Do Low-Fat Diets Actually Work? A Critical Look

Woman Eating a Low Fat MealFor many decades now, the health authorities have told everyone to eat a low-fat diet.

At the time the low-fat dietary guidelines were conceived, people though that saturated fat was a significant cause of heart disease.

This idea was the cornerstone of the dietary recommendations of the past few decades.

This is the reason the large health organizations moved away from meat, eggs and full-fat dairy products (high fat) and towards grains, legumes, fruits and vegetables (low fat, high carb).

The guidelines were based on very weak evidence at the time and many respected scientists objected and said that they could have unforeseen consequences.

Today… these ideas have been thoroughly disproven. Many high quality research studies show that there is in fact no association between saturated fat and cardiovascular disease (1, 2, 3).

But yet the guidelines remain unchanged, even though their scientific foundation has been removed.

The Low-Fat Guidelines And The Obesity Epidemic

The low-fat guidelines were first published in the year 1977. Since then, the major health organizations and the government have not changed their position.

Has this advice been effective against the obesity epidemic? A picture speaks more than a thousand words…

Low Fat Guidelines and Obesity Epidemic

Of course, many things were changing in society at the time and this graph does not prove that the guidelines caused the obesity epidemic, just that it started at the same time the guidelines were published.

However, I personally find it plausible that demonizing fat and giving refined carbs and sugar the green light may have had something to do with it.

Because everyone thought that fat was the root of all evil, all kinds of low-fat junk foods flooded the market.

These foods were loaded with refined carbs, sugar and HFCS, which actually are associated with heart disease, diabetes, obesity and all those diseases that the low-fat diet was meant to treat.

Bottom Line: The low-fat guidelines were first published in the year 1977, around the exact same time the obesity epidemic started.

A Tale of Three Massive Studies

Male Doctor Looking Unhappy

Because the low-fat diet is backed by the government and all the major health organizations, research on it has received a lot of funding.

A few massive studies have been conducted on this diet and I’d like to discuss three of them here.

These are huge randomized controlled trials where people are split into two groups.

One group is placed on a low-fat diet, while the other group doesn’t change anything and serves as a control group.

This is scientific evidence, as good as it gets, evaluating the efficacy of low-fat diets.

The Women’s Health Initiative (WHI)

The Women’s Health Initiative was started by the National Institute of Health back in the year 1991. A part of the study was a low-fat dietary intervention, aimed at reducing obesity, cardiovascular disease and cancer.

The study subjects were 48,835 postmenopausal women, randomized to a low-fat group or a control group. The low-fat group was instructed to eat less fat and increase consumption of fruits, vegetables and whole grains.

After a period of 7.5-8 years, the low-fat group weighed only 0.4 kg (!) less than the control group and there was no difference in the rate of cardiovascular disease or cancer (4, 5, 6, 7).

Multiple Risk Factor Intervention Trial (MRFIT)

MRFIT was another massive study that involved 12,866 men who were at a high risk of having a heart attack. The results were published back in 1982.

If anyone were to benefit from a low-fat diet (if it actually worked), then it would be this group.

These men were instructed to quit smoking, eat less saturated fat and cholesterol and increase their consumption of vegetable oils (a typical low-fat diet). And they did… but it didn’t work.

After a 7 year study period, there was literally no difference in the rate of heart attacks or death, despite the fact that more men in the low-fat group quit smoking. Put simply, the low-fat diet was 100% ineffective (8).

Action For Health in Diabetes (The Look AHEAD Study)

The Action for Health in Diabetes study was an intensive lifestyle intervention study in type II diabetic patients, aimed at reducing the rate of cancer, heart attacks and stroke (9).

It was supposed to be a 13.5 year long study, but they stopped at 9.6 years because they saw that it wasn’t working.

The way this study was different is that it did manage to produce weight loss by intense calorie restriction (total intake of 1200 to 1800 kcal per day) and increased physical activity.

After 9.6 years, the intervention group had lowered their weight by 6%, compared to 3.5% in the control group. Not a massive difference, but significant nonetheless.

The low-fat diet group did lose more weight and improved in some aspects like sleep apnea, mobility and quality of life, but there was no difference in heart disease risk between groups (10).

A low-fat diet can apparently lead to weight loss in diabetic patients, but only if accompanied by exercise and severe calorie restriction. Even if weight loss occurs, the patients don’t live longer and don’t have less heart disease.

Bottom Line: Massive long-term studies show that low-fat diets generally do not reduce the risk of heart disease, cancer or other major lifestyle diseases.

Low-Fat Diets Can Adversely Affect Blood Biomarkers

Boy Eating a Sandwich

Despite low-fat diets being recommended by organizations like the American Heart Association, studies show that they can adversely affect risk factors for heart disease.

I’m sure you’ve heard of LDL being referred to as the “bad” cholesterol. It’s only half true… the size of the LDL particles matters.

The more you have of small particles, the greater your risk of heart disease. If the particles are mostly large, then your heart disease risk is low (11, 12, 13, 14, 15).

The thing with low-fat diets is that they can actually change the LDL from the benign large LDL towards the harmful, artery-clogging small, dense LDL (16, 17, 18).


Some studies also show that low-fat diets can reduce HDL (the good) cholesterol and raise blood triglycerides, another important risk factor (19, 20, 21, 22).

Bottom Line: Low-Fat diets can adversely affect important risk factors for heart disease like LDL pattern, HDL and triglycerides.

Why Don’t Low-Fat Diets Work?

Girl Disgusted by Vegetables

There are a few important parts of the low-fat diet that I think should be effective.

For example, these diets emphasize reduced consumption of refined sugar, replacement of refined grains with whole grains and increased consumption of vegetables.

These changes should all lead to weight loss and a lower risk of heart disease. But why don’t they?

Well… that’s because these diets also gets a few very important things completely wrong.

The low-fat diet advocates reduction of saturated fat, which is harmless and may even improve the blood lipid profile (23, 24, 25).

It also advocates increased intake of vegetable oils, which can contribute to inflammation and an increased risk of heart disease (26, 27, 28, 29).

Another side effect of reducing fat intake is that people avoid animal foods like meat and eggs, which are protein rich and can induce satiety and help with weight loss.

Perhaps the low-fat diet would actually be at least mildly effective if it didn’t tell people to reduce saturated fat and increase vegetable oils. It’s definitely possible.

Bottom Line: The typical low-fat diet gets a few things right, such as a reduction in sugar. But it also gets some important things wrong, such as replacing saturated fat with vegetable oils.

Low-Fat Diets Don’t Address The Underlying Metabolic Problems

A man who needs to lose weight

Obesity is not an isolated issue. It is related to many other metabolic problems like cardiovascular disease and type II diabetes.

These issues are known as the metabolic syndrome, also known as Insulin Resistance Syndrome.

It involves insulin resistance, elevated triglycerides, low HDL cholesterol, abdominal obesity and high blood pressure.

It may be that obesity does not cause these other issues… because many obese people are metabolically healthy and many lean people have these problems (30).

Forced calorie restriction may address a symptom (increased calorie intake and weight gain)… but it is ineffective against the underlying metabolic dysfunction that is the root of the issue.

Bottom Line: The reason low-fat diets don’t work may be that they don’t address the underlying metabolic problems that are making people sick and fat in the first place.

A Diet That is Actually Proven to be Effective

If you’ve been following my blog for a while, then you will know that I’m a big fan of low-carb diets.

Low-carb diets cause more weight loss and improve all the major risk factors for disease much more than low-fat diets (31, 32, 33).

One of the reason low-carb diets are so effective is that they reduce insulin resistance and they address the underlying problem instead of just the symptoms (34, 35).

Even though low-carb diets aren’t any sort of magical solution to these problems, they are at the very least a whole lot better than the low-fat diet that is still pushed despite zero evidence of effectiveness.