Does a low-fat diet improve multiple sclerosis fatigue?

This is a brief review of the study "A low-fat diet improves fatigue in multiple sclerosis: Results from a randomized, controlled trial."

Does a low-fat diet improve MS fatigue? No, not based on this study.

There will be people who say low-fat diets help them, but this does not mean that low-fat diets are universally applicable to MS specifically. The study's title claims that the low-fat diet "improves fatigue in multiple sclerosis," which is misleading. The study's actual results do not fully support that conclusion.

What did the study find?

When everyone who participated in the study was factored in — those who completed the study and those who quit for any reason — there was no change in fatigue. The study's other limits make it impossible to say that a low-fat diet definitely improves fatigue in MS.

Bottom line: No, this study does not prove that a low-fat diet specifically improves MS-related fatigue. Why? Too many other factors could have been involved, including:

• It was a short diet (12 weeks).
• It had a small number of participants who completed the study (33 — 17 in active group, 16 in control group) and 6 dropped out (3 from each group).
• The group not following the diet had some of the same benefits as those who did, which shows that the low-fat diet wasn’t the reason for some of the "positive" effects.
• No study participants or researchers were blinded, so placebo effects and bias are very possible.

What MS diet was studied?

Study diet: Low-fat diet (fat less than or equal to 20% of total daily calories, saturated fat less than 7%) versus wait-list control for 12 weeks

This is similar to, but less restrictive than, the low-fat McDougall diet and Swank diet.

Did the study include all MS types?

It was 92% RRMS, 3% SPMS, and 5% PPMS.

The authors stated that the study's results are broadly generalizable because all participants had “moderate-severe disability in at least one functional domain,” but this doesn’t mean the effects translate into all presentations of MS.

The study included people with fewer mobility/disability considerations, with an average EDSS of 3.78.

It also lacked diversity and information on some important demographics:

• 85% white
• 82% female
• No reporting of financial/economic situation
• No data on participant education

Who did the study exclude?

Exclusions can give important information about who the results can't extend to, since they were left out of the study.

This study's exclusions were:

• Significant depression
• Diabetes or other medical problems causing fatigue
• Pregnancy/breastfeeding
• MS exacerbation or IV steroids within 30 days of screening
• Dietary fat content less than 30% at screening
• Fish/flaxseed oil supplementation within 30 days of screening

What were the study's limitations?

The following are the main limitations of the study. Any of them individually could make the fatigue improvement claims questionable.

• Small sample size due to COVID-19 pandemic and inadequate funding
• Inadequate power to detect impact on fatigue in analysis of all original participants
• Both groups reduced total calorie intake
• Active group did not achieve the target of 20% fat calories
• No data collected on socioeconomic status and education
• Predominantly white population affecting generalizability
• Imbalance in fatigue medication use between groups (42% active vs. 16% control)

When all limitations are combined, the claimed benefits become even less likely.

What were the study's results?

Original study plan with all participants: No significant difference in fatigue between groups (gold standard study outcome)

Modified study plan with only those who followed through (biased sample): The diet group had significantly less fatigue, which is what the study’s researchers reported in the article headline.

What's the difference between original and modified results?

When analysis includes all participants as originally assigned regardless of whether they actually completed the intervention, it provides a more realistic picture of real-world effectiveness. It also gives an idea of how feasible the diet might be. High dropout rates, for example, could indicate that a diet is very hard to follow.

Modified analysis, on the other hand, that removes information from non-compliant participants or those who quit the study, often shows stronger treatment effects. But this doesn't mean the diet is proven to work. It means that the information was diluted to only include those who followed the program, which skews the results, making it harder to see how the outcome might work in the real world or for everyone.

Aren't any fatigue improvements good, though?

There's no way of knowing if there were fatigue improvements from the diet, which is what the study was supposed to do.

Both groups reduced how much food they ate, and changes in food intake can influence energy levels in complex ways. Metabolism isn't separate from MS, and isn't a cut-and-dried equation.

The study did not definitely show that the diet change led to fatigue improvements in those who followed it.

Benefits could have come from:

• Eating less overall (because we have no comparison of how much and what was eaten before)
• Eating foods that were easier to digest (lower-fat ingredients may digest better in some people; this does not mean a low-fat diet improves fatigue or helps MS)
• People paying closer attention to their eating habits (the idea of being watched, even by researchers who are only looking at data, often influences eating behaviors)

Were there any researcher conflicts of interest?

The study did not report any conflicts of interest.

However, Vijayshree Yadav, the study's lead researcher, has studied the McDougall low-fat MS diet for many years. She initially collaborated with McDougall directly and received funding from his foundation to conduct research.^[1] Yadav continues to advocate for a low-fat diet for MS^[2][3] despite research outcomes not definitively proving it works better for MS than other dietary approaches. Yadav does not appear to have studied other diet types, making it possible that there is a confirmation bias or belief about how low-fat diets work for MS.