March, 2014

Research Perspective and Comments & Analysis

Effects of Hass Avocado Intake on Post-Ingestive Satiety, Glucose and Insulin Levels, and Subsequent Energy Intake in Overweight Adults

Authors: Michelle Wien; Ella Haddad; Keiji Oda; Joan Sabaté, Loma Linda University

Study Summary

A randomized crossover feeding study on 26 healthy, overweight adults, conducted at Loma Linda University, was published in the November 2013 Nutrition Journal. The study evaluated whether incorporating fresh Hass avocado in a lunch — either by replacing other foods or by simply adding it to the meal — would influence satiety, blood sugar and insulin response and subsequent food intake.

Participants ate the same standardized breakfast followed by three different lunch meals on three different days:

  • Standard (control) lunch: without avocado (designed to meet individual meal-based calorie and macronutrient needs) that included a salad with Italian dressing, a baguette and cookies.
  • Avocado replacing part of the Italian dressing and cookies lunch: the same standard lunch with approximately one-half of a fresh avocado in place of part of the Italian dressing and cookies (the same total energy and macronutrient content of the standard lunch).
  • Avocado added to a standard lunch: the same standard lunch with approximately one-half of a fresh avocado added (total higher calories and macronutrients).

The study found that when approximately one-half of a fresh avocado was added to a standard lunch, participants felt more satisfied and had a reduced desire to eat following the meal, than when a standard lunch with no avocado was eaten. When the avocado was added to a standard lunch or replaced part of the Italian dressing and cookies in a standard lunch, the immediate post-meal rise in insulin levels was significantly lower than when a standard lunch with no avocado was eaten. No matter which lunch was eaten, the calories consumed throughout the remainder of the day were equivalent.

While the conclusions drawn from this study are too limited to apply to the general public — due to the study size and study limitations noted by the researchers — the results do provide promising clues and a basis for future research to determine avocados’ effects on satiety and glucose and insulin response.

Standard Lunch Results

After 26 healthy, overweight people ate the avocado added to a standard lunch:

They had a reduced desire to eat after the meal compared to eating the standard lunch. The half avocado added to a standard lunch significantly decreased the desire to eat by 40 percent over a three-hour period compared to the standard lunch; and decreased the desire to eat by 28 percent over a five-hour period compared to the standard lunch.

Participants felt more satisfied after the meal compared to eating the standard lunch. The half avocado added increased self-reported subjective feelings of satisfaction by 26 percent over a three-hour period compared to the standard lunch.

The amount of calories and macronutrients consumed at subsequent meals were not statistically different compared to when they ate the standard and avocado-replacing part of the Italian dressing and cookies lunches.

At 30 minutes after the start of the test meal, the avocado added to a standard lunch helped mitigate rises in insulin compared to eating the standard lunch. However, there was no significant difference in insulin levels over a three-hour period. The rise in insulin levels was significantly weakened by 22 percent 30 minutes after the start of the meal, compared to eating the standard lunch. There was also no increase in blood sugar levels beyond what was observed after eating the standard lunch, despite the extra calories and carbohydrates provided by the added avocado.

There was no increase in blood sugar levels beyond what was observed after eating the standard or avocado-replacing part of the Italian dressing and cookies lunches. Adding approximately one-half of a Hass avocado to a standard lunch did not affect blood sugar any more than the standard lunch or avocado replacing part of the Italian dressing and cookies lunch, despite the extra calories and carbohydrates provided by the added avocado.

Results When Avocados Replaced Part of the Italian Dressing and Cookies from a Standard Lunch

After 26 healthy, overweight people ate the avocado-replacing part of the Italian dressing and cookies lunch: The amount of calories and macronutrients consumed at subsequent meals was not statistically different compared to when they ate the standard and avocado added to a standard lunch. It helped mitigate post-meal rise in insulin compared to eating the standard lunch. Thirty minutes after the start of the meal, the rise in insulin levels was significantly weakened by 37 percent, compared to eating the standard lunch. And, blood insulin remained significantly lower over a three-hour period compared to when the standard lunch was eaten.

Researchers concluded that the reduction in the rise in insulin levels observed with the avocado-replacing part of the Italian dressing and cookies lunch is worthy of future exploration in persons with insulin resistance (e.g. metabolic syndrome) and type 2 diabetes to determine if eating avocados can favorably influence glucose homeostasis.

 

Research Complexity Opens Door To More Study

This is a very important study. Not because it impacts the whole industry, but because it reminds us how complex the whole area of dietary research is. This research is making an interesting point: that a calorie is not necessarily just a calorie. Or put another way, although seemingly it should be easy to gain or lose weight by consuming more or less calories over a person’s stasis level, some calories consumed may be more important than others.

It is easy to dismiss as charlatans the various weight-loss “experts” who claim to have found that certain combinations of food, or avoiding certain foods, can alter the mathematical facts about weight gain and weight loss — and, indeed, many of these folks are charlatans. Yet, this study poses a more interesting question. In a sense, the study asks whether human beings eating real food can somehow alter their behaviors based on what they eat. So, in this case, a calorie of avocado has been found to increase satiety, which would be expected to lead to less snacking and thus lower calorie intake.

Obviously these tests always need to be repeated and conducted by many researchers under varying circumstances before they can be said to become part of settled wisdom. This study was done only on 26 healthy overweight adults. So we know nothing about children or people who are not overweight, and 26 people just raise questions; it can’t provide definitive answers.

Still it wasn’t all that long ago that the National Cancer Institute was resisting the use of avocados as part of the 5-A-Day program. Oddly enough, at the time, avocados and coconuts were seen as the evil high fat heavy produce items, whereas today avocados are filled with so-called “good fat,” and coconut water is the health drink of choice.

The truth is that we know very little about nutrition. The United Fresh Produce Association recently issued a statement praising the government’s decision to increase fresh fruit and vegetable availability under the WIC (Women, Infants and Children) program. At the same time, it chastised the USDA for not including white potatoes in the program. Nutritionally, the government has taken the position that white potatoes are akin to rice or pasta — the starch component of a meal — and there is no reason to support white potatoes over rice or pasta.

Now the truth is there is a lot of politics in all this. Whatever the nutritional argument, the decision is made easier for the USDA by the fact that grain and rice interests would object to any policy that favors white potatoes over rice or pasta. It is also true that United’s advocacy is not particularly based on any research that proves  children fed white potatoes rather than, say, rice, have fewer illnesses or lived longer, happier lives, but rather on United’s imperative to represent the whole produce industry. One for all, and all for one.

This avocado study throws another variable in the mix — one which may not always rebound to the benefit of the produce industry. This study ultimately asks the impact of consumption of particular items on the propensity to consume other items, an area almost totally devoid of research. So even if it is true that adding avocado to meals will increase satiety, and even if we can show this would reduce snacking, and (as a consequence) people will consume less calories — thus gaining less weight, be less likely to be obese  and essentially living longer or healthier lives — it still wouldn’t prove that avocado is the optimum or only way to achieve this effect.

Perhaps peanut butter could do this. Or an olive tapenade. Or smashed bananas. Or bacon. Line-caught salmon? Or thinly sliced USDA Prime Grade New York strip? We just don’t know.

One wonders if the willingness by government and industry to promote 100 percent juice products as part of the Fruits & Veggies, More Matters initiative makes sense when considered in light of this study. After all, 8 ounces of Tropicana Pure Premium Orange Juice has 110 calories. An 8.9-ounce orange has 89 calories. Which is likely to provide greater satiety?

A hat tip goes to the Hass Avocado Board for supporting such research and to the researchers at Loma Linda University for exploring the matter. Let us all remember that when it comes to nutrition, what we don’t know can definitely hurt us. So the primary industry initiative should be supporting research to get greater understanding.