diff --git a/code/+nansen/+config/+varmodel/VariableModel.m b/code/+nansen/+config/+varmodel/VariableModel.m
index eabc1490..23c858ab 100644
--- a/code/+nansen/+config/+varmodel/VariableModel.m
+++ b/code/+nansen/+config/+varmodel/VariableModel.m
@@ -162,8 +162,6 @@ function updateDefaultValues(obj)
                 dlModel = nansen.DataLocationModel();
             end
 
-            fileAdapterList = nansen.dataio.listFileAdapters();
-
             for i = 1:numel(obj.Data)
                 if isempty( obj.Data(i).FileAdapter ) || strcmp(obj.Data(i).FileAdapter, str)
                     obj.Data(i).FileAdapter = 'Default';
@@ -179,15 +177,7 @@ function updateDefaultValues(obj)
             if ~isfield(obj.Data, 'DataType') % Infer from file adapter
                 [obj.Data(:).DataType] = deal('');
                 for i = 1:numel(obj.Data)
-                    if ~strcmp(obj.Data(i).FileAdapter, 'Default')
-                        isMatch = strcmp({fileAdapterList.FileAdapterName}, obj.Data(i).FileAdapter);
-                        if any(isMatch)
-                            fileAdapterFcn = str2func(fileAdapterList(isMatch).FunctionName);
-                            obj.Data(i).DataType = fileAdapterFcn().DataType;
-                        else
-                            % pass
-                        end
-                    end
+                    obj.Data(i) = obj.updateVariableDataType(obj.Data(i));
                 end
             else
                 % Normalize existing DataType to char. Todo: support string
@@ -659,15 +649,16 @@ function disableNotifications(obj)
 
     methods (Static) % Todo: Should be moved to a data variable class
         function variableItem = updateVariableDataType(variableItem)
+            % The list holds the data type of every file adapter, also of a
+            % function-based one, which is not a class that can be created
             fileAdapterList = nansen.dataio.listFileAdapters();
             if ~strcmp(variableItem.FileAdapter, 'Default')
                 isMatch = strcmp({fileAdapterList.FileAdapterName}, variableItem.FileAdapter);
                 if any(isMatch)
-                    fileAdapterFcn = str2func(fileAdapterList(isMatch).FunctionName);
-
-                    variableItem.DataType = fileAdapterFcn().DataType;
+                    variableItem.DataType = char(fileAdapterList(find(isMatch, 1)).DataType);
                 else
-                    % pass
+                    % An adapter that is not in the current project keeps
+                    % the data type the item has
                 end
             end
         end
diff --git a/code/+nansen/+dataio/DynamicFileAdapter.m b/code/+nansen/+dataio/DynamicFileAdapter.m
index c02f3e09..bdb486a8 100644
--- a/code/+nansen/+dataio/DynamicFileAdapter.m
+++ b/code/+nansen/+dataio/DynamicFileAdapter.m
@@ -47,11 +47,15 @@
     end
 
     methods (Access = protected)
-        function data = readData(obj)
+        function data = readData(obj, varargin)
+            % Session.loadData passes the variable name and name-value
+            % options to load; the read function takes the file only
             data = obj.ReadFunction(obj.Filename);
         end
 
-        function writeData(obj, data)
+        function writeData(obj, data, varargin)
+            % Session.saveData passes the variable name; the write
+            % function takes the file and the data only
             if ~isempty(obj.WriteFunction)
                 obj.WriteFunction(obj.Filename, data)
             else
diff --git a/code/+nansen/+metadata/+type/Session.m b/code/+nansen/+metadata/+type/Session.m
index 4f104ba7..0e4b1832 100644
--- a/code/+nansen/+metadata/+type/Session.m
+++ b/code/+nansen/+metadata/+type/Session.m
@@ -717,7 +717,17 @@ function updateSessionFolder(obj, dataLocationName, folderPath) %#ok<INUSD>
                 error('NANSEN:Session:MultipleFileAdapters', 'This is a bug. Please report')
             end
 
-            fileAdapterFcn = str2func(fileAdapterList(isMatch).FunctionName);
+            fileAdapterInfo = fileAdapterList(isMatch);
+            if isfield(fileAdapterInfo, 'IsDynamic') && isequal(fileAdapterInfo.IsDynamic, true)
+                % A function-based file adapter is a folder with a
+                % fileadapter.json and read and write functions, not a
+                % class, and DynamicFileAdapter wraps it, as nansen.load does
+                functionName = fileAdapterInfo.FunctionName;
+                fileAdapterFcn = @(filePath, varargin) ...
+                    nansen.dataio.DynamicFileAdapter(functionName, filePath);
+            else
+                fileAdapterFcn = str2func(fileAdapterInfo.FunctionName);
+            end
         end
 
         function viewDataVariable(obj, varName)
diff --git a/tests/+nansen/+unittest/+session/FunctionBasedFileAdapterTest.m b/tests/+nansen/+unittest/+session/FunctionBasedFileAdapterTest.m
new file mode 100644
index 00000000..5293f8c0
--- /dev/null
+++ b/tests/+nansen/+unittest/+session/FunctionBasedFileAdapterTest.m
@@ -0,0 +1,65 @@
+classdef FunctionBasedFileAdapterTest < matlab.unittest.TestCase
+%FunctionBasedFileAdapterTest - Data variables whose file adapter is function-based
+%
+%   A function-based file adapter is a folder with a fileadapter.json and
+%   read and write functions, such as MatFile of the core module. The mock
+%   project gets the variable Values, with the adapter MatFile, and a .mat
+%   file for it in the first session folder.
+%
+%   Run tests:
+%       runtests('nansen.unittest.session.FunctionBasedFileAdapterTest')
+
+    properties
+        Project
+        Session
+    end
+
+    methods (TestClassSetup)
+        function setupProject(testCase)
+            testCase.applyFixture(nansen.fixture.ProjectFixture)
+            testCase.Project = nansen.getCurrentProject();
+
+            % The new item copies the fields of the mock project's item,
+            % which has fields that a default item lacks
+            variableModel = testCase.Project.VariableModel;
+            item = variableModel.Data(1);
+            item.VariableName = 'Values';
+            item.Uuid = nansen.util.getuuid();
+            item.FilePathPattern = '';
+            item.FileNameExpression = 'values';
+            item.FileType = '.mat';
+            item.FileAdapter = 'MatFile';
+            item.DataType = '';
+            variableModel.insertItem(item);
+            variableModel.save()
+
+            testCase.Session = testCase.Project.getSessionObjects( ...
+                testCase.Project.MetaTableCatalog.getMasterMetaTable('session').entries.sessionID{1});
+            Values = magic(3);
+            save(fullfile(testCase.Session.getSessionFolder('MockData'), 'values.mat'), 'Values')
+        end
+    end
+
+    methods (Test)
+        function testInsertedVariableGetsDataTypeOfAdapter(testCase)
+            item = testCase.Project.VariableModel.getItem('Values');
+
+            testCase.verifyEqual(item.DataType, 'struct')
+        end
+
+        function testLoadDataReadsWithFunctionBasedAdapter(testCase)
+            data = testCase.Session.loadData('Values');
+
+            testCase.verifyEqual(data.Values, magic(3))
+        end
+
+        function testSaveDataWritesWithFunctionBasedAdapter(testCase)
+            testCase.addTeardown(@() testCase.Session.saveData('Values', struct('Values', magic(3))))
+
+            testCase.Session.saveData('Values', struct('Values', magic(4)))
+
+            data = testCase.Session.loadData('Values');
+            testCase.verifyEqual(data.Values, magic(4))
+        end
+    end
+end