s.