It is marketed primarily as a low-fat cereal that can be eaten to help one lose weight. It frequently has give-away offers for various health and fitness products or contains dieting information on the back of the box.
The diet that Special K advocates is called "The Special K Challenge." The goal of "The Special K Challenge" is to lose six pounds in two weeks. The diet is as follows: For meal number one you may have a serving of any Special K Cereal with 2/3 cup skim milk and fruit. For meal number two you may have a Special K Protein Meal Bar, a Special K Protein Shake, or another serving of Special K cereal with 2/3 cup skim milk and fruit. Meal number three can be eaten normally. Throughout the day one may consume two Special K snacks choosing from Special K Protein Snack Bars, Special K2O Protein Water Mixes, Special K Cereal Bars, Special K Crackers, or Special K Fruit Crisps. For additional snacks one may consume fruits and vegetables. Drinks may be consumed normally.
In North America, Special K Original has 120 Cal per 31 g cup serving. One serving contains 0.5 g fat, 23 g carbohydrates, 4 g sugar and 6 g protein. In the UK, Special K Original is 17% sugar, meaning a 30 g serving contains 5 g of sugar.
In the UK, an advertising campaign has focused on the results of a study run in collaboration with the Department of Human Sciences at Loughborough University requiring overweight volunteer subjects to replace two meals a day with a bowl of the cereal.  The study found that "after 2 weeks up to 75% of subjects had slimmer waists and hips. The university's scientists concluded that the majority of this was due to fat loss."
In the US, Special K currently comes in ten different varieties:
- Special K Original
- Special K Chocolatey Delight
- Special K Cinnamon Pecan
- Special K Blueberry
- Special K Red Berries
- Special K Vanilla Almond
- Special K Fruit & Yogurt
- Special K Protein Plus
- Special K MultiGrain Oats & Honey
- Special K Low Fat Granola
In the UK & Ireland, Special K similarly currently comes in a number of varieties:
- Special K
- Special K Red Berries
- Special K Purple Berries
- Special K Peaches & Apricots
- Special K Bliss Creamy Berry Crunch
- Special K Bliss Strawberry & Chocolate
- Special K Blueberry Muffin
- Special K Blueberry Cake
- Special K Oats & Honey
- Special K Yoghurty
- Special K Sustain
- Special K Medley
- Special K Chocolate
Special K provides meal replacements in the form of Special K Protein Meal Bars and Special K Protein Shakes.
There are six varieties of Special K Protein Meal Bars:
- Chocolate Peanut Butter
- Double Chocolate
- Chocolatey Chip
- Chocolate Caramel
- Honey Almond
There are three varieties of Special K Protein Shakes:
- Milk Chocolate
- French Vanilla
Special K snacks are marketed as low-fat alternatives to regular snacks. There are several varieties of Kellogg's Special K snacks, including Special K Protein Snack Bars, Special K20 Protein Water Mixes, Special K Cereal Bars, Special K Crackers, Special K Chips, and Special K Fruit Crisps.
There are two varieties of Special K Protein Snack Bars:
- Chocolate Delight
- Chocolate Peanut
There are three varieties of Special K20 Protein Water Mixes:
- Iced Tea
- Pink Lemonade
- Strawberry Kiwi
There are nine varieties of Special K Cereal Bars:
- Peaches & Berries
- Chocolatey Drizzle
- Vanilla Crisp
- Honey Nut
- Chocolatey Pretzel
- Raspberry Cheesecake
- Chocolatey Chip Cookie
There are two varieties of Special K Crackers:
- Savory Herb
There are four varieties of Special K Chips:
- Sea Salt
- Sour Cream & Onion
- Southwestern Ranch
There are two varieties of Special K Fruit Crisps:
Special K Mini Breaks are a low-fat bite-size snack. They come in the following flavours:
- Zesty Lemon
- Mellow Caramel
Wheat bran, soy grits, rice, wheat gluten, soybean oil, soy protein isolate, whole grain wheat, contains 2% or less of sugar, salt, malt flavoring, sucralose, natural and artificial flavor, BHT for freshness. Vitamins and Minerals: Vitamin C (ascorbic acid), vitamin E (alpha tocopherol acetate), niacinamide, reduced iron, vitamin B6 (pyridoxine hydrochloride), vitamin B1 (thiamin hydrochloride), vitamin A palmitate, vitamin B2 (riboflavin), folic acid, vitamin B12.
Special K's original formula is different in Canada from that used in the US. The Canadian formula has a different taste which reflects the "Canadian taste preference" (according to the Consumer Relations department of Kellogg's in Canada) while the American formula is similar to that used in other related products but without nuts or dried fruits.
The Canadian formula includes rice, wheat gluten, sugar/glucose-fructose, defatted wheat germ, salt, malt (corn flour, malted barley), vitamins (thiamin hydrochloride, niacinamide, pyridoxine hydrochloride, folic acid, d-calcium pantothenate), iron, and BHT added to the package material to maintain product freshness.
'Original': Rice, wheat (whole-wheat, wheat flour), sugar, wheat gluten, defatted wheatgerm, dried skimmed milk, salt, barley malt flavouring, vitamin C, niacin, iron, vitamin B6, riboflavin (B2), thiamin (B1), folic acid, vitamin D, vitamin B12.
'New delicious 3 grain recipe' (introduced 2013): Rice (45%), wholewheat (32%), sugar, barley, malted barley flour, barley malt flavouring, salt, vitamin C, niacin, iron, vitamin B6, vitamin B2 (riboflavin), vitamin B1 (thiamin), folic acid, vitamin D, vitamin B12.
Australian and other markets
Controversy in Europe
Denmark has outlawed the addition of vitamins in Kellogg's products since 2004. Danish health officials banned cereals containing added vitamins because, as they claimed, Kellogg's wanted to add extremely high levels of vitamin B6, calcium, folic acid and iron, which would reach toxic levels when eaten on a daily basis. Young children risk liver and kidney damage, while the fetuses of pregnant women can suffer complications from the toxins.
The Dutch television show Keuringsdienst van Waarde, in an episode aired on October 15, 2009, followed up one of Kellogg's Special K nutritional claims, namely the addition of iron. The show provided evidence that the iron was not nutritional ionic iron - as it occurs in natural foods like spinach - but was in fact metallic iron. A Kellogg's telephone helpdesk employee was not willing to discuss the ingredients of their products in general, claiming it was a company secret, although in the show the company was not confronted with the findings. The nutritional experts in the show (a university professor and a general practitioner) agreed that actual metallic iron should not be part of a diet, speculating that it might damage organs. After the airing, the Dutch food authority nuanced the claims made in the TV program, claiming there are no health risks as long as Kellogg's stays within the legal limits. They also challenged the claim that the cereal could contain 'shredded bikes', and responded that iron powder is suitable for human consumption.
The evidence provided during the show follow David Catz's description of an experiment by Dr. Babu George, Sacred Heart University, in which iron is extracted from cereals. The description dates from 1984. As a result of this experiment being published and inquiries being made to the manufacturers, some companies have replaced the metallic iron in their products with an iron compound such as iron(III) phosphate, also called ferric phosphate.
- "Kellogg's Special K Original Cereal: Nutrition facts". Retrieved 2013-05-23.
- "Special K Cereals: Original". Kellogg Company. Retrieved 2013-05-23.
- "Kellogg's Special K Challenge - The science / research". Archived from the original on 2008-12-18.
- "Still not convinced?". Kellogg Company. Retrieved 2013-05-23.
- "Breakfast Cereal For A Healthy You – Special K® Cereals". Kellogg Company. Retrieved 2013-05-23.
- "My Special K: Cereals". Kellogg Company. Retrieved 2013-05-23.
- "Special K". Kellogg Company. Retrieved 2013-05-23.
- Meikle, James; Harding, Luke (2004-08-12). "Denmark bans Kellogg's vitamins". The Guardian. Retrieved 2013-05-23.
- "Uitzending bekijken, Keuringsdienst van Waarde: IJzer extra". teleblik.nl. 2009-10-29. Retrieved 2013-05-23.
- "Keuringsdienst van Waarde: IJzer". uitzendinggemist.nl. 2009-10-15. Retrieved 2013-05-23.
- http://www.vwa.nl/portal/page?_pageid=119,1639824&_dad=portal&_schema=PORTAL&p_news_item_id=24735[dead link]
- Katz, David A. (2006-07-04). "Extracting Iron From Cereal". chymist.com. Retrieved 2013-05-23.